Apparatus and method for heating traction batteries

Abstract

An apparatus for heating a traction battery consisting of a multiplicity of battery cells connected in series, having a converter for feeding a high-frequency alternating current into the traction battery, wherein the converter is connected, by way of a first terminal on the AC voltage side, to a first pole of the traction battery via one or more serial capacitors, is connected, by way of the first terminal on the AC voltage side, to a second pole of the traction battery via one or more serial capacitors, is preferably centrally connected to the series circuit of battery cells via a second terminal on the AC voltage side, and is connected, by way of its terminals on the DC voltage side, to a DC voltage source.

Description

[0001] Device and method for heating traction batteries

[0002] The invention relates to a device and a method for heating traction batteries.

[0003] Electrochemical energy storage systems have become key components in many application areas. In mobile applications – especially electric vehicles – operation over a wide temperature range is required. Low temperatures generally lead to significant limitations in the performance of electrochemical energy storage systems. This limits the use of both existing cell chemistries and new developments. A current example is the replacement of lithium cobalt manganese cells with lithium iron phosphate cells, which is restricted by their less favorable low-temperature behavior. This problem is known for both the former – which are relatively better in this regard – and the solid-state cells currently under development. In addition to limited power output, the rapid charging of these batteries is particularly hindered or even impossible.leads to irreversible damage.

[0004] According to the state of the art, the temperature of the large drive batteries is controlled by means of external liquid cooling. This effort is necessary because the permissible temperature range of the cells is very limited, even in the upper end. In order to use this system to preheat the cells, electrical energy is used to heat the liquid. The majority of this energy must be taken from the drive battery itself, so that its energy content decreases. There is therefore a desire to reduce the amount of energy required. This is possible to a small extent by, for example, additionally utilizing the waste heat from the engines or by a heat pump supplying energy from the ambient air. However, in an efficient electric vehicle, large heat sources are generally not available.

[0005] However, the energy required to preheat a large, heavy traction battery is large, and the time constants are also large. To illustrate the relevant magnitudes, the following figures are helpful: to heat a traction battery weighing 500 kg and 100 kWh (e.g., before charging) by approximately 20°C, approximately 15 kWh are required over a period of approximately 1.5 hours, i.e., an average power of 10 kW. Part of this energy is also required for the coolant itself, since the specific heat capacity of water is high. The disadvantages of this state of the art are therefore both the high energy consumption and the required time, as well as the need for advance planning.

[0006] It is therefore the object of the present invention to enable the required preheating in a shorter period of time and with less energy consumption.

[0007] This object is achieved by the subject matter of the independent claims; preferred embodiments are the subject matter of the subclaims.

[0008] The invention is based on the finding that it is advantageous to concentrate the heating as directly as possible on the active part of the battery, located spatially inside the cells. This is essentially formed by the surface of the electrodes and the liquid or solid electrolyte. In contrast to heat input via the outer casing surface of the cells, with such a solution, the relatively high thermal resistance from the active part to the casing surface is helpful when heat is input from the inside, as it prevents unwanted heat dissipation. Furthermore, the thermal time constant of the active part is significantly smaller than that of the entire cell, including the casing.

[0009] According to the invention, the heating of the active part of the cells is achieved by high-frequency alternating currents which are fed into the drive battery in such a way that its proper operation and the operation of the electronic battery management system preferably remain undisturbed.

[0010] For this purpose, the invention proposes devices and a method which make this possible.

[0011] In particular, it is proposed that the device be retrofittable, allowing it to be connected to any typical traction battery. However, it is also possible to integrate this device into the battery housing.

[0012] In particular, the device is preferably connected to the large number of battery contacts of the internal cell connections (which are present in addition to the actual pole contacts) that are already present in traction batteries for measurement purposes, in order to be able to intervene in the internal workings of the batteries from the outside and non-invasively and to achieve the desired heating there. The heating is achieved by means of injected high-frequency alternating currents, which preferably do not cause any undesirable interference with the direct current operation of the battery. The injected alternating currents are preferably in antiphase pairs and of equal amplitude, and preferably average each other out completely. As a result, the input of the alternating currents is practically imperceptible at the two poles of the battery.

[0013] The device consists of a plurality of series-connected battery cells, with a converter for feeding a high-frequency alternating current into the drive battery, wherein the converter is connected to a first AC-side terminal via one or more series capacitors to a first pole of the drive battery, is connected to the first AC-side terminal via one or more series capacitors to a second pole of the drive battery, is connected via a second AC-side terminal, preferably to the series circuit of battery cells, and is connected to a DC voltage source with its DC-side terminals.

[0014] In addition to this solution with a single converter, which is thus a low-cost variant, the upper or first half of the traction battery can be fed by a first converter and the lower or second half by a second converter. The alternating currents fed in are again preferably in antiphase and of equal magnitude, resulting in equal but opposite-sign voltage drops. From the perspective of the two battery poles, this means that the alternating currents are not perceived externally and do not negatively impact normal operation or battery management. With regard to battery management, it is also advantageous that the alternating currents fed in have a high frequency, which means that potential interference with measured values ​​can be avoided by simple low-pass filters.

[0015] The corresponding device consists of a plurality of series-connected battery cells with at least two converters for feeding a high-frequency alternating current into the traction battery, wherein a first converter is connected to a first terminal on the AC side via one or more series capacitors to a first pole of the traction battery, is connected to the series circuit of battery cells via a second terminal on the AC side, preferably centrally, and is connected to a DC voltage source with its DC voltage terminals, wherein a second converter is connected to a second terminal on the AC side, via one or more series capacitors to a second pole of the traction battery, is connected to the series circuit of battery cells via a second terminal on the AC side, preferably centrally,and is connected to a DC voltage source via its DC side terminals.

[0016] The connection of one or both inverters should preferably be made in the middle.

[0017] The capacitors and optional damping resistors used should preferably be of the same value so that the voltage drops resulting from the fed-in currents compensate each other from the perspective of the battery terminals.

[0018] However, this goal is not mandatory if smaller AC current overlaps and thus voltage overlaps are acceptable. Thus, centralized feeding into the traction battery is also not essential if perfect voltage compensation is not required anyway.

[0019] In principle, heating can also be achieved without compensation, i.e., by a converter supplying only an alternating current that is not compensated by a corresponding alternating current in opposite phase and preferably of equal magnitude from the perspective of the battery terminals, fed into another section of the battery, provided the resulting interference is acceptable. However, it should be noted that the impedance of the drive converter's capacitor typically approaches zero for frequencies above 10 kHz, which would short-circuit the HF components detectable at the terminals.

[0020] Non-symmetrical distribution is also possible. For example, instead of dividing the battery into two equal halves and supplying each half with alternating current of the same magnitude but in phase opposition to the other half, a 1 / 3 to 2 / 3 distribution, etc., is possible.

[0021] The principle according to the invention is not limited to one or two converters. It is also not limited to the battery being divided into just two halves and each of the halves being supplied with opposing alternating current. Instead, the traction battery can be divided into a larger number (N) sections, and each of these sections or segments can then be supplied with alternating current from one or two converters. In other words, each section made up of a predetermined number of battery cells receives alternating current from one or two converters. Preferably, one segment from a neighboring pair of converters is supplied with anti-phase alternating currents so that, from the perspective of the external battery contacts of the respective segment, these currents cancel each other out and therefore do not cause interference.

[0022] The corresponding heating device for heating a traction battery consisting of a plurality of series-connected battery cells preferably has 2xN converters for feeding high-frequency alternating currents into the traction battery, wherein each of the 2xN converters feeds alternating current into one of N sections of the series circuit of battery cells and two converters in each case feed current in antiphase to one another, preferably within a same section of the N sections of the traction battery, such that the resulting voltages cancel each other out, each converter is connected via a first AC-side terminal to one of N first voltage points of the series circuit of battery cells, and via a second AC-side terminal via at least one capacitor to a respective adjacent voltage point of second N+1 voltage points of the series circuit of battery cells,where the same number of battery cells is present between 2 adjacent voltage points, and each of the 2xN converters is connected to a DC voltage source with its DC voltage side terminals.

[0023] In addition to the capacitors, suitable damping resistors are preferably installed in the various devices mentioned in order to counteract possible unwanted vibrations that may arise from interference frequencies of the drive converter.

[0024] The capacitors can preferably be dimensioned to compensate for the voltages of the parasitic inductances in the battery circuit for the selected AC frequency. This corresponds to a resonance tuning, which advantageously reduces the required voltage and apparent power of the converter.

[0025] The invention further relates to a corresponding method.

[0026] Accordingly, a method for heating a traction battery consisting of a plurality of series-connected battery cells is proposed, comprising the following steps: connecting an inverter by means of a first AC-side terminal via a first series circuit comprising a first heating resistor and a first capacitor to a first pole of the traction battery, connecting the inverter by means of the first AC-side terminal via a second series circuit comprising a second heating resistor and a second capacitor to a second pole of the traction battery, connecting the inverter to a second AC-side terminal, preferably centrally, with the series circuit of battery cells, and connecting the inverter to a DC voltage source via the DC-side terminals, and feeding a high-frequency alternating current into the traction battery, wherein the two resulting AC voltages compensate each other,which ensures that the constant output voltage of the drive battery is not disturbed.

[0027] If there are more than two sections fed by 2x N converters, the feed-in is carried out in the same way as described for the device with 2 x N converters.

[0028] The inventive feeding of alternating currents into the interior of the battery, such that these alternating currents compensate for each other and thus do not cause disturbances at the terminals of the traction battery, has, in addition to the desired heating or preheating of the battery, additional positive effects on the service life of such batteries. Therefore, the device and method can also be used during normal operation when heating is no longer necessary. In this situation, the alternating current serves a chemical purpose to increase battery service life.

[0029] The following description provides concrete examples of the temperature increase and power levels the proposed solution is capable of achieving.

[0030] In general, the internal resistance of the traction battery is significantly higher when cold than when warm, and therefore heating can be achieved efficiently when cold. More efficiently than is possible with coolants or due to the battery's continuous operation.

[0031] Preferably, the devices have a control device that determines from a given battery temperature whether preheating is necessary and, if necessary, at what power and frequency charging is required, and adjusts operation accordingly. Operation can be adjusted over time depending on the heating success achieved, ensuring energy-efficient heating.

[0032] The input alternating current preferably has a frequency in the range of 16 - 100 kHz. This range is advantageous because it prevents acoustic interference. The upper limit ensures that the converter's switching frequency requirements are not too high, and switching losses can be kept to a minimum. The exact choice of frequency depends on the size and geometry of the traction battery. Lower frequencies are particularly recommended for large batteries with high parasitic inductances to minimize converter complexity.

[0033] The device may be equipped with a radio facility so that the heating process can be instructed from a remote location.

[0034] The device may have a timer device so that the heating process starts at a predetermined time.

[0035] Preferred embodiments are explained in more detail below with reference to the accompanying figures. They show:

[0036] Fig. 1 : a schematic representation of a circuit according to an embodiment of the invention,

[0037] Fig. 2: an alternative embodiment of the invention using two converters, and Fig. 3: a schematic representation in a general way for an embodiment with 2x N converters.

[0038] Fig. 1 shows a schematic representation of a traction battery 1 consisting of several battery cells 1a-1f connected in series. A total of six cells are shown, but it will be clear to those skilled in the art that this number may amount to several hundred cells.

[0039] The traction battery 1 has a P-pole 2 and an N-pole 3. Typically, such batteries, especially for measurement purposes, have additional terminals located at regular intervals between the battery cells. Fig. 1 shows such a terminal 4, from which the central potential of the six battery cells shown can be tapped.

[0040] A circuit can be connected to the battery shown in this way, which feeds alternating current into the cells to heat the interior of the battery. The corresponding circuit can be retrofitted or switched on as needed, or it can already be present on the battery. The circuit has at least one converter 5, which is connected to a DC voltage source (not shown) via its DC voltage-side terminals 6. The source can also be the traction battery itself; however, due to the high voltage of this battery, a low-voltage auxiliary battery is recommended.

[0041] The converter 5 is connected to the central terminal 4 of the traction battery via its one AC-side terminal 7, and to the P-pole of the battery via a capacitor 9 and the N-pole of the traction battery via a capacitor 10. Preferably and optionally, a damping resistor 11, 12 is also connected in series to combat unwanted vibrations.

[0042] During operation, the converter feeds an alternating current i1 into the upper half of the battery and an alternating current i2 into the lower half. Circuits 13 and 14 illustrate this opposing behavior. With identically dimensioned capacitors 9, 10 and the optional damping resistors 11, 12, it is possible to achieve currents i1 and i2 that are equal in magnitude but in phase opposition. As a result, these alternating currents cancel each other out in voltage from the perspective of battery terminals 2 and 3, preventing any unwanted disruption to normal battery operation or battery management.

[0043] It is clear to the expert that the capacitors 9, 10 can also be series circuits consisting of several capacitors.

[0044] In practice, the battery cells 1a-1f may comprise 100 or more cells, and additional connection points 4 may be provided at regular intervals between an equal number of battery cells.

[0045] By feeding in the alternating currents, the internal resistance 1f, 1h shown schematically ultimately causes the battery to heat up. Instead of heating the casing or coolant, only the active part of the battery is heated, thus avoiding unnecessary energy consumption. As discussed below, the internal resistance increases with falling temperature, thereby improving heating performance. This means that the inventive principle works best precisely where it is intended to be used, namely when the battery is cold, so that the useful power of the battery can be quickly increased to a solid value by heating. Not only the inherent internal resistances 1g, 1h are shown, but also, for the sake of completeness, such inductive components 1i, 1k.

[0046] Fig. 2 shows an embodiment analogous to Fig. 1, but with two converters 25, 26. Each of the two converters is powered by a DC power source via its DC voltage side terminals. On the AC voltage side, one terminal of each converter is connected to the central battery terminal 24. The second AC voltage side terminal of the converter 25 is connected via a capacitor 27a (or a series circuit of capacitors) to the P-pole 22 of the traction battery 21. Accordingly, the other AC voltage side terminal of the converter 26 is connected via a capacitor 27b (or a series circuit of capacitors) to the N-pole 23 of the traction battery 21, which in turn consists of a plurality of battery cells 21a-f connected in series. Optionally, damping resistors 28a, 28b may be present again, as well as a switch 29 for selectively connecting the branches connected to the battery poles.

[0047] Fig. 3 generalizes the embodiments to 2 x N converters. With an even number of 2 x N converters, two converters each can supply alternating current to one of N sections of the battery 31. Two converters always feed into one of the N sections, with this occurring in two halves of each section. This allows for section-by-section heating of the battery.

[0048] The traction battery 31 can thus be divided into, for example, 20 sections. The compensating behavior of adjacent converters can be seen from the circles 37 shown. The converters 33, 34, 35, 36 are only shown schematically and as examples; in fact, a much larger number of converters can be used. It can be seen that pairs 33, 34 and 35, 36 of converters with an AC voltage side connection are connected together to a connection point 31a, 31d. With its respective second connection, each converter in a pair is connected via a capacitor 33a, 34a, 35a, 36a to the next connection point of the traction battery, as shown in Fig. 3. Converter 33 is therefore connected via capacitor 33a to the outermost connection, i.e. battery pole 32, in order to span a certain number of battery cells, 2 cells in the example.Accordingly, converter 34 is connected to the next terminal 31b via a capacitor and spans the same number of battery cells. The four cells shown accordingly thus form a section of the battery that can be specifically heated via two converters. The same applies to converters 35, 36 and capacitors 35a, 36a, as well as to all other converters not shown.

[0049] Fig. 3 is only schematic. Naturally, all converters are powered on their DC side by energy sources. Additional damping resistors analogous to Fig. 1 and Fig. 2 are possible, as are switches like those in Fig. 2, which can connect the return branches of each converter pair or even between converter pairs.

[0050] Overall, the ideal solution is to ensure that all alternating currents fed into the grid cancel each other out, at least from the perspective of the main poles 23 and 33, so that normal battery operation is not disrupted. This means that the currents should be in phase opposition and equal in magnitude. However, currents from converters that are not arranged directly next to each other can also cancel each other out.

[0051] It will be apparent to those skilled in the art that all embodiments follow the same basic idea and thus details of one embodiment can also be applied to the other embodiment.

[0052] A quantitative estimate of the heating achievable with the invention can be made based on known, typical data from the rapid charging of traction batteries, because this also involves current-induced internal heating. Unlike the invention, however, this involves a direct current:

[0053] When fast charging the above battery with P a de = 4 • Ppahrt = 400kW, which is permissible for a high-quality battery according to the state of the art, will therefore be a current of 800A and an internal “heating power” of PvLade = (800A) 2 • 0.0050 = 32 kW.

[0054] If this charge is applied to a battery with the initial temperature v a= 25°C, the battery generally reaches an internal temperature of approximately 60°C after just 15 minutes, meaning the charging current must be reduced. The temperature gradient is therefore approximately 35km / 15min « 2.3°C per minute. Because the thermal time constant of such a large battery is greater than one hour, the internally generated heat cannot be dissipated to the outside to any significant extent in the above-mentioned time period. It is therefore possible and useful to compare the adiabatic temperature gradient below for estimation purposes. Furthermore, it will be assumed below that - to protect the battery - the relative overvoltage during charging is not kept higher than during rapid charging as described above. Whether this conservative marginal operating limit should be adhered to depends on the specific battery technology and is not generally applicable.

[0055] Heating the battery will generally only be carried out if the internal resistance of the battery is significantly increased – e.g., already at least twice the above-mentioned nominal value of Ri = 0.050. Under these starting conditions, a heating alternating current of 400A will be applied, in accordance with the above-mentioned conservative current limitation. The internal power that heats the active part of the battery will therefore

[0056] PVHeating = (400A) 2 • 0.1Q = 16kW. The temperature gradient will therefore be (2.3k / min), ie: approx. 1.65°C per

[0057] Minute. This value is significantly higher than the values ​​achievable with external heating of the battery. This leads to significantly reduced energy consumption and shorter, more easily plannable preheating times.

[0058] To achieve significant heating during ferry operation – due to the power required for propulsion – very high battery power would be required, as the following comparison shows. Driving at P Fa hrt = 100kW, which would not be achievable even when driving continuously at v = 200km / h on the motorway, leads to an internal power loss of

[0059] P v = (200A) 2 • 0.1Q = 4kW ie a temperature gradient of approximately 0.3°C per minute. In realistic driving conditions with e.g. P F driving 33kW on the highway (with recommended speed v=130km / h) the temperature gradient would already be negligible at approx. 0.046°C per minute.

[0060] A significant advantage of the invention lies in the fact that a useful heating of the battery can be achieved in a short period of time compared to the battery's thermal time constant. In this adiabatic heating regime, no significant energy is lost through heat dissipation in the housing, cooling circuit, or surroundings.

Claims

Patent claims 1. Device for heating a traction battery consisting of a plurality of series-connected battery cells, with: a converter for feeding a high-frequency alternating current into the traction battery, wherein the converter is connected to a first AC-side terminal via one or more series capacitors to a first pole of the traction battery with a first AC-side terminal, is connected to a second pole of the traction battery with the first AC-side terminal via one or more series capacitors, is connected via a second AC-side terminal, preferably centrally, to the series connection of battery cells, and is connected to a DC voltage source with its DC-side terminals.

2. A device for heating a traction battery consisting of a plurality of series-connected battery cells, comprising: at least two converters for feeding a high-frequency alternating current into the traction battery, wherein a first converter is connected to a first terminal on the AC side via one or more series capacitors to a first pole of the traction battery, is connected to the series circuit of battery cells via a second terminal on the AC side, preferably centrally, and is connected to a DC voltage source via its DC voltage terminals, wherein a second converter is connected to a second terminal on the AC side via one or more series capacitors to a second pole of the traction battery, is connected to the series circuit of battery cells via a second terminal on the AC side, preferably centrally,and is connected to a DC voltage source via its DC side terminals.

3. Device for heating a traction battery consisting of a plurality of series-connected battery cells, comprising: 2xN converters for feeding high-frequency alternating currents into the traction battery, wherein each of the 2xN converters feeds alternating current into one of N sections of the series circuit of battery cells, and 2 converters each feed current in antiphase to each other, preferably within a same section of the N sections of the traction battery, such that the voltage drops of all cells caused by the high-frequency alternating currents cancel each other out, each converter is connected via a first AC-side terminal to one of N first voltage points of the series circuit of battery cells, and via a second AC-side terminal is connected via at least one capacitor to a respective adjacent voltage point of second N+1 voltage points of the series circuit of battery cells, wherein the same number of battery cells is present between each two adjacent voltage points,and each of the 2xN inverters is connected to a DC voltage source via its DC side terminals.

4. Device according to one of the preceding claims, wherein the alternating currents fed in are each of equal magnitude but in antiphase, such that no high-frequency components arise at the battery poles.

5. Device according to one of the preceding claims, wherein damping resistors are connected in series with the capacitors.

6. Device according to one of the preceding claims, wherein all capacitors have the same values.

7. Device according to one of the preceding claims, wherein the alternating currents have frequencies in the range of 16-100 kHz.

8. Device according to one of the preceding claims, wherein the circuit is integrated into the battery housing.

9. Device according to one of the preceding claims, wherein the converter has a transformer on the DC voltage side in order to be able to be fed by the drive battery.

10. Device according to one of claims 2-9, wherein a plurality of converters are formed by a single converter with a transformer having a plurality of potential-separated output windings, wherein a plurality of output AC voltages can be generated by the potential-separated output windings.

11. Method for heating a traction battery consisting of a plurality of series-connected battery cells, comprising the following steps: Connecting a converter by means of a first AC-side terminal via one or more capacitors to a first pole of the traction battery, connecting the converter by means of the first AC-side terminal via one or more capacitors to a second pole of the traction battery, connecting the converter to a second AC-side terminal, preferably centrally to the series circuit of battery cells, and Connecting the inverter to a DC voltage source via the DC side terminals, and Feeding high-frequency alternating currents into the traction battery.

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