Calibration of equalization system in battery system

Abstract

The invention relates to a method for calibrating a passive equalization system in a battery system comprising a plurality of lithium-ion battery cells and a battery management device, a battery cell unit formed by a single battery cell or by a plurality of battery cells connected in parallel is provided with a discharge circuit having a load resistor R i , which is a calibration parameter, and each battery cell unit is connected in series into a group string, and the battery management device is arranged for measuring the voltage U i of each battery cell unit and for manipulating the discharge circuit at selectable points in time in order to discharge the battery cell unit i in a controlled manner through the load resistor R i , wherein the method comprises the following steps: - manipulating the discharge circuit of the battery cell unit i for taking out a charge Q i over a discharge duration t i , and determining t i , Q i , and the voltage time curve U i (t); - determining R i as (I).

Description

Technical Field

[0001] This invention relates to a method for calibrating an equalization system in a battery system. Background Technology

[0002] Battery system:

[0003] Battery systems for electric or hybrid electric vehicles consist of multiple individual secondary battery cells, typically lithium-ion battery cells, connected in parallel and series, and are controlled by a battery management system (BMS).

[0004] In particular, a Battery Management System (BMS) has the function of monitoring operational data such as cell voltage, state of charge (SoC), aging level (SoH), current, and temperature, as well as controlling the charging and discharging of cells. Other tasks of a BMS include thermal management of the battery system, protection of individual cells, and prediction of the remaining lifespan of individual cells based on recorded operational data.

[0005] In a battery system, individual battery cells can be connected in series to achieve a desired voltage, such as 200V to 400V. Alternatively, to increase capacity, multiple battery cells can be connected in parallel in groups, and the resulting battery packs can then be connected in series. From a BMS perspective, parallel-connected battery packs behave like individual battery cells in terms of voltage or SoC monitoring, as well as in terms of balancing, which will be described in more detail below. Therefore, in the following text, both individual battery cells and groups of parallel-connected individual battery cells will be collectively referred to as "battery cell units".

[0006] balanced:

[0007] A key function of a Battery Management System (BMS) is so-called balancing, which refers to the balancing of the state of charge (SoC) of individual battery cells or battery cell groups. The SoC of an individual battery cell may deviate from that of the rest of the battery cell group due to increased self-discharge caused by uneven temperature distribution or manufacturing fluctuations.

[0008] This imbalance becomes apparent through drift in individual battery cell voltages and can lead to shortened cell lifespan and accelerated wear. The same applies to groups of individual battery cells connected in parallel, which outwardly behave as individual cells with correspondingly larger capacities. Equalization restores balance by making the state of charge of the battery cells (i.e., individual cells or groups of cells) equal to each other.

[0009] Generally, balancing methods can be divided into active and passive methods. In active balancing, charge is transferred from battery cells with a higher state of charge (SOC) to those with a lower SOC. This can be achieved using charge transfer elements such as capacitors, coils, and / or voltage converters. In passive balancing, excess charge in battery cells with a higher SOC is simply dissipated through resistors (shunts) until the state of charge is balanced.

[0010] The charge converted (i.e., consumed and potentially supplied by each cell during equalization) and its distribution across the individual cells of the battery system during equalization provide conclusions about the degree of self-discharge, which in turn indicates the state of aging (SoH) and, if necessary, the risk of internal short circuits. Therefore, a method for accurately determining the equalization charge is needed.

[0011] Task proposal:

[0012] In principle, the equalization charge can be determined by the individual cell voltage, the duration of the equalization circuit's operation, and the characteristics of the equalization circuit itself. In the case of passive equalization, the equalization current can be calculated from the resistance value R of the load resistor (shunt) and the voltage curve U(t) measured during equalization, i.e., I(t) = U(t) / R, and the integral over the duration of the equalization system's operation provides the charge flowing through it.

[0013] The challenge here is that, while the voltage curve and time are known with good accuracy, the accuracy of charge determination depends on the tolerance of the load resistor. For cost reasons, using a high-precision load resistor or separately remeasuring the precise resistance value is not considered for most applications.

[0014] Therefore, a calibration method is needed to determine the equalization charge with high accuracy, which can be implemented in a pre-configured battery system with passive equalization—in which the precise resistance value of the load resistor is unknown—without incurring significant overhead. Preferably, the method should also be implementable in the field or during ongoing operation, without requiring specialized laboratory-grade equipment. Summary of the Invention

[0015] The present invention relates to a method for calibrating a passive equalization system in a battery system, the battery system comprising multiple lithium-ion battery cells and a battery management unit (BMU).

[0016] In the battery system used according to the present invention, battery cell units, formed by a single battery cell or a group of multiple battery cells connected in parallel, are connected in series to form a string. Each battery cell unit (i.e., a single battery cell or a group of battery cells connected in parallel) is provided with a discharge circuit, which has a load resistance R. i, where R i The value is a calibration parameter. The BMU is also set to measure the voltage U of each individual battery cell. i The discharge circuit is manipulated at selectable times to allow the load resistor R to pass through. i The battery cell i is discharged in a controlled manner.

[0017] The present invention includes the following steps:

[0018] -During discharge duration t i The discharge circuit that controls the battery cell unit i;

[0019] - Determine the discharge duration t i Charge Q extracted from inside i and voltage-time curve U i (t);

[0020] - Determine R i for

[0021]

[0022] To determine Q i As an alternative, in particular, consideration is given to providing a known charge and subsequently dissipating it through an equalization system, and utilizing the known differential capacitance C of the individual cells. i =dQ i / dU i Calculations are made based on voltage.

[0023] The load resistance R can be determined using the calibration method according to the present invention. i The precise value of this calibration method can accurately determine the amount of charge flowing during equilibration, which can then be used for diagnostics (e.g., initial micro-short circuits). This calibration method can also be used repeatedly throughout the battery system's lifespan without requiring a workshop visit. Attached Figure Description

[0024] Figure 1 The diagram schematically illustrates the structure of a string of individual battery cells, each equipped with a discharge circuit and a voltage measuring device.

[0025] Figure 2 This schematically illustrates how Q is determined by providing and subsequently dissipating a predetermined charge. i The structure of time. Detailed Implementation

[0026] The structure of a battery system using the method according to the invention and embodiments of the method itself according to the invention are described in more detail below.

[0027] Battery system and balancing:

[0028] A battery system using the method according to the invention includes a plurality of lithium-ion battery cells and a battery management system (BMS), wherein each battery cell unit, formed by a single battery cell or a group of battery cells connected in parallel, is provided with an balancing circuit. The battery management system is configured to perform charge balancing, i.e., equalization, at predetermined time points. For this purpose, the balancing circuit is manipulated in a battery cell or group of battery cells whose voltage is higher than at least one other battery cell or group of battery cells, so as to draw charge from that battery cell or group of battery cells until the battery cell voltages are balanced with each other.

[0029] Equalization typically occurs during stationary phases, such as after charging and when the battery system is not under load. If the battery system is installed in an electric vehicle, equalization can be performed at any time outside of driving operations, preferably immediately after the energy storage unit is charged. In hybrid electric vehicles or plug-in hybrid electric vehicles, driving operations using an internal combustion engine are also taken into consideration. According to the invention, there are no particular limitations on the timing and exact method of equalization, as long as the charge transferred to each battery cell during equalization can be determined by the BMS.

[0030] During passive balancing, charge is drawn from battery cells with higher cell voltage (and therefore higher SOC) and dissipated over the load resistor (shunt). Figure 1 The diagram shows a simplified schematic of this passive balancing circuit for N battery cells connected in series. For each battery cell i, the cell voltage U is monitored by the BMS. i In addition, each battery cell is equipped with a shunt circuit, which includes at least one switch S controlled by the BMS. i (Such as MOSFET) and the original parallel resistor (shunt) R i .

[0031] To maintain low equipment costs, no current I was provided for direct measurement of the equalization circuit. i The possibility of [something]. Instead, it is determined by the resistance value R. i and the voltage curve U measured during the equalization period i (t) Calculate the equilibrium current as I i (t)=U i (t) / R. The integral over time provides the charge flowing through.

[0032] Determining the load resistance:

[0033] The calibration method according to the present invention is used to accurately determine the resistance value R. i This is to allow for precise determination of the balancing current and the charge flowing through it. The current flowing through the load resistor during balancing is typically I.i =U i / R i , among which, U i It refers to the state of charge (SOC) of a single battery cell. i It is a function of ) and therefore does not need to remain constant over time, but depends on the charge Q that has flowed. i Therefore, the charge is calculated as follows:

[0034] Q i =∫I i dt = 1 / R i *∫U i dt

[0035] As described above, the battery management device is capable of measuring U with high precision. i And record U over time as needed i This is so that, for example, the state of charge (SOC) of individual battery cells can be monitored.

[0036] This invention is based on the concept of determining the calibration parameter R using the above formula. i The method is as follows: determine the operation duration (discharge duration) t of the discharge circuit. i The charge Q flowing through i and voltage curve U i (t). Therefore, R i It can be calculated as

[0037] R i =1 / Q i *∫U i dt.

[0038] The necessary measurements and calculations are performed by the battery management system, which is configured to monitor voltage and control discharge circuitry.

[0039] To determine the flowing charge Q i For example, one could consider providing a known charge and then removing that charge through a discharge circuit, or calculating the charge from the differential capacitance and the voltage curve during discharge.

[0040] Q is determined by providing a known charge. i :

[0041] Confirm Q i The first possibility is to provide a known charge Q, which is based on the increase in voltage U caused by the increase in the state of charge of the individual battery cells. i Increase the voltage. Then manipulate the discharge circuit until the increased voltage drops back to its initial value. Therefore, the state of charge (SOC) of the battery cell is again the same as before the charge was supplied, i.e., the charge Q flowing during discharge. i Corresponding to the provided charge Q. Schematic structure in Figure 2 As shown in the image.

[0042] This embodiment of the method according to the present invention includes the following steps:

[0043] (1) Determine the initial voltage U of each battery cell i in the string through the battery management device. i,0 ;

[0044] (2) At the predetermined time t L A pre-known charging current I is applied to the string to provide a known charge Q = ∫Idt for each individual battery cell;

[0045] (3) Take out the previously provided charge Q i =Q, the method is: manipulate the discharge circuit until the initial voltage U is reached again. i,0 This makes the discharge duration t i Satisfying condition U i (t i )=U i,0 ;

[0046] (4) Determine R i for

[0047]

[0048] Where t i (U=U i,0 The duration of the discharge is denoted as U, after which the voltage drops back to its initial value. i,0 .

[0049] First, in step (1), the voltage U is measured. i,0 The voltage U i,0 It is a measure of the initial SOC of a single battery cell, which must also be equal to the final SOC at the end of the subsequent step (3).

[0050] Then, in step (2), the entire string is charged with a defined charging current over a defined time period. This step can be performed using a conventional charger and differs from normal charging only in that the battery system is not fully charged, but only a known charge Q is provided, which is calculated by integrating the charging current over time.

[0051] There are no particular restrictions on the charging method. For example, charging can be performed with a constant current or a constant voltage. Only the time curve of the charging current I needs to be measured to calculate the charge. To control the charging process, the battery system or charging device inherently includes a current measuring device that can be used to determine the charge. Figure 2In the simplified embodiment shown, the current measuring device is integrated into the battery system (“S-Box”). If necessary, a high-precision current measuring device can be introduced into the charging circuit to determine the charge with high accuracy.

[0052] Step (2) does not require physical access to individual battery cells; instead, it can be implemented in the field using the existing battery system with common charging equipment. At most, a high-precision current measuring device may be required as an additional accessory.

[0053] Since the string is formed by only battery cells connected in series, the current flowing through each battery cell is the same, and therefore the charge provided by each battery cell is approximately the same, which can be calculated as Q = ∫Idt.

[0054] After charging is complete, due to slight differences in the voltage of individual battery cells, slow charge exchange may occur between the cells, causing the charges to drift away from each other over time. However, in the method according to the invention, this effect is negligible based on the slow time scale, especially if step (3) is performed immediately following step (2).

[0055] By increasing the SOC based on the provided Q, after step (2), the cell voltage in the battery cell relative to U i,0 Increase. In step (3), the battery cell is discharged by manipulating the discharge circuit until U is reached again. i,0 And therefore the initial SOC. Here, the dissipated charge Q i Therefore, it is equal to the charge provided in step (2).

[0056] The load resistance R is then calculated in step (4) by recording the voltage curve during discharge and integrating it over time. i The value is:

[0057]

[0058] No special laboratory equipment is required, nor is any external action taken on the battery system itself.

[0059] Determining Q using known differential capacitance i :

[0060] As an alternative, Q i It can also be generated by the differential capacitor C stored in the battery management device. i =dQ i / dU i Determine or obtain the charge / voltage related data Q that is required by the stored SOC. i (U iThe calculation is performed by differentiating the voltage. This method according to the invention uses a differential capacitor C. i The implementation method in this case includes the following steps:

[0061] (1) Manipulate the discharge circuit so that at a predetermined time t i Through resistor R i Discharge each individual battery cell i, and simultaneously measure the voltage U during the discharge. i (t) is used to obtain the voltage-time curve;

[0062] (2) From C i and U i (t) determines the predetermined time t i The charge Q extracted during the period i for:

[0063]

[0064] (3) Determine R i for:

[0065]

[0066] In step (1), the battery cell is discharged again in a controlled manner, and the voltage curve during discharge is measured. However, unlike the first scheme, the charge extracted is not known in advance, but must be determined in step (2) by the known differential capacitance C. i and the measured voltage curve U i (t) is calculated. Differential capacitance C i It is either stored in the battery management system itself, or dynamically calculated from a pre-known no-load characteristic curve.

[0067] Finally, in step (3), R is determined in a manner similar to that of the first embodiment. i .

Claims

1. A method for calibrating a passive equalization system in a battery system, the battery system comprising multiple lithium-ion battery cells and a battery management device, in, Each battery cell unit, consisting of a single battery cell or a group of multiple battery cells connected in parallel, is provided with a discharge circuit, wherein the discharge circuit has a load resistor R. i The load resistance is a calibration parameter, and the individual battery cells are connected in series to form a string. Furthermore, the battery management device is configured to measure the voltage U of each individual battery cell. i And to manipulate the discharge circuit at selectable times so as to pass through the load resistor R i Discharge the battery cell i in a controlled manner. The method includes the following steps: - The initial voltage U of each battery cell i in the string is determined by the battery management device. i,0 ; - at the scheduled time t L A pre-known charging current I is applied to the string to provide a known charge Q=∫Idt for each individual battery cell; - During the discharge duration t i The discharge circuit of the battery cell i is manipulated to extract charge Q. i And determine t i Q i and voltage-time curve U i (t), such that the method includes removing the previously provided charge Q. i =Q, the method is: manipulate the discharge circuit until the initial voltage U is reached again. i,0 , making t i Satisfying condition U i (t i )=U i,0 ; - Determine R i for ； Among them, t(U=U i,0 This indicates the duration of the equalization circuit's operation, after which the voltage drops back to its initial value U. i,0 .

2. The method according to claim 1, wherein, The differential capacitance C of each battery cell i =dQ i / dU i Stored in the battery management device, wherein dQ i The method represents a change in electric charge and includes the following steps: 1) Manipulate the discharge circuit so that at a predetermined time t i Through resistor R i Discharge each individual battery cell i, and simultaneously measure the voltage U during the discharge. i (t) is used to obtain the voltage-time curve; 2) By C i and U i (t) determines the predetermined time t i The charge Q extracted during the period i for ； 3) Determine R i for 。 3. A battery system with passive balancing, the battery system comprising multiple lithium-ion battery cells and a battery management device, in, Each battery cell unit, consisting of a single battery cell or a group of multiple battery cells connected in parallel, is provided with a discharge circuit, wherein the discharge circuit has a load resistor R. i Furthermore, each of the aforementioned battery cells is connected in series to form a string. Furthermore, the battery management device is configured to measure the voltage U of each individual battery cell. i And to manipulate the discharge circuit at selectable times so as to pass through the load resistor R i Discharge the battery cell i in a controlled manner. The battery system is configured to implement the method according to claim 1 or 2.

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