Patent Application
20130147434
The present invention relates to a method for balancing states of charge of a battery having a plurality of battery cells, to a battery management system which comprises the method according to the invention, to a battery, in particular a lithium-ion battery, having the battery management system according to the invention, and to a motor vehicle having the battery according to the invention. The battery can be used, in particular, to drive the motor vehicle.
In the future it is expected that, for stationary applications such as, for example, wind turbine systems, emergency current units or island networks, as well as in vehicles such as, for example, hybrid vehicles or pure electric vehicles, battery systems will increasingly be used on which stringent requirements are made in terms of useful energy content, charging/discharging efficiency and reliability.
In order to be able to meet with the requirements with respect to available energy content, maximum power and overall voltage, many individual battery cells are connected in series and partially additionally in parallel. For example, in hybrid vehicles and electric vehicles, battery packs are used comprising Li ions or NiMH technology, which battery packs are composed of a large number of electrochemical cells connected in series.
Such a large number of battery cells which are connected in series entail a number of problems. For safety reasons and in order to achieve sufficient accuracy in the measurement of the voltage, the cell voltages of the individual battery cells must be measured individually and checked for compliance with upper limits and lower limits. Because of the series connection of the battery cells, the same current flows through all the battery cells, i.e. the quantity of the charge which is extracted during discharging and input during charging is also identical for all the battery cells. As a result, the cell with the lowest capacity limits the overall charge of the pack. Since as a rule there is a direct relationship between the loss of capacity and the increase in internal resistance for electrochemical cells, the cell with the lowest capacity generally also has the lowest power. If the capacity of a battery cell therefore differs from that of another, for example due to aging, the battery cells with a relatively high capacity can only be charged as far as the battery cell with the lowest capacity. Furthermore, the defect in an individual battery cell causes the entire battery to fail because the current can no longer flow through the defective battery cell and therefore through the battery.
A measure of the quantity of energy stored in a battery cell is what is referred to as the state of charge (SOC). It is to be noted here that the initial states of charge of the battery cells when they are assembled to form a battery will never be precisely the same. Furthermore, owing to a certain product variation during manufacture, the battery cells always differ slightly in their parameters and therefore also in their reaction of the state of charge to a current which is impressed from the outside. These differences can increase further due to aging of the battery cells. It is known to use a battery management system to monitor the states of charge of a battery. In addition to the monitoring of safety, the longest possible service life of the battery is to be ensured and it is to be ensured that the states of charge of the individual cells are matched to one another. This is done by suitable cell balancing. Cell balancing or balancing of the states of charge is generally performed resistively. For this purpose, a resistor and a switching element are provided for each cell in order to be able to discharge individual cells in a targeted fashion. DE 10 2006 022 394 A1 discloses a device for balancing the charge of an energy source with a plurality of cells, in which balancing process the cells are connected to a discharging unit which has the purpose of balancing charges and at least partially discharges the cells. However, according to the prior art, it is also possible to carry out the cell balancing capacitively, i.e. with connected capacitors, or inductively, that is to say by means of connected inductors. In these two cases, energy can be exchanged between the cells with a limited efficiency while in the case of resistive cell balancing energy can only be converted into heat, and is therefore lost.
For hybrid vehicles, a high performance level is advantageous at any time, both in the charging direction for the purpose of recuperation and in the discharging direction for the purpose of acceleration. It is known that the maximum permitted charging power decreases as the state of charge rises, while the maximum permitted discharging power increases. For this reason, according to the prior art it would be desirable to operate a battery pack for hybrid vehicles in a state of charge of 50%. However, as a rule, in practice an operating window is used, for example between 40% and 60% of the state of charge. For what are referred to as “plug-in hybrids”, the operating window is correspondingly larger, for example 10% to 90% of the state of charge.
A contemporary balancing strategy attempts to achieve a continuously equal state of charge of all the cells. For this purpose, as a rule, balancing to an identical quiescent voltage of all the cells is performed. In the case of new cells with a virtually identical capacity, this strategy is justified. However, in the case of cells with differing capacities, such as occur due to production variations and aging, this balancing strategy leads to unnecessary energy losses as a result of the balancing.
According to the invention, a method for balancing states of charge of a battery having a number of N battery cells is presented, which method firstly comprises the step of determining the individual cell capacities Capn of all the n=1 to N cells. This is followed by the determination of a k-th cell with the smallest cell capacity Capk of the n=1 to N cells and the determination of the individual states of charge SOCn of all the n=1 to N cells. The determination of the depth of discharge DODn of all the n=1 to N cells then follows. This can preferably take place according to the equation DODn=Capn−SOCn*Capn. In a further step, a target depth of discharge DODTarget,n is then determined for all the n=1 to N cells, with DODTarget,n=Capn/2+DODk−Capk/2. From the target depth of discharge, the target state of charge SOCTarget,n is determined for all n=1 to N cells from SOCTarget,n=1−DODTarget,n/Capn. After this, the deviation of the state of charge SOC, of one cell from its target state of charge SOCTarget,n is determined for all the n=1 to N cells according to ΔSOCTarget,n=SOCn−SOCTarget,n. In a further step, the minimum deviation ΔSOCmin of the state of charge SOC, of a cell from its target state of charge SOCTarget, is determined for all n=1 to N cells from ΔSOCmin=min (ΔSOCTarget,n). This is then followed by the discharging of at least one of the n=1 to N cells for which the following applies: ΔSOCTarget,n−ΔSOCmin>X where X is a parameter for which X≧0 applies. If ΔSOCTarget,n−ΔSOCmin≦X then applies for all of the n=1 to N cells, the method is then ended. However, if this does not apply, that is to say for at least one cell ΔSOCTarget,n−ΔSOCmin>X, then the process returns to the step of determining the individual states of charge and the subsequent steps are repeated.
In other words, the states of charge of N individual electrochemical battery cells of a battery are balanced. The method according to the invention having the features of the independent claim has the advantage that the energy losses are reduced compared to the prior art within the scope of balancing of the states of charge. In this context, at the same time the efficiency in the target charge state is optimized. The duration of the balancing is reduced. There is no need to use any further hardware for balancing. The method can be implemented as a computer program using software, and can therefore be implemented cost-effectively.
In the discharging step, all the cells for which ΔSOCTarget,n−ΔSOCmin>X applies can preferably be discharged, and even more preferably they can be discharged simultaneously. This reduces the duration of the balancing further.
The return to the step of determining the states of charge can preferably be performed at predefined time intervals. However, it can also take place continuously.
The state of charge is preferably specified in percentages, and the parameter X is preferably greater than zero and smaller than 6%, even more preferably smaller than 3%.
The method can advantageously also be used for capacitively and inductively balancing states of charge. In addition, after the discharging step and before the step of returning to the determination of the states of charge, it then comprises the step of feeding in the discharge current of the discharged cells in order to charge at least one of the n=1 to N cells for which the following applies: ΔSOCTarget,n−ΔSOCmin<Y, where X>Y.
In one preferred exemplary embodiment, all the cells for which ΔSOCTarget,n−ΔSOCmin<Y applies are charged, and are more preferably charged simultaneously.
Accordingly, a battery management system is proposed for a battery, in particular an Li-ion battery which comprises the method according to the invention which is described above.
Analogously, a battery, in particular an Li-ion battery, is specified, with a battery management system according to the invention, and a motor vehicle with a battery according to the invention is specified.
Advantageous developments of the invention are specified in the dependent claims and described in the description.
Exemplary embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.
In said drawings:
The present invention relates to a method for balancing the states of charge of individual cells of a battery or of a battery pack, in particular of an Li-ion battery of an Li-ion battery pack.
A contemporary balancing strategy of the prior art attempts to achieve a state of charge of all the cells which is always constant. For this purpose, balancing is generally carried out to an identical quiescent voltage of all the cells. In new cells with almost identical capacities, this strategy is justified. However, in the case of cells with different capacities, such as occur due to production variations and aging, this balancing strategy leads to unnecessary energy losses as a result of the balancing. This is illustrated by
In the following method, the state of charge is given as a percentage with “SOC” (State of Charge), where 100% corresponds to the state in which the cell is fully charged. The capacity of a cell is abbreviated with “Cap” and is given in the text which follows using the unit “Ah”. The depth of discharge is abbreviated with DOD (Depth of Discharge), and is also given in “Ah”. Given a depth of discharge of 0Ah, the cell is fully charged. The depth of discharge is calculated from DOD=(1−SOC)*Cap.
For the purpose of balancing all the N cells of a battery pack in such a way that, despite different capacities, in the case of 50% state of charge of the total battery pack they always have an individual state of charge of 50%, the following method can be advantageously applied:
ΔSOCTarget,n=SOCn−SOCTarget,n;
ΔSOCmin=min ΔSOCTarget,n;
ΔSOCTarget,n−ΔSOCmin>X,
The above method relates in this context firstly to the resistive balancing of states of charge. In the case of resistive balancing, the cells can merely be discharged, with the result that in the case of resistive balancing, only the discharging in step S9 is significant. In another exemplary embodiment, the capacitive or inductive balancing is also described, during which cells can also be charged.
In one preferred embodiment, the method according to the invention is part of a battery management system of a battery. In the text which follows, it is therefore described by way of example that the method according to the invention is carried out by means of a battery management system. However, this does not occur in a restrictive fashion but rather merely in an exemplary fashion.
The respective variables which are necessary for the course of the method can be subsequently calculated mathematically or determined or measured in some other way, for example by means of specification sheets.
In Step 1, the cell capacities are preferably determined or measured by means of a battery management system. In the next Step S2, the cell with the lowest cell capacity is then determined from the acquired data. Owing to the series connection of the cells, the cell with the lowest capacity limits the total charge of the pack. Since, as a rule, there is a direct relationship between loss of capacity and an increase in the internal resistance for electrochemical cells, the cell with the lowest capacity generally also has the lowest power. Therefore, in the method according to the invention, the balancing is performed with respect to the capacity or depth of discharge of the cell with the lowest capacity. In Step S3, the state of charge of each individual electrochemical cell of the battery is determined or measured. This is in turn preferably carried out using the battery management system. The depth of discharge is then calculated from the individual state of charge by means of the equation specified in Step S4, or is determined or measured in some other way. In Step S5, a desired target depth of discharge is then calculated therefrom for each cell. This target depth of discharge is calculated here as a function of the capacity and the depth of discharge of the cell with the lowest capacity. The target state of charge is then calculated from the target depth of discharge according to the equation specified above in Step S6. For balancing, in Step S7 the deviation of the present state of charge from the target state of charge ΔSOCTarget,n=SOCn−SOCTarget,n for each cell is determined or calculated. The minimum deviation ΔSOCmin is then determined from the deviations (S8). If the difference between the deviation ΔSOCTarget,n=SOCn−SOCTarget,n and the minimum deviation ΔSOCmin is then greater than a parameter X which is predefined or can be selected, at least one of those cells for which this applies is discharged (S9). All the cells are preferably discharged simultaneously. In Step S10 it is checked whether in fact a cell is discharged or is to be discharged. If the answer is affirmative, the process jumps back to the step S3 and the method is resumed from there. However, if a cell no longer has to be discharged, the process jumps to the next step and the method is ended (S11).
In this context, the jump back in Step S10 can occur continuously (if the computing time is short compared to the discharging process) or else discretely at fixed time intervals, for example 1-10 times per second. However, depending on the accuracy of the method, a time period of minutes may also be appropriate. It is therefore an iterative method in which the parameter X specifies precisely how balancing is to be performed. The parameter X may be an element of the real or natural positive numbers including the zero. X can be smaller than 10%, and X is preferably smaller than 6%, and even more preferably smaller than 3%. A relatively high parameter speeds up the method, but makes the adaptation less precise. A low parameter leads to a relatively low bandwidth of the individual states of charge after the balancing but a longer adaptation period.
Table 1 shows an exemplary embodiment of the method according to the invention for the resistive balancing of a battery, in particular of an Li-ion battery with six individual cells. The parameter X for the discharging is, for example, 4% here. As is apparent from Table 1, the cell 3 is the cell with the lowest capacity, here 4.3 Ah and has a state of charge of 64%. What is referred to as the target depth of discharge DODTarget of the individual cells is then a function of this smallest capacity and of the instantaneous depth of discharge of the respective cell with the minimum capacity. In the example shown, the cell 5 has a lower state of charge of 60% compared to 64% in cell 3. For this reason, the cell 5 also has the value with the lowest deviation from the target state of charge, specifically a negative deviation of −3%. Since the difference between the deviation of the target state of charge from the actual state of charge and the minimum deviation is zero for the cell 5, this cell is not discharged. In the case of a deviation threshold value X of 4%, in the present example, the cells 1, 2, 4 and 6 are discharged in the first step. The further profiles of the iterative method are not illustrated.
For the purpose of comparison,
On the other hand, after discharging to 50% of the state of charge of the total pack all the n=1 to N cells would also have a 50% state of charge, given a correspondingly low value of the deviation threshold value X. If the state of charge of the cell with the smallest capacity in Table 1 is changed to 50%, the target state of charge for all cells is, in fact, also 50%, as is shown by Table 2. This follows from the fact that the depth of discharge DOD is a function of the state of charge SOC. According to DOD=(1−SOC)*Cap, the depth of discharge for the cell k with the lowest capacity and a state of charge of 50% is precisely half the capacity of the cell, that is to say DODk=Capk/2. The following then applies for the target depth of charge of each further cell of the cells DODTarget,n=Capn/2. The target depth of charge therefore has the value of half the individual capacity. The target state of charge is then 50%, as desired.
In a further exemplary embodiment, the method is also adapted for capacitive and inductive balancing of the states of charge. In this context, in Step S9 of the method the discharge current of the discharged cells is used for charging other cells. At least one of the cells for which ΔSOCTarget,n−ΔSOCmin<Y applies is then charged, where X>Y. All the cells for which the relationship applies are preferably charged together. The parameter Y can be freely selected within the specified limits with respect to the parameter X.
The method according to the invention is preferably implemented as software in a battery management system of a battery. This software solution permits the method to be implemented in a way which is as cost-effective and flexible as possible.
The battery with the corresponding battery management system may be part of a motor vehicle. It is preferably part of a drive system of a motor vehicle and coupled to the engine of the motor vehicle.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 002 326.4 | Feb 2010 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/050060 | 1/4/2011 | WO | 00 | 11/5/2012 |
