Calibration of a Balancing System in a Battery System

Abstract

A method is provided for calibrating a passive balancing system in a battery system which has a plurality of lithium ion cells and a battery management system, in which cell units consisting of individual cells or parallel-connected groups of a plurality of cells are each provided with a discharge circuit having a load resistance Ri representing the calibration parameter, and the cell units are serially connected in series. The battery management system is designed to measure the voltage U.sub.i of each cell unit and to actuate the discharge circuit at a selectable time in order to discharge the cell unit in a controlled manner via the load resistance R.sub.i. The method includes actuating the discharge circuit of the cell unit for a discharge time t.sub.i in order to remove a charge Q.sub.i, and determining t.sub.i, Q.sub.i and the voltage characteristic over time U.sub.i(t); and determining R.sub.i.

Claims

1.-4. (canceled)

5. A method for calibrating a passive balancing system in a battery system comprising a plurality of lithium-ion cells and a battery management system, wherein cell units composed of individual cells or parallel-connected groups of a plurality of cells are each provided with a discharge circuit having a load resistor R.sub.i representing a calibration parameter, and the cell units are connected in series in strings, and wherein the battery management system is configured to measure a voltage U.sub.i of each cell unit and to actuate the discharge circuit at a selectable time in order to discharge the cell unit in a controlled manner via the load resistor R.sub.i, wherein the method comprises the steps of: actuating the discharge circuit of the cell unit for a discharge time t.sub.i in order to draw a charge Q.sub.i, and determining t.sub.i, Q.sub.i and the voltage profile over time U.sub.i(t); and determining R.sub.i as: $R_i=\frac{1}{Q_i}{\int }_0^{t_i}{U}_i\left(t\right)\mathrm{dt}.$

6. The method according to claim 5, further comprising: determining an initial voltage U.sub.i,0 of each cell unit of the string by way of the battery management system; applying a previously known charging current I to the string for a predetermined time t.sub.L in order to supply the known charge Q=∫Idt to each cell unit; drawing the previously supplied charge Q.sub.i=Q by virtue of the discharge circuit being actuated until the initial voltage U.sub.i,0 is reached again, with the result that the discharge duration t.sub.i meets the condition U.sub.i(t.sub.i)=U.sub.i,0; determinatiNG R.sub.i as: $R_i=\frac{1}{Q}\underset{0}{\overset{t_i\left(U=U_{i,0}\right)}{\int }}{U}_i\left(t\right)\mathrm{dt}$ wherein t(U=U.sub.i,0) represents the duration of actuation of the balancing circuit after which the voltage has fallen back to the initial value U.sub.i,0.

7. The method according to claim 5, wherein the differential capacitance of each cell unit is C.sub.i=dQ.sub.i/dU.sub.i, wherein dQ.sub.i represents the change in the charge, and is stored in the battery management system, the method further comprising the steps of: actuating the discharge circuit in order to discharge each cell unit via the resistor R.sub.i for a predetermined time t.sub.i, with simultaneous measurement of the voltage U.sub.i(t) during the discharge in order to obtain the voltage profile over time; determining the charge Q.sub.i drawn during the predetermined time t.sub.i from C.sub.i and U.sub.i(t) as: $Q_i=\int C_{i}d{U}_i=\underset{0}{\overset{t_i}{\int }}{C}_i\frac{d{U}_i\left(t\right)}{dt}dt$ determining R.sub.i as: $R_i=\frac{1}{Q_i}\underset{0}{\overset{t_i}{\int }}{U}_i\left(t\right)\mathrm{dt}.$

8. A battery system with passive balancing, comprising: a plurality of lithium-ion cells; and a battery management system, wherein cell units composed of individual cells or parallel-connected groups of the plurality of cells are each provided with a discharge circuit having a load resistor R.sub.i, and the cell units are connected in series in strings, wherein the battery management unit is configured to measure the voltage U.sub.i of each cell unit and to actuate the discharge circuit at a selectable time in order to discharge the cell unit in a controlled manner via the load resistor R.sub.i, wherein the battery system is configured to: actuate the discharge circuit of the cell unit for a discharge time t.sub.i in order to draw a charge Q.sub.i, and determines t.sub.i, Q.sub.i and the voltage profile over time U.sub.i(t); and determine R.sub.i as: $R_i=\frac{1}{Q_i}{\int }_0^{t_i}{U}_i\left(t\right)\mathrm{dt}.$

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] FIG. 1 schematically shows the structure of a string of cell units which are each provided with a discharge circuit and a voltage measurement device.

[0021] FIG. 2 schematically shows the structure during the determination of Q.sub.i through supplying and subsequently dissipating a previously known charge.

DETAILED DESCRIPTION OF THE DRAWINGS

[0022] The following text describes in more detail the structure of the battery system in which the method is used and the embodiments of the method itself.

Battery System and Balancing

[0023] The battery system in which the method is used comprises a plurality of lithium-ion cells and a battery management system (BMS), wherein cell units composed of individual cells or parallel-connected groups of cells are each provided with a balancing circuit. The battery management system is set up to execute a charge equalization, that is to say to carry out a balancing operation, at predefined times. To this end, in a cell or cell group, the cell voltage of which is raised in comparison with at least one other cell or cell group, the balancing circuit is actuated in order to draw charge from this cell or cell group until the cell voltages are equalized.

[0024] The balancing is typically carried out during a rest phase, for example after charging, and at a time at which the battery system is not subject to any loading. If the battery system is installed in an electric vehicle, the balancing can be carried out at an arbitrary time, other than during driving operation, preferably directly after the charging of the store. In a hybrid electric vehicle or plug-in hybrid electric vehicle, driving operation using the combustion engine is also considered. According to the invention, the time and the exact method of balancing are not specifically limited, provided that the charge transferred in the balancing operation for each cell can be ascertained by the BMS.

[0025] In passive balancing, charge is drawn from the cell with an increased cell voltage (and thus increased SOC) and said charge is dissipated to a load resistor (shunt). A simplified schematic illustration of such a passive balancing circuit for the case of N series-connected cells is shown in FIG. 1. For each cell i, the cell voltage U.sub.i is monitored by the BMS. In addition, each cell is provided with a shunt circuit, which comprises at least one switch S.sub.i (for example a MOSFET), which is controlled by the BMS, and the actual parallel resistor (shunt) R.sub.i.

[0026] In order to keep the apparatus-based outlay low, an option for directly measuring the current I.sub.i in the balancing circuit is not provided. Instead, the balancing current is calculated as I.sub.i(t)=U.sub.i(t)/R from the resistance value R.sub.i and the voltage profile U.sub.i(t) measured during the balancing. Integration over time provides the charge that flowed.

Determination of the Load Resistance

[0027] The calibration method is used to accurately determine the resistance value R.sub.i in order to be able to precisely ascertain the balancing current and the charge that has flowed. The current flowing via the load resistor during balancing is generally I.sub.i=U.sub.i/R.sub.i, wherein U.sub.i is a function of the state of charge (SOC.sub.i) of the cell unit and consequently does not have to be constant over time but depends on the charge Q.sub.i that has already flowed. The charge is thus calculated as:

Q.sub.i=∫I.sub.idt=1/R.sub.i*∫U.sub.idt.

[0028] As stated above, the battery management system is capable of measuring U.sub.i to a high degree of precision and if necessary plotting it over time in order for example to be able to monitor the state of charge (SOC) of the cell unit.

[0029] The invention is based on the idea of ascertaining the calibration parameter R.sub.i based on the above formula by virtue of determining the duration of the actuation of the discharge circuit (discharge duration) t.sub.i, the charge Q.sub.i that has flowed and the voltage profile U.sub.i(t). R.sub.i can then be calculated as:

[0030] The measurements and calculations required for this are performed by the battery management system, which is configured to monitor the voltage and control the discharge circuit anyway.

[0031] In order to determine the charge Q.sub.i that has flowed, for example the supplying of a known charge and its subsequent drawing via the discharge circuit are considered, or the calculation of the charge from the differential capacitance and the voltage profile during the discharge.

Determination of Q.SUB.i .by Supplying a Known Charge

[0032] A first option for determining Q.sub.i consists in supplying a known charge Q, which leads to an increase in the voltage U.sub.i on account of the increase of the state of charge of the cell unit. Then, the discharge circuit is actuated until the increased voltage has fallen back to the starting value. The state of charge (SOC) of the cell unit is then again also the same as before the charge was supplied, that is to say the charge Q.sub.i that flowed during discharge corresponds to the charge Q supplied. The schematic structure is shown in FIG. 2.

[0033] This embodiment of the method according to the invention comprises the following steps:

(1) determination of the initial voltage U.sub.i,0 of each cell unit i of the string by way of the battery management system;
(2) application of a previously known charging current I to the string for a predetermined time t.sub.L in order to supply the known charge Q=∫Idt to each cell unit;
(3) drawing of the previously supplied charge Q.sub.i=Q by virtue of the discharge circuit being actuated until the initial voltage U.sub.i,0 is reached again, with the result that the discharge duration t.sub.i meets the condition U.sub.i(t.sub.i)=U.sub.i,0;
(4) determination of R.sub.i as:

[00002]$R_i=\frac{1}{Q}\underset{0}{\overset{t_i\left(U=U_{i,0}\right)}{\int }}{U}_i\left(t\right)\mathrm{dt}$

wherein t.sub.i(U=U.sub.i,0) represents the discharge duration after which the voltage has fallen back to the initial value U.sub.i,0.

[0034] First of all, in step (1), the voltage U.sub.i,0 is measured, which represents the measure for the initial SOC of the cell unit, which must also be equal to the final SOC when the subsequent step (3) has ended.

[0035] Subsequently, in step (2), the entire string is charged with a defined charging current for a defined period. This step can be done 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, which is calculated by integrating the charging current over time, is supplied.

[0036] The charging method is not specifically limited. For example, the charging can take place with a constant current or a constant voltage. It is only necessary to measure the profile of the charging current I over time in order to be able to calculate the charge. To control the charging process, the battery system or the charging device is provided with a current measurement device anyway, which can be used for determining the charge. In the embodiment shown in FIG. 2, the current measurement device is integrated into the battery system (“s-box”). If necessary, a highly precise current measurement device can be introduced into the charging circuit in order to be able to ascertain the charge with a high degree of accuracy.

[0037] Step (2) does not require any physical access to the individual cell units but can be carried out using the installed battery system in field deployment using conventional charging devices. At most, a highly precise current measurement device may be required as additional equipment.

[0038] Since the string consists purely of series-connected cell units, the current flowing through each cell and thus in good approximation also the supplied charge of each cell unit is the same and can be calculated as Q=∫Idt.

[0039] After the charging has ended, if necessary there may be a slow exchange of charge between the cells on account of slightly different cell voltages, with the result that the charges can drift apart from one another over time. However, this effect is negligible in the method according to the invention due to the slow timescale, in particular when step (3) is carried out directly after step (2).

[0040] By increasing the SOC on account of the supplied charge Q, after step (2) the cell voltage in the cell units is increased with respect to U.sub.i,0. In step (3), the cell units are discharged during actuation of the discharge circuit until U.sub.i,0 and thus the original SOC is reached again. The charge Q.sub.i dissipated here is thus equal to the charge supplied in step (2).

[0041] By plotting the voltage profile during the discharge and integrating over time, the value of the load resistance R.sub.i is then calculated in step (4) as:

[00003]$R_i=\frac{1}{Q}\underset{0}{\overset{t_i\left(U=U_{i,0}\right)}{\int }}{U}_i\left(t\right)\mathrm{dt}.$

[0042] No specific laboratory equipment is required and no measures to be executed externally at the battery system itself are required.

Determination of Q.SUB.i .Based on the Previously Known Differential Capacitance

[0043] As an alternative, Q.sub.i can also be determined from the differential capacitance C.sub.i=dQ.sub.i/dU.sub.i, which is stored in the battery management system, or can be calculated from the stored charge/voltage correlation data Q.sub.i(U.sub.i), which are required anyway in order to ascertain the SOC, by differentiating according to the voltage. This embodiment of the method according to the invention using the differential capacitance C.sub.i comprises the following steps:

[0044] (1) actuation of the discharge circuit in order to discharge each cell unit i via the resistor R.sub.i for a predetermined time t.sub.i, with simultaneous measurement of the voltage U.sub.i(t) during the discharge in order to obtain the voltage profile over time;

[0045] (2) determination of the charge Q.sub.i drawn during the predetermined time t.sub.i from C.sub.i and U.sub.i(t) as

[00004]$Q_i=\int C_{i}d{U}_i=\underset{0}{\overset{t_i}{\int }}{C}_i\frac{d{U}_i\left(t\right)}{dt}dt$

[0046] (3) determination of R.sub.i as:

[00005]$R_i=\frac{1}{Q_i}\underset{0}{\overset{t_i}{\int }}{U}_i\left(t\right)\mathrm{dt}.$

[0047] In step (1), the cell is again discharged in a controlled manner and the voltage profile during discharge is measured. However, in contrast to the first version, the charge drawn is not previously known but must be calculated in step (2) from the previously known differential capacitance C.sub.i and the measured voltage profile U.sub.i(t). The differential capacitance C.sub.i is either stored itself in the battery management system or it is calculated on the fly from the previously known no-load characteristic curve.

[0048] In step (3), the determination of R.sub.i is finally carried out in an analogous manner to the first embodiment.