Processes and systems for discharge

Abstract

The current disclosure relates to processes and systems for over-discharging of rechargeable battery cells, battery packs, and battery modules. The processes comprise providing one or more battery cells to be discharged; applying a discharge current to the one or more battery cells; discharging the one or more battery cells until it has reached a cell voltage of 0 V, and over-discharging the one or more battery cells below 0 V.

Description

[0001] PROCESSES AND SYSTEMS FOR DISCHARGE

[0002] Technical field

[0003] The present disclosure relates to processes and systems for discharging of battery cells, battery packs, and battery modules.

[0004] Background

[0005] Through the rapid advancement of electrification in many industries, like the automotive or aviation industry, there is a growing demand for batteries. As batteries have a limited lifetime, there will soon be a rapid increase in used, aged, or end-of-life batteries that have to be disposed of. Considering the significant use of rare minerals for production of batteries, recycling of used-up batteries is indispensable.

[0006] Rechargeable (also referred to as "secondary") batteries, including battery cells, battery modules and battery packs, find widespread use as electrical power supplies and energy storage systems. In particular in the transportation sector, to achieve the goal of the Intergovernmental Panel on Climate Change (IPCC) to limit global warming to 1.5°C, electric vehicles (EVs) powered by renewable energy have been tapped as the primary means to achieve decarbonization. As a result of the push from policymakers and global awareness, the number of electric vehicles bought and used worldwide will significantly increase in the next years and, as a consequence thereof, also the number of batteries at their end of life will significantly rise.

[0007] Batteries include heavy metals and may release harmful gases if improperly disposed. Thus, recycling of batteries is imperative to meeting the IPCC goal. Furthermore, several of the elements used in EV batteries, such as lithium, cobalt, nickel, aluminum, copper and manganese, are valuable, in particularly considering the increasing demand for electric vehicles one the one side, and the scarcity of resources and the difficult mining conditions on the other side. Thus, saving resources, minimizing pollution and lowering cost are also driving factors for the recycling of batteries. Current battery recycling processes follow a sequence of discharging and / or disassembly, hydro- or pyrometallurgy, metal extraction and product preparation. Hydrometallurgy, which is considered more environmentally friendly than pyrometallurgy, is preceded by crushing the batteries. For safety reasons, discharging of the batteries is crucial, because during the crushing process the anode and cathode materials could meet, thereby causing a short circuit or self-ignition. If the voltage potential between the anode and cathode is high enough, it could cause thermal runaway, that is, rapid, uncontrolled release of the stored electrical energy, causing the cell to open explosively or to cause a fire.

[0008] Due to inherent fire and explosion hazard of for example lithium ion batteries for electric vehicles and other industrial solutions, particularly at their end of life, discharging is an important step in ensuring the safety of the recycling process. Today, electrical discharging of battery packs is largely a manual or semi-automatic process. With the forecasted 3000% increase in lithium ion battery production in the coming decade, it is estimated that 1.16 million electric vehicle batteries will be at their end of life in Europe by 2030. Therefore, it is increasingly important that a safe and rapid automated process be industrialized.

[0009] Accordingly, enhanced methods for discharging batteries to be recycled are needed.

[0010] Summary

[0011] An object of the present disclosure is to provide new processes which seek to mitigate, alleviate, or eliminate the above-identified deficiencies in the art and disadvantages singly or in any combination. This object is obtained by a process for over-discharging of rechargeable battery cells to be recycled, the battery cells having positive and negative terminals forming electric poles of the battery cell, the process comprising: providing one or more battery cells to be discharged; applying a discharge current to the one or more battery cells; discharging the one or more battery cells until it has reached a cell voltage of 0 volts (V), and overdischarging the one or more battery cells below 0 V. In the present disclosure, the over-discharging of the one or more battery cells below 0 V may be by applying an electric charge of more than 2% of a capacity of each battery cell. In some aspects, a system is provided for over-discharging of rechargeable battery cells to be recycled, the system comprising: a plurality of battery cells, modules or packs connected in series in a first circuit; a plurality of voltage sensors for measuring a voltage of the plurality of battery cells, modules or packs, wherein each battery cell, module or pack of the plurality of battery cells, modules or packs is connected to one voltage sensor; a plurality of bypass switches, wherein each battery cell, module or pack of the plurality of battery cells, modules or packs is arranged with one bypass switch in a second circuit for bypassing said battery cell, module or pack in the first circuit; a plurality of disconnection switches, wherein each battery cell, module or pack of the plurality of battery cells, modules or packs is arranged with one disconnection switch for disconnecting said battery cell, module or pack from the first circuit of the system; a converter connected to the plurality of battery cells, modules or packs in the first circuit. In the present disclosure, the system may be configured to perform the above-mentioned process for overdischarging of rechargeable battery cells to be recycled.

[0012] Other objects and advantages will become apparent to those skilled in the art from a review of the ensuing detailed description, which proceeds with reference to the following illustrative drawings, and the attendant claims. The steps of any process disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

[0013] Brief description of the drawings

[0014] The above and other aspects of the present invention will now be described in more detail, with reference to the appended figures. The figures are not necessarily to scale, and generally only show parts that are necessary in order to elucidate the inventive concept, wherein other parts may be omitted or merely suggested.

[0015] Figure 1 illustrates battery cells having positive and negative poles, where Figure 1A depicts a cylindrical cell and Figure IB a prismatic cell.

[0016] Figure 2 is a flow chart illustrating a process according to the present disclosure. Figure 3 illustrates a system for performing the process according to the present disclosure.

[0017] Figure 4 illustrates a single cell test of a process of the present disclosure.

[0018] Figure 5 illustrates the energy and the charge during the polarity reversal of a process of the present disclosure.

[0019] Figure 6 illustrates a two-part discharge process, where additional discharge was needed to reach complete internal short circuit.

[0020] Figure 7 illustrates a discharge test where the discharge current was regularly interrupted to monitor the voltage rebound in a cell.

[0021] Figure 8 depicts results of discharge tests performed using different discharge currents.

[0022] Detailed description

[0023] The present disclosure provides fast and safe methods / processes for discharging of a battery to be recycled, including battery cells, battery modules or battery packs, by providing a process for over-discharging of rechargeable battery cells to be recycled, such that a polarity reversal induced internal short circuit of the battery cells occurs. Typically, battery cells to be recycled are discharged before manually / mechanically externally short circuiting the cells to remove remaining voltage. The battery cells may be discharged down to 0% SOC (state of charge), which may correspond to an average cell voltage in the range of 2.5 to 2.8 V, before short circuiting.

[0024] Alternatively, in some processes, the battery cells are further deep discharged below 0% SOC, such as down to 0 V, before short circuiting. However, it has been found that over-discharging the batteries below 0 V results in polarity reversal and internal short circuit of the battery cells, which thus removes the need for external short circuit of the cells. By forcing the cell into polarity reversal using an active current to push the cell beyond 0 V, internal short circuit may be attained due to cell material breakdown.

[0025] Thus, the present disclosure provides methods for discharging of a battery to be recycled including electrically over-discharging one or more battery cells of the battery below 0 V. By performing this method of over-discharging such that an internal short circuit occurs, the need to short circuit the battery cell after the battery cell has been discharged in order to prevent any voltage rebound is removed, and thereby increased throughput and lower costs are attained.

[0026] Aspects of the present disclosure will be described more fully hereinafter with reference to the accompanying drawings. The methods disclosed herein can, however, be realized in many different forms and should not be construed as being limited to the aspects set forth herein. Like numbers in the drawings refer to like elements throughout. The features of various embodiments can be combined to form further exemplary aspects of the present disclosure that may not be explicitly described or illustrated. Further it is to be understood that the words and terms employed herein are used for describing specific embodiments only, and is not intended to be limiting, since the scope of the present invention is defined by the appended claims and equivalents thereof.

[0027] The terminology used herein is for the purpose of describing particular aspects of the disclosure only, and is not intended to limit the disclosure. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0028] In some aspects the non-limiting term "battery" or "batteries" is used, which is intended to include a battery cell or battery cells. In some aspects the terms battery module or battery modules are used, which typically contain a plurality of battery cells, or the terms battery pack or battery packs, which typically contain a plurality of battery modules. The term "battery" is used for secondary batteries, i.e., rechargeable batteries. Such batteries are usually provided as modules, which may include multiple battery cells that are connected in series and / or in parallel, or packs, which may include one or more battery modules and associated control circuitry. The processes herein refer to one or more rechargeable battery cell, which are typically present in a battery pack, wherein the one or more battery cells may be present in a battery module present in the battery pack.

[0029] The term "cell voltage" refers to the voltage of the battery cell, which is determined by the chemical reactions in the battery cell, the concentrations of the battery cell components, and the polarization of the battery cell. The voltage calculated from equilibrium conditions is typically known as the nominal cell voltage. The cell voltage may also be referred to as the "operating voltage". The cell voltage may be measured using any suitable method in the art, such as using a multimeter. Similarly as for the cell voltage, the terms "module voltage" and "pack voltage" refer to the voltage of a battery module and a battery pack, respectively.

[0030] In the related art the term "discharging" for the sake of recycling is sometimes used both to refer to discharging down to 0% state of charge (SOC) and down to 0 V. In the context of this disclosure, the term "discharging" is understood as fully discharging a battery down to a voltage of 0 V, or about 0 V. As there might be a small margin of uncertainty in the process, a slight deviation from 0 V will still be considered as that the battery is fully discharged in the processes of the present disclosure. Such full discharging usually makes a battery unusable, as internal processes in the battery cells prevent recharging afterwards, but is important when the discharging is a first step of a recycling process. This type of discharging is thus only intentionally performed when the battery is to be recycled, and is otherwise avoided to prevent premature failure of the battery. Fully discharging a battery down to 0 V comprises "normal" discharging to about 0% SOC and "deep" discharging from 0% SOC to about 0 V. Thus, as the discharging step continues down to 0 V, the term "deep discharging" may be used. For the normal discharging, the cell voltage may be measured or estimated / calculated and correlated to the SOC. For example, the pack voltage is measured and said value is used to calculate the average cell voltage. For Li-ion battery cells the working range is from about 2.7 V to 4.2 V, wherein 0% SOC corresponds to 2.7 V and 100% SOC to 4.2 V. The exact voltages vary between cell types and manufacturers. However, 2.5 V to 2.8 V typically corresponds to 0 % SOC for most battery cells. When said calculation indicates that the average cell voltage is in the range of 2.5 to 2.8 V, then it is estimated that the battery is close to 0% SOC. For the deep discharging, the cell voltage to about 0 V may be measured, as discussed above.

[0031] Furthermore, in the context of this disclosure, the term "over-discharging" is understood as discharging a battery below 0 V, i.e. down to negative voltages. "Extreme over-discharging" may also be used interchangeably. This over-discharging requires an active discharge current, and cannot occur spontaneously, or by using a resistor.

[0032] The method of over-discharging of the battery cells will result in that the polarity of the electrical poles of the one or more battery cells is reversed, which may be referred to as "pole reversal" or "polarity reversal" of the battery cells. Pole / polarity reversal refers to a condition where anode potential becomes larger than the cathode potential, which is further discussed in detail below. The polarity reversal may also be referred to as "reverse potential". Polarity reversal occurs when the cell voltage subceeds 0 V (at negative voltages), and is confirmed by the anode potential becoming larger than the cathode potential. During the polarity reversal, the battery is charged (charge is put into the battery), but in reverse (backwards). After polarity reversal has occurred, internal short circuit of the battery cell will follow. If discharge continues after polarity reversal has occurred, the insulating separator layer between the electrodes will fail, leading to internal short circuit. Internal short circuit may typically occur between -10 to -20 % SOC, or around a cell voltage of -1.5 to -2.2 V (depending on the battery cell). A slight rise in cell voltage may also be registered after the internal short circuit occurs. Internal short circuit may be confirmed by terminating the discharge upon which no voltage rebound takes place.

[0033] The term "capacity" of a battery cell is the total amount of electricity generated due to electrochemical reactions in the battery cell. It is the ability of the battery cell to store a certain charge and is expressed in ampere hours (Ah). For example, a constant discharge current of 1 C (5 A) can be drawn from a 5 Ah battery for 1 hour.

[0034] One problem arising in battery recycling is that the charge level of returned batteries is usually unknown. Due to the high charge density of modern batteries, significant amounts of energy can be stored in the battery at the time it enters a recycling facility. If released in an uncontrolled manner, such stored energy can present a significant electrical shock or fire hazard. Returned batteries are therefore subjected to a controlled discharge process as a first step in the recycling process. In this process, energy stored in the battery can either be dissipated as heat, when the battery is discharged through a resistor, or can be harvested for re-use, when the battery is discharged through a DC / DC converter or similar circuitry.

[0035] The standard technique for electrical discharge is using static or dynamic resistance. This resistance is typically in the form of an electronic load which can be applied to a source to discharge it. The device consists of a resistor or a group of resistors, and an electronic control system. With this device, a constant current (CC), constant resistance (CR), constant voltage (CV) or constant power (CP) can be set to discharge the battery. In the case of the resistor, the energy removed from the battery is converted to heat. Modern electronic loads include a 'regenerative' feature with an AC / DC inverter or DC / DC converter so the energy from the battery can be recovered for use within the facility grid or for other energy storage. Many of these processes aim to discharge the batteries completely (down to 0 V), referred to as "deep discharge" or full discharging, or nearly completely, to avoid thermal runaway when crushing the batteries. When electrical discharge is completed, the cell is normally short-circuited in order to crush it safely, and to prevent voltage rebound. The batteries after deep or full discharge to slightly above, or close to, 0 V, tend to show a rebound effect, where the output voltage increases again even after the battery has been discharged to 0 V. Thus, an external short circuit needs to be applied after discharging to 0 V to prevent the rebound effect before the battery is to be recycled in a next step.

[0036] During the discharging process, heat is generated in the battery. This heat mainly comprises heat created by ohmic losses and by exothermic chemical processes when the battery charge approaches 0 V. Overheating of a battery may lead to a pressure increase in individual battery cells, which may cause a battery to swell and eventually burst or explode. Overheating of a battery may also result in a thermal runaway, in which a cascade of exothermic chemical reactions eventually causes the battery to burn. To avoid such hazards, a general approach is to discharge returned batteries with a low current. Known strategies include constant current (CC), constant resistance (CR), constant voltage (CV) or constant power (CP) control. Depending on the charge capacity of the battery, such low-current discharge can take up to 24 hours.

[0037] Current electrical discharging methods are applied either on individual battery cells or on a group of battery cells with matched initial voltage and / or capacity. These discharging methods typically comprise individual resistors, MOSFETs (metal-oxide- semiconductor field-effect transistors), and / or energy converters for each of the battery cells to be discharged. The former two components, i.e., resistors and transistors, have low controllability and they readily warm up, and thereby waste, heat generated in the discharging process, especially at high discharging rates. As a result, it is often required to cool down such systems and / or have a slow and long deep discharging process.

[0038] On the other hand, application of individual energy converters for electrical discharging of battery cells is attractive, as they utilize heat from the discharging process. However, the high cost of the energy converters and necessity to combine them with resistors and switches makes this discharging method impractical for large-scale applications.

[0039] Furthermore, discharging battery cells to 0 V is often not sufficient to prevent the rebound effect. For all above-mentioned methods, it is needed to have an additional step after the discharge process in which each of the battery cells is short-circuited.

[0040] It would thus be desirable to provide a discharge method and system for batteries to be recycled which enables a higher throughput of batteries in an industrial battery recycling process. It would also be desirable to provide a discharge method and system for batteries with a better control of related hazards and which reduces costs. Furthermore, a discharge method which can be applied on a plurality of battery cells regardless of their initial voltage and / or capacity and avoid the necessity of manual / external short-circuit to prevent the rebound effect in discharged batteries is still needed. These objectives, and related advantages, are achieved by subjecting the batteries to the process for over-discharging of the present disclosure.

[0041] Battery cells in general comprise an anode, cathode, separator and electrolyte. The electrolyte acts as a conductor allowing ions to move between the positive electrode (cathode) and the negative electrode (anode) and in the reverse, in an oxidation and reduction reaction, respectively. In lithium-ion secondary batteries (LIBs), lithium ions move from the anode to the cathode during discharge. The separator is meant to allow the transfer of ions but not to allow a short circuit between the anode and cathode. Furthermore, a battery cell typically comprises a casing for housing the electrodes and the electrolyte, current collectors or terminals and various safety devices, such as polymer gaskets, and one or more vents or valves for venting during thermal runaway. The positive terminal is connected to the cathode, e.g., by an aluminum tab, whereas the negative terminal is connected, e.g., by a copper tab, to the anode. The positive and negative terminals form the electric poles of the battery cell. The terms "electric pole" or "pole" and "terminal" are used interchangeably herein. The battery cell also has a "polarity", which means that one electric pole is the positive pole, and the other electrical pole is the negative pole. The positive pole or the positive terminal has a higher voltage as compared to the negative pole or the negative terminal. When the polarity of the electrical poles is reversed, the positive pole or the positive terminal has a lower voltage as compared to the negative pole or the negative terminal.

[0042] Both cylindrical and prismatic cells are widely used, where cylindrical cell containers typically have two components: a large cylindrical can and a positive terminal cap. The basic structure of such a cylindrical battery is shown in Figure 1A, where the positive and negative poles are shown. The prismatic cells typically are constructed with a metal casing having two external terminals as illustrated in Figure IB. Pouch cells are also used, which comprise repeated stacks of multiple layers of cell components enclosed in an aluminum-coated plastic film. The pouch cells comprise two external terminals, one positive and one negative, similar to prismatic cells. Each terminal is welded to all current collectors of corresponding polarity in a pouch cell.

[0043] Batteries in general are characterized by their capacity and operating voltage. The operating voltage of a battery cell is limited by the potential difference across the terminals of the battery when no current is being drawn, also known as the open circuit voltage (Voc). For safe operation, battery cells are typically limited to an operating voltage of 4.2 V. Over a given voltage, a cell can only deliver a specific amount of electric charge or current, this is known as the capacity (Q) of the battery which is measured in ampere hours (Ah). The capacity is limited by the electrode, particularly cathode materials.

[0044] Whether a battery cell is fully charged or not is monitored using the state of charge (SOC). The SOC is defined as the available electric charge over the full capacity of the battery. At 100% SOC a battery cell is fully charged and has its full capacity. At 0% SOC, the cell has little capacitance left and can supply little current. 0% SOC typically means that about 2.5 - 3 V remains, such as 2.5-2.8 V. Based on the SOC and type or size of a battery cell, the energy content of the battery cell may be derived.

[0045] In the present discharge process, the draw-backs of the discharging methods of the prior art is overcome by a process for over-discharging of rechargeable battery cells to be recycled, comprising providing one or more battery cells to be discharged, such as individual cells, or cells in battery packs or modules; applying a discharge current to the one or more battery cells, wherein the discharge current may be applied to a system of battery cells; discharging the one or more battery cells until they have reached a cell voltage of 0 V and continuing the discharge process for over-discharging the one or more battery cells below 0 V. The over-discharging continues until an internal short circuit of the cell has occurred. If the cell is present in a system, the individual cell may have a cell voltage below 0 V, while the total voltage of the system may still be positive. Thus, a benefit of having a number of cells in series during discharge is that the polarity of the total system voltage can be maintained positive, which simplifies the converter and control system. When internal short circuit has occurred, the cell may be disconnected from the system and swapped for a new cell to be discharged. This gives rise to an efficient discharging process and system for battery cells to be recycled.

[0046] Several effects are present upon discharging a battery cell below 0 V. At negative voltages a polarity reversal occurs in a battery cell, wherein the polarity reversal represents the condition where the anode potential becomes larger than the cathode potential.

[0047] Once the polarity is reversed, the positive pole becomes negative and the negative pole becomes positive, i.e., the polarity of the electrical poles is reversed.

[0048] In order to achieve the polarity reversal at 0 V and a subsequent internal short circuit, a discharge current should be actively pushed through the battery cell until the cell voltage is below 0 V and until the internal short circuit has occurred. If the discharge current is not actively pushed through the battery cells, i.e., if only a passive discharge, such as a resistor, is applied to the battery cell, the polarity reversal will not occur. The polarity reversal is also referred to as "reverse potential". Further discharging of the battery cell beyond the polarity reversal results in the dissolution and oxidation of the copper (Cu) current collector to Cu2+in the battery cell. The dissolution and oxidation of the Cu collector takes place when the anode potential reaches certain potential, e.g., around 3.4 ~ 3.5 V versus Li / Li+. The dissolved copper is then deposited on the cathode, the separator, and the anode in the battery cell, and thereby the dissolved copper forms a contact between the two electrodes and an internal short circuit within the battery cell occurs. Internal short circuit may be detected by measuring that voltage rebound does not occur, i.e., that the cell voltage does not rebound to more than about 0.05 V. This remaining small rebound is due to the fact that a perfect short circuit cannot be reached in reality. The short circuit will have some resistance, leaving an initial voltage over the poles when disconnected. That voltage will eventually disappear as the small remaining energy slowly discharges.

[0049] To achieve an internal short circuit in a battery cell, a fraction of the capacity of the battery cell needs to be applied during the polarity reversal (during the overdischarging) by subjecting the battery cell to an external influence, e.g., to a current. As the current is applied during the polarity reversal, the charge is put into the battery cell, and thereby the battery is charged in reverse. Once the current has been stopped after a certain amount of time, the battery cell will rebound to a positive voltage, which should not be more than about 0.05 V.

[0050] The inventors have realized that discharging a battery cell below 0 V, i.e., beyond the polarity reversal and attaining internal short circuits therein, enables both quick discharge of the battery cell and removes the need to short circuit the battery cell externally after the battery cell has been discharged. Furthermore, the discharge method of the present invention can be applied to a plurality of battery cells regardless of their initial voltage and / or capacity. It is thereby possible to perform the current discharge method on a series of connected battery cells in a system, for efficient discharge of a plurality of battery cells. Alternatively, battery modules or battery packs can be used with this method.

[0051] Thus, by performing the process for over-discharging, the throughput increases and less resources are spent per discharged battery. Thus, the new process provides an increased throughput and asset utilization, recovery of 95% of the energy in the battery, controlled and supervised discharge during both the "discharge" phase and the "over-discharge" phase. The new process also provides utilization of series of connected battery cells with same or different initial voltage and / or capacity, and removes the step of short circuiting the discharged battery cells. Accordingly, an improved discharge process is provided and, by over-discharging of rechargeable battery cells to be recycled, an increased throughput is reached in the recycling discharge system.

[0052] Hence, an improved process for over-discharging of rechargeable battery cells to be recycled is provided, the battery cells having positive and negative terminals forming electric poles of the battery cell, the process comprising: providing (SI) one or more battery cells to be discharged; applying (S2) a discharge current to the one or more battery cells; discharging (S3) the one or more battery cells until they have reached a cell voltage of 0 volts, V, and over-discharging (S4) the one or more battery cells below 0 V. Both discharging (S3) and over-discharging (S4) include subjecting the one or more battery cells to external influence to discharge the one or more battery cells, wherein said external influence may or may not be varied during the discharge process. In an example, subjecting the one or more battery cells to external influence includes subjecting the one or more battery cells to a discharge current, which may or may not be varied during the discharge process. Thereby, the discharge method removes energy from the one or more battery cells, which energy may be recovered using an AC / DC inverter or DC / DC converter, as described above. As such energy recovering equipment is quite expensive, it is beneficial to use only one converter for a plurality of battery cells as in this method.

[0053] Several effects are present upon discharging a battery cell below 0 V, such as a polarity reversal and an internal short circuit. The polarity reversal occurs at the instance when the voltage subceeds 0 V, when the anode potential becomes larger than the cathode potential.

[0054] Further discharging beyond the polarity reversal results in the dissolution and oxidation of the copper (Cu) current collector to Cu2+in the battery cell at around 3.4 ~ 3.5 V versus Li / Li+. The dissolved copper forms a contact between the two electrodes by its deposition on all parts of the battery cell (i.e., cathode, separator, and / or anode) and an internal short circuit within the battery cell occurs.

[0055] The one or more battery cells used in the process may be present in one or more battery packs or battery modules. In the battery modules and battery packs, all battery cells are in polarity reversal at the same time. Thus, the voltage and current of a battery module or a battery pack may be measured, and then the discharge current may be applied. Continuous monitoring of the voltage during the process is performed for the battery module or the battery pack instead of monitoring the voltage of each battery cell in the battery module or the battery pack. During the polarity reversal, the battery module or the battery pack may be charged with at least 2% of the total capacity of the battery module or the battery pack. In an embodiment, the battery module or the battery pack may be charge with 3-7% of the total capacity of the battery module or the battery pack. Once the complete internal short circuit has occurred and the rebounded average cell voltage is < 0.05 V for each cell of the battery module or the battery pack, the process can be terminated and the battery module or the battery pack can be removed from the discharge process / system.

[0056] The present disclosure reaches one or more of the above objectives by providing a system (100) for over-discharging of rechargeable battery cells to be recycled, wherein the system comprises: a plurality of battery cells, modules or packs (10) connected in series in a first circuit; a plurality of voltage sensors (20) for measuring a voltage of the plurality of battery cells, modules or packs, wherein each battery cell, module or pack of the plurality of battery cells, modules or packs is connected to one voltage sensor; a plurality of bypass switches (30), wherein each battery cell, module or pack of the plurality of battery cells, modules or packs is arranged with one bypass switch in a second circuit for bypassing said battery cell, module or pack in the first circuit; a plurality of disconnection switches (25), wherein each battery cell, module or pack of the plurality of battery cells, modules or packs is arranged with one disconnection switch for disconnecting said battery cell, module or pack from the first circuit of the system; and a converter (50) connected to the plurality of battery cells, modules or packs in the first circuit.

[0057] Example operations

[0058] The proposed methods will now be described in more detail referring to Figure 2. It should be appreciated that Figure 2 comprises some operations which are illustrated with a solid border and some operations which are illustrated with a dashed border. The operations which are illustrated with solid border are operations which are comprised in the broadest example embodiment. The operations which are illustrated with dashed border are example embodiments which may be comprised in, or a part of, or are further embodiments which may be taken in addition to the operations of the broader example embodiments. It should be appreciated that the operations do not need to be performed in order. Furthermore, it should be appreciated that not all of the operations need to be performed.

[0059] Figure 2 illustrates a process for over-discharging of rechargeable battery cells to be recycled, said battery cells having positive and negative terminals forming electric poles of the battery cell, the process comprising: providing (SI) one or more battery cells to be discharged; applying (S2) a discharge current to the one or more battery cells; discharging (S3) the one or more battery cells until they have reached a cell voltage of 0 V, and over-discharging (S4) the one or more battery cells below 0 V. Thus, one or more battery cells are subjected to a discharge current, such as an initial discharge current, to discharge the battery cells down to 0 V. The discharge is then continued by over-discharging to negative voltages, i.e., a cell voltage of below 0 V.

[0060] It has been shown (see example section below) that discharging have to continue below 0 V for the internal short circuit to occur, and if aborted too soon before it has occurred, will lead to voltage rebound (above 0.05 V per cell). It has been shown that applying an electric charge of more than 2% of the maximum capacity (the ability of a battery / battery cell to store a certain charge) of each battery cell (i.e., 0.02*Q Ah, where the capacity is Q), such as 3-7 %, typically will be enough for internal short circuit to occur. Discharging by applying a specific level of charge, such as charge of more than 2% of the capacity of the cell, has the advantage of being based on controllable parameters, specifically the amount of current and time. This makes the discharge process predictable and thereby also more efficient than controlling the process by e.g. levels of applied energy. Moreover, the process is more convenient for performing, especially for series of connected cells. Thus, in some embodiments the process for over-discharging may further comprise overdischarging (S4) the one or more battery cells by applying an electric charge of more than 2% of the capacity of each battery cell. In an embodiment, the electrical charge is in the range of 3-7% of the capacity of each battery cell. For a pack, the voltage and current of the whole pack may be monitored / measured, and the capacity fraction used will be based on the entire pack, with no need to monitor individual cells, i.e. it may be charged with e.g. 3-7 % of the total pack capacity. The same applies for a module. As an example, for a 160 Ah battery, the needed electric charge during the over-discharge (S4) is at least 4 Ah.

[0061] During the process for over-discharging of rechargeable battery cells to be recycled, the battery cells are subject to external influence, e.g., to a discharge current. For example, the discharge current may be set based on C-rates, especially for the overdischarging step S4. The C-rate in this regard is a measure of the rate at which a battery is discharged relative to its maximum capacity (Q). A 1C rate means that the discharge current will discharge the entire battery from 100% of the capacity to 0% of the capacity in 1 hour. For a battery with a capacity of 100 Ah, this equates to a discharge current of 100 Ampere (A). In an embodiment, the discharge current may be up to 1.5C , i.e., discharge current < 1.5C, wherein C equals a discharge current (A) to fully discharge the battery within 1 hour. The discharge current may be for example 1C, or lower. In some embodiments, the discharge current is in the range of 7.5-240 A, depending on the battery to be discharged. As an example, for a 160 Ah battery, the discharge current can be as high as 1.5 C , which corresponds to 240 A. For even larger battery cells, the discharge current can be higher. For a small cylindrical 5 Ah battery, the discharge current can be even 1.5 C, which corresponds to 7.5 A. For a pack, the discharge current used will be based on the entire pack. The same applies for a module.

[0062] The discharge current may or may not be varied during the process, where an initial discharge current is used in step S3. In some embodiments, the discharge current in step S3 and / or S4 is constant. Thus, the discharge current can be constant or variable during the process or any of its steps, such as during the discharging (S3) step or the over-discharging (S4) step. The discharging step (S3) and the overdischarging step (S4) can comprise continuous monitoring (S31, S41) of the voltage of the battery cell through one or more sensors. In some embodiments, determining that internal short circuit has most likely occurred may be based on measured cell voltage. It is likely that an internal short circuit has occurred when a battery cell has reached a certain negative voltage, or a certain negative SOC. Thus, in some embodiments, the over-discharging (S4) of the battery cell results in the battery cell having a voltage in the range of -0.2 to -2.5 V, such as -0,5 to -1.8 V.

[0063] The duration of the over-discharging step (S4) depends on the battery to be discharged. In some embodiments, the processing time of the over-discharging step may be up to 60 minutes. However, if a higher current / C-rate may be used, the time may be significantly lower, such as a few minutes (e.g. 2-8 minutes). In such a fast process, the problems related to heat and / or pressure build-up may be avoided. As an example, for a small cylindrical 5 Ah battery, the over-discharging (S4) step can be performed in around 3 minutes if the 7% of the capacity of the battery has been applied using the discharge current of 1.5 C (7.5 A) during the over-discharging step. The processing time for the complete discharging process, including discharging (S3) down to 0 V, will depend on the battery and the initial SOC. In addition, the time needed for the over-discharge (below 0 V) to attain a complete short circuit depends on how aggressive the discharge is. When the discharge current is such that the voltage is slightly below 0, a longer time may be needed compared to if the voltage is far below 0 V.

[0064] The duration of the complete discharge process, including the discharging (S3) and the over-discharging (S4) step, may be up to 60 minutes per each battery cell, such as 45 minutes per each battery cell. Typically, the first step, S3, down to 0 V will take longer time than the over-discharging step S4. As an example, for a small cylindrical 5 Ah battery and the discharge current of 1.5 C (7.5 A), the discharging (S3) step can be performed in about 40 minutes, and the over-discharging (S4) step can be performed in around 5 minutes if the 7% of the capacity of the battery has been applied in the over-discharging (S4) step.

[0065] During the over-discharging step as described above, the electrical polarity of the battery cell is reversed when the cell reaches a negative cell voltage. Thus, in some embodiments over-discharging (S4) the one or more battery cells comprises discharging the one or more battery cells until the electrical poles of the one or more battery cells are reversed. One may measure the voltage of the battery cell to confirm that polarity reversal has been attained, and for how long it had been charged in reverse polarity.

[0066] Further application of the discharge current leads to an internal short circuit. Thus, in some embodiments, over-discharging (S4) the one or more battery cells comprises discharging the one or more battery cells until internal short circuit of said cells has occurred. This internal short circuit may occur once the battery cell is below -10% state of charge, SOC. Internal short circuit may also be detected by a slight increase in cell voltage during the discharge. Thus, reaching a minimum voltage followed by a slow rise in voltage may indicate that internal short circuit has occurred. This may be detected in the continuous voltage monitoring of the battery cell (module or pack). It may also be detected / confirmed by measuring that voltage rebound of a cell is below a voltage level, such as below 0.05 V.

[0067] The process for over-discharging of rechargeable battery cells to be recycled may further comprise continuously or repeatedly monitoring a battery cell and system thereof, including status of a battery cell through one or more sensors by determining one or more status parameters. Monitoring may include monitoring the cell voltage, the discharge current used, and the charge and energy applied to the battery. Monitoring may include monitoring a status of the battery by determining or measuring one or more status parameters, including at least one of output voltage, output voltage change rate, temperature, temperature change rate, and / or emission behaviour of the battery. The monitoring may further include determining a charge level of the battery from the output voltage, output voltage change rate, temperature, and / or temperature change rate of the battery. In an embodiment, the process for over-discharging of rechargeable battery cells to be recycled may further comprise continuously monitoring (S31, S41) the voltage of the one or more battery cells through one or more sensors during discharging (S3) and overdischarging (S4).

[0068] Internal short circuit may be detected by measuring that voltage rebound does not occur. In one example this may be determined by detecting that the voltage of a cell does not rebound to more than about 0.05 V. This small rebound is due to the fact that a "perfect" internal short circuit cannot be reached in reality. The internal short circuit will have some resistance, leaving an initial voltage over poles when disconnected, which will eventually disappear as the small remaining energy slowly discharges. Thus, the method may comprise steps of allowing the battery cell to selfrebound to 0 V and confirming that the rebounded voltage is < 0.05 V. If the rebounded voltage is < 0.05 V, the battery cell may be removed from the discharge process / system. If the rebounded voltage is > 0.05 V, further discharging of the battery cell by applying an external discharge current may be performed.

[0069] Once the internal short circuit has occurred the discharge process may be terminated (S5), which may include removing or disconnecting the battery cell from the process or system, which may be followed by replacing the battery cell with a new battery cell to be discharged. Typically, the discharging process for overdischarging of rechargeable battery cells to be recycled, including discharging (S3) and over-discharging (S4), may have a process time in the range up to 6 hours per cell, such as 45 minutes per cell.

[0070] The process for over-discharging of rechargeable battery cells can further comprise steps of allowing (S6) the one or more battery cells to self-rebound (to around 0 V if internal short circuit has occurred) and confirming (S7) that complete internal short circuit of the one or more battery cells have occurred, such as determining that the rebounded voltage is < 0.05 V for each battery cell, where "complete" internal short circuit refers to that the short circuit has been present long enough in a cell to deplete the energy from the battery cell. If the rebounded voltage is < 0.05 V, the battery cell may be removed (S8) from the discharge process. If the rebounded voltage is > 0.05 V, further continued discharging of the battery cell may be performed by applying an external discharge current, i.e., the step S2-S4 can be repeated until the rebounded voltage is < 0.05 V. If several cells are present in a pack or module, then the average rebounded voltage should be < 0.05 V per cell. Thus, the rebounded voltage should be < X*0.05 V, where X is the number of battery cells.

[0071] After the rebounded voltage is < 0.05 V and the battery cell has been removed (S8), the battery cell can be sent (S9) to a next step in a battery cell recycling process, wherein a next step in a battery cell recycling process may comprise crushing the one or more cells, before metal extraction.

[0072] The process can be performed correspondingly using battery packs or battery modules. However, for the battery packs or the battery modules the voltage and current may be measured and monitored for the whole pack / module without monitoring each battery cell in the pack / module. The over-discharging (S4) of the battery pack or the battery module is done by applying an electric charge of more than 2% of the capacity of the whole pack / module, such as in the range of 3-7% of the capacity of the whole pack / module. Other parameters are the same as described above for battery cells.

[0073] Figure 3 illustrates a system for performing the process according to the present disclosure. The system comprises a plurality of battery cells, modules or packs (10) connected in series in a first circuit. The plurality of battery cells (10) can be also arranged in trays, and a plurality of such trays (10) can be connected in series in the first circuit. The trays are holders for the plurality of battery cells making it easy to transport them. In an example, each tray typically holds between 20 and 150 battery cells, depending on the size of the battery cells. Even if the battery cells are arranged in trays, each battery cell is monitored and / or treated individually. Once all battery cells in a tray are discharged, such as that the rebounded voltage of each battery cell in the tray is < 0.05 V, the tray can be disconnected. Alternatively, one might use a jig to arrange a plurality of battery modules or battery packs therein. The number of the battery cells, arranged as individual battery cells, in trays, in battery packs, in battery modules, or in jigs can be as high as several hundreds.

[0074] The system further comprises a plurality of voltage sensors (20) for measuring a voltage of the plurality of battery cells, modules, or packs. Each battery cell, module, or pack of the plurality of battery cells, modules, or packs is connected to at least one voltage sensor, such that every individual cell / module / pack may be monitored individually. Each voltage sensor (20) may then continuously monitor the voltage of the battery cell, module, or pack to which it is connected. Accordingly, each battery position is monitored by a voltage sensor and controlled by a bypass switch and a disconnection switch that enable swap in and out of individual batteries during continuous discharge. The voltage sensor give feedback to the control system about the progress of the battery discharge, allowing the system to determine when the discharge sequence is finalized for that battery cell. The system also comprises a plurality of bypass switches (30) and a plurality of disconnection switches (25), such that every individual cell / module / pack / tray / jigs also may be individually bypassed and / or disconnected from the system, such as for swapping the cell / module / pack / tray / jig for a new entity (cell / module / pack / tray / jig) to be recycled once the present one has reached complete internal short circuit. The switches may be arranged on each battery cell in a module / pack / tray or jig, or one per whole entity. Each battery cell, or each battery cell in a module, pack, jig or tray, of the plurality of battery cells, modules, packs or trays, is arranged with one bypass switch in a second circuit for bypassing said battery cell, module, pack, jig or tray in the first circuit (one second circuit per entity). In addition, each battery cell, module, pack or tray of the plurality of battery cells, modules, packs or trays is arranged with one disconnection switch for disconnecting said battery cell, module, pack, jig or tray from the first circuit of the system.

[0075] The system further comprises a converter (50) connected to the plurality of battery cells, modules, packs, trays or jigs in the first circuit. The converter may be any suitable converter in the art, such as AC / DC or DC / DC, and may perform energy recovery. The system may be configured to arrange the plurality of entities, such as battery cells, modules, packs, jigs or trays, to be discharged in series, and to discharge the plurality of battery cells, module, packs, jigs or trays using a discharge current from the converter (50) while continuously monitoring each battery cell voltage, module voltage, or pack voltage, using a voltage sensor (20) connected to the battery cell, module, or pack.

[0076] The system as described above utilizes series of connected battery cells, battery modules or battery packs, to perform discharge, polarity reversal and internal short circuit of the battery cells, battery modules or battery packs. The battery cells / modules / packs are connected in series in the system and there is therefore no need to match the voltages, as the case would be for parallel connected cells / modules / packs.

[0077] The system may also be configured to, upon detecting that a battery cell voltage, module voltage or pack voltage of one of the plurality of battery cells, modules, or packs has passed below 0 V and that internal short circuit has occurred in the battery cell(s), close the bypass switch (30) of said battery cell, module, or pack and open the disconnection switch (25) of said battery cell, module, or pack to disconnect the battery cell, module, or pack from the discharge current. Once the disconnection switch (25) is open it is possible to remove the disconnected cell, module, or pack from the system, and optionally to replace (swap in and out) the removed cell, module, or pack with a new cell, module, or pack to be discharged, which may be performed during continuous operation. Thereby, the system may be configured to perform the process for over-discharging of rechargeable battery cells to be recycled, as described above.

[0078] Accordingly, an efficient system for discharging of a plurality of entities may be attained, which system provides fast and efficient discharge of said entities, short discharge time, and which does not require manual or external short circuit to be performed after removal from the system. It may be noted that even though individual entities (e.g. cells) have negative voltages, the net voltage is still positive. The system may be built for individual entities (battery cells, battery packs and / or battery modules). During the polarity reversal, the entities (battery cells, battery modules, or battery packs) may be charged with at least 2% of the total capacity of the entity (battery cells, modules, or packs). In an embodiment, the battery cells, the battery modules, or the battery packs may be charged with 3-7% of the total capacity of the battery cell, module or pack. Once the complete internal short circuit has occurred and the rebounded cell voltage is < 0.05 V for each battery cell of the one or more battery cells, the process can be terminated, and the battery cell can be removed from the system. For a module or pack, the process may be terminated once an average cell voltage of each battery cell in a module or pack is < 0.05 V, such that the total rebounded voltage of the module or pack is below a total voltage of 0.05 V times the number of cells in the module or pack. As an example, for a battery pack with 100 battery cells, the rebounded cell voltage should be on average < 0.05 V for each of 100 battery cells, i.e., the rebounded voltage of the battery pack as a whole should be < 5 V. Alternatively, an entity (a plurality of battery cells, battery packs and / or battery modules) may be placed in a tray or on a jig. During the polarity reversal, the battery cells, battery modules, or battery packs in the tray or on the jig may be charged with at least 2% of the total capacity of the battery cells, modules, or packs in the tray or on the jig. In an embodiment, the battery cells, the battery modules, or the battery packs in the tray may be charge with 3-7% of the total capacity of the battery cells, the battery modules or the battery pack in the tray. Once the complete internal short circuit has occurred and the rebounded voltage is < 0.05 V for each battery cell of the one of more battery cells, or each battery cell in the modules or packs, in the tray or on the jig, the process can be terminated and the tray or the jig can be removed from the system.

[0079] The system may have a large capacity (high volume cell discharge), having hundreds of entities (e.g. cells) connected in series to one converter which actively discharges the cells, which entities may be bypassed, disconnected and swapped when internal short circuit has occurred in the individual entity, without stopping the operation of the system, i.e. the remaining entities will still be discharged since the total voltage in the (first) circuit will still be positive. Thus, only one converter may be needed for simultaneous discharge and short circuit of a large plurality of cells. It may be built for individual cells or cells placed in trays, for simple swapping. Since e.g. the cells are not matched and continuously swapped in and out, the total voltage can remain high. The system also has a beneficial setup for recovering the electrical energy of the cells at a low cost. Since the battery cells / modules / packs can be continuously swapped in and out in the system, the system enables recovering the electrical energy at a low cost, while keeping manual handling minimal. Furthermore, as the battery cells / modules / packs are not matched, the total voltage remaining in the battery cells can be high, and this system enables utilization of all energy.

[0080] EXAMPLES

[0081] Example 1, a single cell test

[0082] A single cell test of the process of the present disclosure was performed, starting with a cell at 0.85 V, as illustrated in Figure 4. A constant discharge current of 10 A was used until polarity reversal and internal short circuit occurred. As may be seen in Figure 4, the total discharging time was around 16 minutes, after which the discharge current was stopped. The over-discharging below 0 V started after about 2 minutes, and after about 10 minutes, a slow rise in the cell voltage may be observed. This may be interpreted as that internal short circuit starts to form (about to occur). The discharging continued for a few minutes to ensure complete internal short circuit, and it may be seen that essentially no voltage rebound occurred after the discharge current stopped. As may be seen in Figure 4, the cell is quickly deep discharged (and over discharged), without the need of additional or external short circuit. The method also applies correspondingly for packs or modules.

[0083] Example 2, energy and charge based discharge

[0084] A discharge process of a 160 Ah capacity battery cell was performed, starting from an initial cell voltage slightly above 3,5 V, as illustrated in Figure 5. A discharge current of 20 A was used in the process. It could be seen that 0 V (deep discharge) was reached after about 215 minutes, whereafter polarity reversal and charging of the battery begun, up to a total charge of 12 Ah after about 35 minutes of overdischarging. The energy into the battery during polarity reversal was about 4.5 Wh (watt hours), calculated from the point of polarity reversal (i.e. the over-discharging below 0 V).

[0085] After the 35 minutes of over-discharging, the current was turned off, and the cell rebounded to around 0 V, thus showing no voltage rebound above 0.05 V, hence confirming that complete internal short circuit did occur. It may be seen that the temperature, even though rising slightly from about 28 to 40 °C, did not reach any high or critical level during this time.

[0086] Example 3, repeated over-discharging

[0087] In this example, which results are illustrated in Figure 6, a 160 Ah capacity battery cell with an initial cell voltage of around 2.5 V was discharged using a varied discharge current, starting at 0 A, and quickly going up to 30 A, such that the cell voltage reached 0 V (polarity reversal) already after less than 2 minutes, after which the discharge current was fluctuating between 15-20 A. Due to the properties of the converter, constant current at this voltage level was not possible to achieve. At a cell voltage below 0 V polarity reversal and charging of the battery begun, up to a charge of 4 Ah and an energy of 6 Wh after about 15 minutes of over-discharging. The discharge was stopped, and the cell rebounded to a cell voltage above 0.05 V, indicating that satisfactory (complete) internal short circuit was not yet reached. Thus, a second round of discharge begun using a discharge current around 20 A for another 10 minutes, adding another 4 Ah of charge (up to a total of 8 Ah) after which the cell rebound showed that satisfactory internal short circuit had occurred. Thus, it was concluded that 4 Ah of charge was not enough for the present battery cell, but that a total of 8 Ah was.

[0088] The results from the different experiments are summarized in Table 1 below, where it may be seen that a good correlation of complete internal short circuit and charge may be seen, while no good correlation on energy is seen. Table 1 shows some parameters measured during the over-discharge (polarity reversal) of a 160 Ah battery for a first and a second round of discharging for examples 2 and 3, including the total from both rounds, or the single round, where applicable.

[0089] Table 1.

[0090] Example 4, discharge tests

[0091] In this example, discharge tests were performed such that the discharge current was regularly interrupted to monitor the voltage rebound and to determine when complete internal short circuit had been reached in each studied battery cell. An example of these discharge tests is illustrated in Figure 7. As may be seen in Figure 7, 0 V (deep discharge) of a battery cell was reached after about 52 minutes when a discharge current of -20 A was applied. Although the applied discharged current was constant, some fluctuations in the discharge current were observed, for example around 0 V of the battery cell. After around 5 minutes of over-discharging the battery cell below 0 V, the discharge was stopped and the battery cell was allowed to self-rebound. The cell rebounded to a cell voltage of around 0.8 V within 10 minutes, indicating that satisfactory (complete) internal short circuit was not yet reached. Thus, a second round of discharge begun using a discharge current of -20 A for another 2-3 minutes after which the cell was again allowed to self-rebound for about 20 minutes. The cell rebounded to a voltage around 0.4 V and thus satisfactory (complete) internal short circuit was again not reached. The steps of discharging for 2-3 minutes followed by allowing the cell to self-rebound for about 20 minutes were repeated two times. However, the cell rebounded to a voltage slightly higher than 0.05 V and satisfactory (complete) internal short circuit was not reached. The discharging current was thus applied again, for around 1 minute, after which the cell was allowed to self-rebound for around 20 minutes. No voltage rebound or the rebounded voltage < 0.05 V was observed. Thus, it was concluded that the battery cell had reached the complete internal short circuit during the last round of applying the discharge current (indicated with an arrow in Figure 7).

[0092] Accordingly, the above-described discharge tests enabled to determine when complete internal short circuit had been reached in each battery cell. Moreover, these tests enabled to calculate charge (Ah) and energy (Wh) needed to achieve the complete internal short circuit in the battery cell, as well as to evaluate the corresponding charge and energy values for different cells and discharge currents.

[0093] The discharge tests were performed on two different types of prismatic cells, 70 Ah capacity battery cells and 160 Ah capacity battery cells, subjected to discharge currents in the range of 0.1 to 0.9 C . The tests were performed with two types of prismatic cells, where charge (Ah) and energy (Wh) needed to achieve complete internal short circuit (ISC) in the battery cells were determined for different discharge currents (i.e., different C-rates). Obtained data were normalized to the average of the data set. Results of the discharge tests are depicted in Figure 8. As it may be seen in Figure 8, the charge needed to achieve complete internal short circuit is weakly dependent on the applied discharge current (i.e., C-rate). On the other hand, the energy needed to achieve complete internal short circuit is dependent on the applied C-rate. Thus, it was concluded that controlling internal short circuit in a battery cell by means of charge is more reliable, efficient and convenient than controlling internal short circuit by using energy, especially for series of connected cells.

[0094] In the drawings and specification, there have been disclosed example aspects of the disclosure. However, many variations and modifications can be made to these aspects without substantially departing from the principles of the present disclosure. Thus, the disclosure should be regarded as illustrative rather than restrictive, and not as being limited to the particular aspects discussed above. Accordingly, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation.

[0095] The description of the example embodiments provided herein have been presented for purposes of illustration. The description is not intended to be exhaustive or to limit example embodiments to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of various alternatives to the provided embodiments. The examples discussed herein were chosen and described in order to explain the principles and the nature of various example embodiments and its practical application to enable one skilled in the art to utilize the example embodiments in various manners and with various modifications as are suited to the particular use contemplated. The features of the embodiments described herein may be combined in all possible combinations of processes, products, and systems. It should be appreciated that the example embodiments presented herein may be practiced in any combination with each other.

[0096] It should be noted that the word "comprising" does not necessarily exclude the presence of other elements or steps than those listed and the words "a" or "an" preceding an element do not exclude the presence of a plurality of such elements. It should further be noted that any reference signs do not limit the scope of the claims, that the example embodiments may be realized in the broadest sense of the claims.

[0097] Itemized embodiments

[0098] 1. A process for over-discharging of rechargeable battery cells to be recycled, the battery cells having positive and negative terminals forming electric poles of the battery cell, the process comprising: providing (SI) one or more battery cells to be discharged; applying (S2) a discharge current to the one or more battery cells; discharging (S3) the one or more battery cells until they have reached a cell voltage of 0 volts, V, and over-discharging (S4) the one or more battery cells below 0 V.

[0099] 2. The process according to item 1, wherein over-discharging (S4) the one or more battery cells comprises discharging the one or more battery cells until the polarity of the electrical poles of the one or more battery cells are reversed.

[0100] 3. The process according to items 1-2, wherein the over-discharging comprises: over-discharging (S4) the one or more battery cells by applying an electric charge of more than 2% of a capacity of each battery cell.

[0101] 4. The process according to item 3, wherein the electric charge is in the range of 3-7% of the capacity of each battery cell.

[0102] 5. The process according to items 1-4, wherein the discharge current is below 1.5 C wherein 1 C equals a discharge current needed to fully discharge the battery cell from 100% capacity to 0% capacity in 1 hour.

[0103] 6. The process according to items 1-5, wherein the discharge current is in the range of 7.5-240 A. 7. The process according to items 1-6, wherein the discharge current is constant.

[0104] 8. The process according to items 1-7, wherein over-discharging (S4) the one or more battery cells comprises over-discharging each battery cell to a voltage in the range of -0.2 to -2.5 V, such as -0.5 to -1.8 V.

[0105] 9. The process according to items 1-8, wherein the one or more battery cells are discharged until an internal short circuit in the one or more battery cells occurs.

[0106] 10. The process according to item 9, wherein the internal short circuit occurs when the battery cell is below -10% state of charge, SOC.

[0107] 11. The process according to item 9, further comprising: terminating (S5) the discharge process after the internal short circuit of the one or more battery cells has occurred.

[0108] 12. The process according to items 1-11, wherein discharging (S3) and overdischarging (S4) comprise: continuously monitoring (S31, S41) the voltage of the one or more battery cells through one or more sensors.

[0109] 13. The process according to any one of items 1-12, wherein the process further comprises: allowing (S6) the one or more battery cells to self-rebound; confirming (S7) that complete internal short circuit of the one or more battery cells has occurred, when the rebounded cell voltage is < 0.05 V; and removing (S8) the one or more battery cells from the discharge process.

[0110] 14. The process according to items 1-13, wherein the one or more battery cells is present in one or more battery pack or battery module. 15. A system (100) for over-discharging of rechargeable battery cells to be recycled, the system comprising: a plurality of battery cells, modules or packs (10) connected in series in a first circuit, a plurality of voltage sensors (20) for measuring a voltage of the plurality of battery cells, modules or packs, wherein each battery cell, module or pack of the plurality of battery cells, modules or packs is connected to one voltage sensor, a plurality of bypass switches (30), wherein each battery cell, module or pack of the plurality of battery cells, modules or packs is arranged with one bypass switch in a second circuit for bypassing said battery cell, module or pack in the first circuit, a plurality of disconnection switches (25), wherein each battery cell, module or pack of the plurality of battery cells, modules or packs is arranged with one disconnection switch for disconnecting said battery cell, module or pack from the first circuit of the system, a converter (50) connected to the plurality of battery cells, modules or packs in the first circuit.

[0111] 16. The system according to item 15, wherein the system is configured to: arrange the plurality of battery cells, modules or packs to be discharged in series, discharge the plurality of battery cells, module or packs using a discharge current from the converter while continuously monitoring each battery cell voltage, module voltage, or pack voltage using a voltage sensor connected to the battery cell, module or pack, detect that a battery cell voltage, module voltage or pack voltage of one of the plurality of battery cells, modules or packs has passed below 0 V and that internal short circuit has occurred in the battery cell(s), close the bypass switch and opening the disconnection switch of said battery cell, module or pack to disconnect the battery cell, module or pack from the discharge current, remove the disconnected cell, module or pack from the system, replace the removed cell, module or pack with a new cell, module or pack to be discharged. 17. The system according to items 15-16, wherein the system is configured to perform the process of any one of items 1-14.

Claims

Claims1. A process for over-discharging of rechargeable battery cells to be recycled, the battery cells having positive and negative terminals forming electric poles of the battery cell, the process comprising: providing (SI) one or more battery cells to be discharged; applying (S2) a discharge current to the one or more battery cells; discharging (S3) the one or more battery cells until they have reached a cell voltage of 0 volts, V, and over-discharging (S4) the one or more battery cells below 0 V by applying an electric charge of more than 2% of a capacity of each battery cell.

2. The process according to claim 1, wherein over-discharging (S4) the one or more battery cells comprises discharging the one or more battery cells until the polarity of the electrical poles of the one or more battery cells are reversed.

3. The process according to claims 1 or 2, wherein the electric charge is in the range of 3-7% of the capacity of each battery cell.

4. The process according to claims 1-3, wherein the discharge current is below 1.5 C wherein 1 C equals a discharge current needed to fully discharge the battery cell from 100% capacity to 0% capacity in 1 hour.

5. The process according to claims 1-4, wherein the discharge current is in the range of 7.5-240 A.

6. The process according to claims 1-5, wherein the discharge current is constant.

7. The process according to claims 1-6, wherein the one or more battery cells are discharged until an internal short circuit in the one or more battery cells occurs.

8. The process according to claim 7, wherein the internal short circuit occurs when the battery cell is below -10% state of charge, SOC.

9. The process according to claim 7, further comprising: terminating (S5) the discharge process after the internal short circuit of the one or more battery cells has occurred.

10. The process according to claims 1-9, wherein discharging (S3) and overdischarging (S4) comprise: continuously monitoring (S31, S41) the voltage of the one or more battery cells through one or more sensors.

11. The process according to any one of claims 1-10, wherein the process further comprises: allowing (S6) the one or more battery cells to self-rebound; confirming (S7) that complete internal short circuit of the one or more battery cells has occurred, when the rebounded cell voltage is < 0.05 V; and removing (S8) the one or more battery cells from the discharge process.

12. The process according to claims 1-11, wherein the one or more battery cells is present in one or more battery pack or battery module.

13. A system (100) for over-discharging of rechargeable battery cells to be recycled, the system comprising: a plurality of battery cells, modules or packs (10) connected in series in a first circuit, a plurality of voltage sensors (20) for measuring a voltage of the plurality of battery cells, modules or packs, wherein each battery cell, module or pack of the plurality of battery cells, modules or packs is connected to one voltage sensor, a plurality of bypass switches (30), wherein each battery cell, module or pack of the plurality of battery cells, modules or packs is arranged with one bypass switch in a second circuit for bypassing said battery cell, module or pack in the first circuit, a plurality of disconnection switches (25), wherein each battery cell, module or pack of the plurality of battery cells, modules or packs is arranged with one disconnectionswitch for disconnecting said battery cell, module or pack from the first circuit of the system, a converter (50) connected to the plurality of battery cells, modules or packs in the first circuit, wherein the system is configured to perform the process of any one of claims 1-12.

14. The system according to claim 13, wherein the system is configured to: arrange the plurality of battery cells, modules or packs to be discharged in series, discharge the plurality of battery cells, module or packs using a discharge current from the converter while continuously monitoring each battery cell voltage, module voltage, or pack voltage using a voltage sensor connected to the battery cell, module or pack, detect that a battery cell voltage, module voltage or pack voltage of one of the plurality of battery cells, modules or packs has passed below 0 V and that internal short circuit has occurred in the battery cell(s), close the bypass switch and opening the disconnection switch of said battery cell, module or pack to disconnect the battery cell, module or pack from the discharge current, remove the disconnected cell, module or pack from the system, replace the removed cell, module or pack with a new cell, module or pack to be discharged.

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