WO2017219136A1

WO2017219136A1 - Method of balancing a multi-cell battery

Info

Publication number: WO2017219136A1
Application number: PCT/CA2017/050755
Authority: WO
Inventors: David Lokhorst; Ian RUMLEY
Original assignee: Corvus Energy Inc.
Priority date: 2016-06-21
Filing date: 2017-06-20
Publication date: 2017-12-28

Abstract

There are provided a method and system for predictively balancing a multi-cell battery. The system determines a need to balance the cells of the battery. The system may then balance the cells of the battery while the battery is in use, by estimating a time required to balance each unbalanced cell. Once balancing is complete, the estimated balancing times may be updated based on the effectiveness of the previous balancing, so that a subsequent balancing is more accurate. The system may identify periods during which the battery is at rest, at which points the method may determine any new need for balancing, and may estimate new balancing times / update previous estimated balancing times if required.

Description

METHOD OF BALANCING A MULTI-CELL BATTERY

Field of the Disclosure

[0001] The present disclosure relates to a method of balancing a multi-cell battery, such as a rechargeable lithium-ion battery as used in the maritime industry.

Background to the Disclosure

[0002] One type of rechargeable battery is a lithium-ion battery, comprising a number of serially connected cells. The battery is typically housed in an enclosure to form a battery module. During normal operating conditions, electrical energy is converted to and stored as chemical energy during charging, and stored chemical energy is converted to electrical energy during discharging.

[0003] The cells within the module differ slightly In capacity and other parameters due to acceptable manufacturing tolerances. During use, and especially after repeated charging and discharging of the battery (i.e. after repeated charging-discharging cycles), the slight differences in the ceils will gradually cause the cells to settle at voltages that differ from each other. Since the cells are arranged in series, this will result in lower effective capacity, as charging must be stopped when the cell with the highest voltage reaches its upper voltage limit, and discharging must be stopped when the cell with the lowest voltage reaches its lower voltage limit. To overcome this restriction In effective capacity, it is well known that lithium-ion cells can be "balanced" from time to time. This is typically implemented by means of a balancing circuit that is permanently mounted within the module. To balance the cells of a battery, the charge contained in those cells with a relatively higher open circuit voltage is dissipated until all cells have an open circuit voltage that is roughly equal, or "balanced".

[0004] Open circuit voltage, or OCV, is the voltage at which the cell settles after a period of rest (I.e. when no current is flowing or the cell is not part of an electrical circuit). It typically takes 10- 30 minutes of rest for a lithium-ion cell to asymptotically reach OCV. Some batteries are designed with cells in a series-paraiiel arrangement. For example, a battery module may have 24 battery ceiis, arranged as 12 groups, each of the groups consisting of 2 cells in parallel. Such a group of cells connected in parallel is called a "series element". The cells within a series element will generally have equal voltage due to the fact they are directly connected in parallel. The voltage of a series element (i.e. the voltage of all of the cells within the series element) is called the "series element voltage", or SEV. The series elements will differ in voltage for the same reasons that single cells in series differ in voltage. Therefore, both series only and series-parallel arrangements of batteries will require balancing, in the parlance of the industry, a series element may have 1 , 2, 3, or more cells. From this point forward In the disclosure, as is typical in the industry, SEV is used as the voltage of a single element measured at the terminals, regardless of whether the battery is arranged as series-connected cells or series-parallel connected cells.

[0005] In industries where sustained electrical power output Is important, such as the maritime industry, it is key that the batteries be operated with as little interruption as possible. In particular, battery packs (comprising multiple battery modules connected in series) on a vessel such as a ship are often run around the clock In order to power the ship's various functions. One particular issue with the operation of such batteries is the requirement to take a battery module (or indeed an entire battery pack) offline In order to balance the cells. This is because, when balancing the cells, it is usual for the SEV of each cell to be continuously measured so that the balancing can be monitored and stopped when the desired voltage is reached. For example, US 8,004,246 B2 and US 201 1 /068744 A1 describe balancing systems in which the voltages of the cells are read in order to monitor the progress of the balancing. However, measuring the voltage of a cell (or series element) while the battery is running (i.e. is being discharged) can lead to misleading voltage readings since, when electrical current Is applied to the cell, its SEV deviates from its OCV due to internal resistance of the cell. Furthermore, the internal resistance differs among cells due to acceptable manufacturing tolerances and age effects. Therefore, the relative SEVs while charging and discharging differ from the relative OCVs of those cells or series elements. Thus, is it usual when balancing the cells for the battery to be taken offline such that it is no longer providing a power output. Balancing can be a relatively slow process, and as a result the corresponding offline time can in some cases be significant (upwards of one hour).

[0006] There is therefore a need In the art for new and improved methods of balancing battery cells, directed at addressing some of the current drawbacks in prior art balancing systems. Summary of the Disclosure

[0007] In a first aspect of the disclosure, there is provided a method of balancing a multi-cell battery. The method comprises determining whether any of the cells of the multi-cell battery meet an unbalanced condition. The method further comprises Identifying one or more of the cells that are determined to meet the unbalanced condition as unbalanced cells. The method further comprises, for each unbalanced cell, estimating a balancing time being an estimated time for balancing the unbalanced cell. The method further comprises balancing each unbalanced ceil according to its estimated balancing time.

[0008] Thus, a cell may be balanced without the need to monitor its SEV. The balancing will cease once the estimate balancing time for the cell has expired. The cell may be discharged/charged in the usual way simultaneously as it is being balanced (i.e. balancing may take place at the same time that the battery is operating). The battery may have any number of cells. The cell may be a series-element comprising multiple individual cells in parallel.

[0009] The determining may comprise measuring a first voltage across each cell of the multi- ceii battery. The determining may be based on a measured state of charge of each cell of the multi- ceil battery. The state of charge of a cell may have a known relationship to the cell's open circuit voltage. A cell may be identified as an unbalanced cell if its measured first voltage is, by a predetermined amount, greater or less than the measured first voltage of another cell. The predetermined amount may be 5mV.

[0010] Balancing an unbalanced cell may comprise adjusting an amount of charge contained in the unbalanced ceil.

[001 1] An estimated balancing time may be based on an estimated balancing rate and a difference between the measured first voltage of the unbalanced cell and the measured first voltage of another cell, which may also be an unbalanced cell. The measured first voltage of the other cell may be the lowest measured first voltage of the cells of the multi-cell battery. The estimated balancing rate may be a preset value, and may be adjustable by a user. Each cell may have a different estimated balancing rate assigned to it. A typical estimated balancing rate may be 5mV/hour.

[0012] The measured first voltages may comprise open circuit voltages.

[0013] The balancing may comprise discharging each unbalanced cell for its estimated balancing time. The discharging may comprise dissipating charge contained in the unbalanced ceil using a balancing circuit connected to the unbalanced cell.

[0014] The balancing may comprise transferring charge from a first unbalanced cell to a second unbalanced cell. The first unbalanced cell may have a greater measured first voltage than the second unbalanced cell.

[0015] The method may further comprise updating one or more estimated balancing times.

The method may further comprise balancing one or more unbalanced cells according to the one or more updated estimated balancing times. The updating may comprise determining whether to carry out the updating based on one or more balancing criteria. The one or more balancing criteria may comprise one or more of: a period of time elapsed since the balancing of each unbalanced cell according to its estimated balancing time; and a number of charge-discharge cycies undergone by the multi-cell battery since the balancing of each unbalanced cell according to its estimated balancing time. The period of time may be measured from the start of the balancing of each unbalanced cell. The number of charge-discharge cycles may be measured from the start of the balancing of each unbalanced cell. The period of time may be 1 to 7 days. The number of charge-discharge cycles may be 5 to 50. A charge-discharge cycle may be defined as when the battery module undergoes a complete discharge of one or more of its cells (possibly all its cells), and a complete charge of one or more of its ceils (possibly ail its cells).

[0016] Thus, provided that the battery module has not undergone an excessive number of charge-discharge cycies since its cells were last balanced, and provided that a not excessive amount of time has elapsed since the ceils were last balanced, the previous estimated balancing times may updated and used in a subsequent balancing phase or step. [0017] The updated estimated balancing rate may be based on an estimated balancing rate and a difference between: a voltage measured across the unbalanced cell after the balancing of the cell according to its estimated balancing time; and a voltage measured across another cell of the multi-cell battery. In other words, the updated estimated balancing rate may be based on an estimated balancing rate used for a first balancing phase (i.e. the balancing of each unbalanced cell according to its estimated balancing time). The updated estimated balancing rate may be further based on a difference between a voltage of an unbalanced cell after having been balanced and a voltage of another ceil of the multi-cell battery. The voltage of the other cell may be the lowest measured voltage of the cells. Thus, if an initial balancing of a cell according to an initial balancing rate resulted In insufficient balancing of the ceil, then the estimated balancing rate for that cell may be decreased. This may allow the estimated balancing time for the ceil to be increased, thereby increasing the likelihood that, following a subsequent balancing of the cell, the cell will be sufficiently balanced.

[0018] The updated estimated balancing time may be further based on a second voltage measured across the unbalanced cell and a second voltage measured across another cell of the multi- cell battery. The measured second voltage of the other cell may be a lowest measured second vo!tage of the ceils of the multi-cell battery. The measured second voltages may be open circuit voltages.

[0019] The method may further comprise, prior to the determining of whether any of the ceils of the multi-cell battery meet an unbalanced condition, determining that voltages measured across each cell of the multi-cell battery have reached a steady state. The measured voltages may comprise open circuit voltages of the cells of the multi-cell battery. The determining may comprise waiting a relaxation time before measuring the voltages across each cell. The relaxation time may be from between 5 minutes and 30 minutes. The relaxation time may be a time required for the SEV of a cell to asymptotically reach the cell's OCV (open circuit voltage).

[0020] The method may further comprise repeating at least the determining step when voltages measured across each cell of the multi-ceil battery have reached a steady state. The measured voltages may comprise an SEV of each cell. The steady state may be reached when the SEV of a cell asymptotically reaches the cell's OCV (open circuit voltage).

[0021 ] The multi-cell battery may undergo multiple charge-discharge cycles during the balancing of each unbalanced cell according to its estimated balancing time. Alternatively, during the balancing of an unbalanced cell, the battery module may undergo less than one charge-discharge cycle.

[0022] The battery module may comprise a capacity of between 45 to 55 Ampere-hours.

[0023] The battery module may be configured to provide a power output of between 450 to

550 kW.

[0024] The battery module may be configured to provide a discharge current of between 450A to 550A.

[0025] In a further aspect of the disclosure, there is provided a system for balancing a multi- cell battery. The system comprises a balancing circuit connected in parallel to each cell of the multi- cell battery. The system further comprises one or more processors. The one or more processors are configured to determine whether any of the cells of the multi-cell battery meet an unbalanced condition. The one or more processors are further configured to identify one or more of the cells that are determined to meet the unbalanced condition as unbalanced cells. The one or more processors are further configured to, for each unbalanced cell, estimate a balancing time being an estimated time for balancing the unbalanced cell. The one or more processors are further configured to control the balancing circuit to balance each unbalanced cell according to its estimated balancing time.

[0026] The one or more processors may be further configured to update one or more estimated balancing times. The one or more processors may be further configured to control the balancing circuit to balance one or more unbalanced cells according to the one or more updated estimated balancing times. The one or more processors may be further configured to determine whether to carry out the updating based on one or more balancing criteria. The one or more balancing criteria may comprise one or more of: a period of time elapsed since the balancing of each unbalanced cell according to its estimated balancing time; and a number of charge-discharge cycles undergone by the multi-cell battery since the balancing of each unbalanced cell according to its estimated balancing time.

[0027] The one or more processors may be further configured, after controlling the balancing circuit to balance each unbalanced cell, to further control the balancing circuit to balance one or more ceils of the multi-cell battery when voltages measured across each cell of the multi-cell battery have reached a steady state.

[0028] In a further aspect of the disclosure, there is provided a non-transitory computer- readable medium, having machine-readable instructions stored thereon, the instructions configured when read by a machine to cause the steps of any one of the above-described methods to be carried out,

[0029] Thus, there are provided a method and system for predictively balancing a multi-cell battery. The system determines a need to balance the cells of the battery. The system may then balance the cells of the battery while the battery is in use, by estimating a time required to balance each unbalanced cell. Once balancing Is complete, the estimated balancing times may be updated based on the effectiveness of the previous balancing, so that a subsequent balancing is more accurate. The system may identify periods during which the battery Is at rest, at which points the method may determine any new need for balancing, and may estimate new balancing times / update previous estimated balancing times if required.

Brief Description of the Drawings

[0030] Detailed embodiments of the disclosure will now be described in conjunction with the accompanying drawings of which:

[0031] Figure 1 is a circuit diagram of a system for balancing a multi-cell battery, in accordance with an embodiment of the disclosure; [0032] Figures 2A-2C are schematic representations of cells at various states of charge; and

[0033] Figure 3 is a flowchart showing a method of balancing a multi-cell battery, in accordance with an embodiment of the disclosure.

Detailed Description of Specific Embodiments

[0034] The present disclosure seeks to provide an improved method of balancing a multi-cell battery. While various embodiments of the disclosure are described below, the disclosure is not limited to these embodiments, and variations of these embodiments may well fall within the scope of the disclosure which is to be limited only by the appended claims.

[0035] Turning to Figure 1 , there is shown a circuit diagram of a system 100 for balancing a multi-cell battery, in accordance with an embodiment of the disclosure. System 100 comprises a battery module 105 having a plurality of serially connected lithium-ion cells 1 10a-d. Note that although In the present embodiment cells 1 10a-d are represented as single cells, in other embodiments each of cells 1 10a-d may be a series element comprising one or more cells in parallel (as discussed above). In addition, although the embodiment of Figure 1 shows four cells in series arrangement, the disclosure embraces battery modules with any number of cells.

[0036] Each cell 110a-d is connected in parallel to a corresponding balancing circuit 1 12a-d comprising a corresponding transistor Q_rQ₄ and a corresponding resistor R_rI¾. Balancing circuits 112a-d are configured to provide controlled balancing of cells 1 10a-d, as will be described in more detail below. Control electronics 1 18 (including a programmable component such as firmware) is connected to balancing circuits 112a-d and comprises a number of components such as a processor 120, memory 122, and other electronic components for controlling operation of transistors Q_rQ₄ and measuring the voltages V_rV₄ across cells 1 10a-d (for diagrammatic clarity, voltage pickups are not shown but are well known in the art).

[0037] To better understand the disclosure, the well-known procedure for cell balancing is as follows, using system 100 as a reference. First, it is generally necessary to stop current flow between the positive and negative terminals 124 and 126 and wait a settling time (also referred to as a relaxation time; typically 5 minutes or more) In order to accurately measure the cell voltages V_rV₄ to determine if the ceils are "in balance". If any cell voltages differ by a predetermined amount, then the control electronics determines that cell balancing is required. If balancing is required, the main battery current through the positive and negative terminals is kept very low, typically below 5% of the rated current of the battery. The transistor corresponding to the cell that has a higher-than-threshold voltage is activated, causing current to flow through the resistor corresponding to that cell. This current is referred to as the "balancing current". The resistor is chosen to have a high resistance so that the balancing current is small and the voltage reading remains accurate. The transistor is held active and the balancing current flows until the control electronics monitoring the voltage detects that the balancing is complete (i.e. the voltage across the cell has been balanced with the voltages across the other ceils), at which time the transistor is deactivated. The battery module may then be used again.

[0038] For example, if the battery is left at rest for 10 minutes and V₃ is measured by the control electronics to be 4110mV, while the voltages at celis 1 , 2 and 4 are all measured to be at 4100mV, then the transistor Q₃ is activated. A balancing current will begin to flow through R₃ and 0₃ and will slowly deplete cell 3. This causes the voltage V₃ to slowly drop, until it is within a tolerance (about 5m V) of the voltages Vi, V₂ and V₄, at which time V₃ is 4105mV and Q₃ is deactivated. The balancing current stops and the battery module can be re-used.

[0039] In practice, for battery packs comprising multiple battery modules, each with multiple cells, the control electronics typically selects a "voltage setpoint" which is the lowest cell voltage in the entire collection of modules. Then, the transistors for all cells whose voltages exceed the voltage setpoint are simultaneously activated, and all modules are balanced simultaneously. This type of balancing Is referred to in the art as passive balancing. Active balancing, which is also contemplated within the scope of the disclosure, Is carried out using a circuit configured to transfer charge from one ceil to another, without 'bleeding out' the energy through a resistor.

[0040] There will now be described a method of balancing a multi-cell battery, in accordance with an embodiment of the disclosure. The method may be carried out by software, stored on a memory (such as memory 122) and executed by a processor (such as processor 120) using traditional balancing circuits (for example the one described above in connection with Figure 1 ). The method, described below in a connection with Figure 3, will also be described in conjunction with Figures 2A- 2C. Figures 2A-2C are schematic representations of the states of charge of cells 1 10a-d of system 100. In Figure 2A, battery module 105 (and thus cells 1 10a-d) has undergone a number of charge- discharge cycles sufficient to bring about a charge imbalance. In particular, in Figure 2A the series element voltage (or SEV) of cells 1 10a-d is as follows: V₁>V₂>V₄>V₃. For clarity of description, the various states of charge of cells 110a-d are not shown to scale.

[0041 ] Now turning to Figure 3, which shows a method 200 of balancing cells in a multi-cell battery, according to an embodiment of the disclosure, at step 210 a command is received at control electronics 1 18 to begin method 200. The command may be received from a pack controller, which is a controller configured to control multiple ones of serially connected battery modules 105. The pack controller may be programmed to initiate a "Start Balancing" command every time a predetermined period of time elapses, or once a predetermined number of charge-discharge cycles of battery module 105 have occurred. In some embodiments, the "Start Balancing" command may only be triggered if the state of charge of cells 110a-d is above a relatively high threshold, say 80%.

[0042] At step 211 , processor 120 waits a "relaxation period". A relaxation period is a period of time during which battery module 105 has operated at below a threshold current for a minimum period of time (for example less than 5% rated current for at least 5 minutes), and all SEVs are measured to be within excess of a fixed minimum value, such as 4000mV.

[0043] At step 212, processor 120 measures the series element voltage (SEV) across each cell 1 10a-d. In the case of Figure 2A, processor 120 measures the SEV of cell 1 10a as Vi , the SEV of cell 1 10b as V₂, the SEV of cell 110c as V₃, and the SEV of cell 1 10d as V₄.

[0044] At step 213a, processor 120 determines whether the time elapsed since the previous balancing phase Is less than a fixed value, Tu (generally 1 to 7 days). If the time elapsed since the previous balancing phase is not less than Tu, then the method proceeds to step 214. If the time elapsed since the previous balancing phase is less than Tu, then the method proceeds to steps 213b and 213c which are discussed in more detail below. [0045] At step 214, processor 120 determines a voltage setpoint from among the measured

SEVs. In the present embodiment, the voltage setpoint corresponds to the lowest measured SEV of ceils 1 10a-d, though in other embodiments the voltage setpoint may be defined differently, Thus, in the case of Figure 2A, processor 120 determines that the voltage setpoint (voltage setpoint 1 1 1 ) is equal to V₃, The method then proceeds to step 216.

[0046] At step 216, processor 120 determines whether an unbalanced condition is met. An unbalanced condition is met if any of the measured SEVs is greater than voltage setpoint 1 1 1 by a predetermined amount, V_threshold (for example 5mV). If no unbalanced condition is met (i.e. if each measured SEV is within V_threshold of voltage setpoint 1 1 1 ), then the method proceeds to step 217 where the method ends. In Figure 2A, voltage setpoint 1 1 1 is defined as V₃. V₄-V₃ < V_threshold, arid therefore cell 1 10d is determined to not be unbalanced. However, V_rV₃ > V_threshold and V₂-V₃ > V_threshold, and therefore cells 1 10a and 1 10b are Identified as unbalanced cells. In some embodiments, for example where active balancing Is to be used, cell 1 10c (with an SEV corresponding to voltage setpoint 1 1 1 ) may also be identified as an unbalanced cell.

[0047] As an unbalanced condition has been met for cells 1 10a and 110b, the method proceeds to step 222 where processor 120 estimates a balancing rate K_balance for each unbalanced cell. In one embodiment, K_balance is initially set for each cell by a user, and may be for example 5mV/hour. Each cell 1 10a-d may be Initially set with the same K_balance, or each cell 1 10a-d may have a different K_balance- The balancing rate is an estimated balancing rate since, during balancing of the unbalanced cell, the corresponding balancing circuit of the unbalanced cell, and the SEV, result in the actual balancing rate varying with time.

[0048] At step 222, processor 120 also estimates a balancing time for each of the unbalanced cells, i.e. unbalanced cell 1 10a and unbalanced cell 1 10b. The estimated balancing time is an estimated time for balancing the unbalanced cell such that the SEV of the unbalanced cell is brought within V_threshold of voltage setpoint 1 1 1 , Each estimated balancing time T_balance is calculated according to the following relationship:

[0049] The method then proceeds to step 224 where processor 120 commences balancing of cells 1 10a and 1 10b according to their respective estimated balancing times. That is, each of cells 1 10a and 1 10b is balanced by activating transistors Q₁ and Q₂ in balancing circuits 1 12a and 1 12b such that charge contained in cells 1 10a and 110b is dissipated through resistors R, and R₂. Simultaneously to balancing of cells 110a and 110b, battery module 105 may be operated in the usual way. This includes discharging or charging through main battery terminals 124 and 126 as may be required by the battery-powered system to which battery module 105 is connected. Thus, cells 1 10a and 110b may balance at the same time battery module 105 provides electrical power in the usual way. Battery module 105 may undergo multiple charge-discharge cycles during the balancing of cells 1 10a and 110b.

[0050] At step 226, processor 120 determines when T_balancing for each unbalanced celi expires, at which point method 200 proceeds to step 228 where method 200 ends.

[0051 ] Turning to Figure 2B, there is shown the states of charge of cells 110a-d after a first phase of balancing is complete, and once battery module 105 has been recharged. It can be seen that cell 110d has reached a maximum state of charge before cells 1 10a-c. It can also be seen that cells 1 10a, 1 10b and 110c have been balanced to a degree since the SEVs of cells 1 10a, 110b and 110c are closer to one another. In particular, V'rV'₃ < V<-V₃ and V2-V3 < V₂-V₃. Furthermore, V'₃>V₃, indicating that cell 1 10c is closer to a maximum state of charge after the balancing operation.

[0052] Method 200 may be repeated following a first balancing phase (I.e. following an iteration of method 200). As described above, at step 210 a "Start Balancing" command is received at processor 120. At step 21 1 , processor 120 waits a relaxation period before proceeding to step 212. At step 212, the SEV of each of celis 1 10a-d is measured. At step 213a, processor 120 determines whether the time since the start of the previous balancing phase (i.e. the time that has passed since the start of the previous balancing at step 224 in the previous iteration of method 200) is less than T_u. In this case, let us assume that the time since the previous balancing phase is still less than Tu.

[0053] The method then proceeds to step 213b where processor 120 determines whether the number of charge-discharge cycles that have occurred since the start of the previous balancing phase

(I.e. since the start of step 224 in the previous iteration of method 200) is less than a predetermined number of charge-discharge cycles, C_u (typically 5 to 20 cycles). If the number of charge-discharge cycles that have occurred since the start of the previous balancing phase is not less than C_u, then method 200 proceeds to step 214 without first passing to step 213c. If the number of charge- discharge cycles that have occurred since the start of the previous balancing phase is less than Cu, then method 200 proceeds to step 213c.

[0054] At step 213c, processor 120 updates the initial estimated balancing rates K_balance for cells 1 10a and 1 10b. It can be seen from Figures 2A and 2B that Ν/Ί - V_threshold > V'_3l and therefore the initial K_balance for ceii 1 10a was insufficient to fully balance cell 1 10a. The initial estimated balancing rate K_balance for cell 1 10a is therefore adjusted (decreased) accordingly to account for the fact that cell 110a balanced more slowly than anticipated by the initial K_balanc-_e- Similarly, for cell 1 10b, V'₂ - V_threshold > V'_3i and therefore K_balance for cell 1 10b was insufficient to fully balance cell 1 10b. The initia K_balance for eel! 1 10b is therefore adjusted (decreased) accordingly to account for the fact that cell 110b balanced more slowly than anticipated by the initial K_balance- Note that in other embodiments, K_balance may be increased if the initial estimated balancing time resulted in unbalanced ceils 'overbalancing' such that their SEV became lower than the voltage setpoint. Step 213c is therefore only carried out if the previous balancing phase was sufficiently recent that battery module 105 is not likely to have gone out of balance since the previous balancing phase.

[0055] Method 200 then proceeds to step 214. At step 214, processor 120 determines a voltage setpoint from among the measured SEVs. in the case of Figure 2B, processor 120 determines that the voltage setpoint (voltage setpoint 115) is equal to V₃. The method then proceeds to step 216.

[0056] At step 216, processor 120 determines whether an unbalanced condition is met. If no unbalanced condition is met (i.e. if each measured SEV Is within V_threshold of voltage setpoint 1 15), then the method proceeds to step 217 where the method ends. In Figure 2B, voltage setpoint 1 15 is defined as V₃. V4-V3 < V_threshold, and therefore cell 11 Qd is determined to not be unbalanced. However, V'rV'3 > V_threshold and V2-V3 > V_threshold , and therefore cells 110a and 1 10b are identified as still unbalanced cells. In some embodiments, for example where active balancing is to be used, cell 110c (with an SEV corresponding to voltage setpoint 1 15) may also be identified as an unbalanced cell. [0057] As an unbalanced condition has been met for cells 1 10a and 1 10b, the method proceeds to step 222 where processor 120 retrieves the updated balancing rates for each

unbalanced cell (as generated in step 213c). At step 222, processor 120 also estimates an updated balancing time for each of the still unbalanced cells, i.e. unbalanced cell 1 10a and unbalanced cell 1 10b. The updated estimated balancing time is an estimated time for balancing the unbalanced cell such that the SEV of the unbalanced cell is brought within of voltage setpolnt 1 15. Each

updated estimated balancing time is calculated according to the following relationship:

[0058] The method then proceeds to step 224 where processor 120 commences balancing of cells 1 10a and 1 10b according to their respective updated estimated balancing times. That is, each of cells 1 10a and 1 10b is balanced by activating transistors Q₁ and Q₂ in balancing circuits 1 12a and 1 12b such that charge contained in cells 1 10a and 1 10b is dissipated through resistors R< and R₂. Simultaneously to balancing of cells 1 10a and 1 10b, battery module 105 may be operated in the usual way. This includes discharging or charging through main battery terminals 124 and 126 as may be required by the battery-powered system to which battery module 105 is connected. Thus, cells 1 10a and 1 10b may balance at the same time battery module 105 provides electrical power in the usual way. Battery module 105 may undergo multiple charge-discharge cycles during the balancing of cells 1 10a and 1 10b.

[0059] At step 226, processor 120 determines when for each unbalanced cell

expires, at which point method 200 proceeds to step 228 where method 200 ends.

[0060] Turning to Figure 2C, there is shown the states of charge of cells 1 10a-d after the second phase of balancing is complete, and once battery module 105 has been recharged. It can be seen that each of V"-,-V"₄ is within Thus, in the next iteration of method 200, no

unbalanced condition will be met and method 200 will proceed from step 216 to step 217 (assuming that in the meantime no further imbalance affects cells 1 10a-d). [0061 ] The above-described method is an example of an algorithm that may be implemented in order to balance ceils in a multi-cell battery. The number of steps taken may be greater or fewer than those described, and the order of the steps may be varied.

[0062] Method 200 may be repeated again with a certain frequency, such as after a set number of charge-discharge cycles of battery module 105 or a set period of time. It is typical to initiate balancing once every 5 to 30 days. In one embodiment, following an iteration of method 200, processor 120 may opportunistically wait for a sufficient relaxation time to occur before initiating a further iteration of method 200. This may be done by monitoring the SEVs of cells 1 10a-d continuously, and by re-initiating method 200 as soon as a sufficient relaxation period happens to occur in the course of norma! operation of battery module 105. In typicai operation, this may occur frequently enough to keep the cells balanced without requiring the user to undertake a deliberate balancing activity. One a relaxation time has been determined to occur, the next iteration of method 200 may begin immediately at step 212.

[0063] As discussed above, a relaxation time may be defined as a period of time that passes until the SEVs of the battery module's cells enter a "steady state". A steady state may be defined as a state In which the SEVs of cells 110a-d asymptotically reach the OCV (open circuit voltage) of cells 1 10a-d. At this point method 200 may be re-initiated, and processor may run through method 200 starting at step 212.

[0064] One or more example embodiments have been described by way of illustration only.

This description is been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope of the claims. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the claims. It is furthermore contemplated that any part of any aspect or embodiment discussed in this specification can be implemented or combined with any part of any other aspect or embodiment discussed in this specification.

Claims

1 . A method of balancing a multi-cell battery, comprising:

determining whether any of the cells of the multi-cell battery meet an unbalanced condition; identifying one or more of the cells that are determined to meet the unbalanced condition as unbalanced cells;

for each unbalanced cell, estimating a balancing time being an estimated time for balancing the unbalanced cell; and

balancing each unbalanced cell according to Its estimated balancing time.

2. The method of claim 1 , wherein the determining comprises measuring a first voltage across each cell of the multi-cell battery.

3. The method of claim 2, wherein a cell is Identified as an unbalanced cell if its measured first voltage is, by a predetermined amount, greater or less than the measured first voltage of another cell.

4. The method of any one of claims 1 to 3, wherein balancing an unbalanced cell comprises adjusting an amount of charge contained in the unbalanced cell.

5. The method of any one of claims 2 to 4, wherein an estimated balancing time is based on an estimated balancing rate and a difference between the measured first voitage of the unbalanced cell and the measured first voltage of another cell.

6. The method of claim 5, wherein the measured first voltage of the other ceil is the lowest measured first voltage of the cells of the multi-cell battery.

7. The method of any one of claims 2 to 6, wherein the measured first voltages comprise open circuit voltages.

8. The method of any one of claims 1 to 7, wherein the balancing comprises discharging each unbalanced cell for its estimated balancing time.

9. The method of claim 8, wherein the discharging comprises dissipating charge contained in the unbalanced cell using a balancing circuit connected to the unbalanced cell.

10. The method of any one of claims 2 to 9, wherein the balancing comprises transferring charge from a first unbalanced cell to a second unbalanced cell, the first unbalanced cell having a greater measured first voltage than the second unbalanced ceil.

1 1 . The method of any one of claims 1 to 10, further comprising:

updating one or more estimated balancing times; and

balancing one or more unbalanced cells according to the one or more updated estimated balancing times.

12. The method of claim 1 1 , wherein the updating comprises determining whether to carry out the updating based on one or more balancing criteria.

13. The method of claim 12, wherein the one or more balancing criteria comprise one or more of: a period of time elapsed since the balancing of each unbalanced cell according to its estimated balancing time; and a number of charge-discharge cycles undergone by the multi-cell battery since the balancing of each unbalanced cell according to its estimated balancing time.

14. The method of any one of claims 1 1 to 13, wherein an updated estimated balancing time is based on an updated estimated balancing rate and a difference between a second voltage measured across the unbalanced cell and a second voltage measured across another cell of the multi-cell battery.

15. The method of claim 14, wherein the updated estimated balancing rate is based on an estimated balancing rate and a difference between: a voltage measured across the unbalanced cell after the balancing of the cell according to its estimated balancing time; and a voltage measured across another ceil of the multi-cell battery.

16. The method of any one of claims 1 to 15, further comprising, prior to the determining of whether any of the cells of the multi-cell battery meet an unbalanced condition, determining that voltages measured across each cell of the multi-cell battery have reached a steady state.

17. The method of ciaim 16, wherein the measured voltages comprise open circuit voltages of the cells of the multi-cell battery.

18. The method of claim 16 or 17, wherein the determining comprises waiting a relaxation time before measuring the voltages across each cell, the relaxation time being from between 5 minutes and 30 minutes.

19. The method of any one of claims 1 to 18, further comprising repeating at least the determining step of claim 1 when vo!tages measured across each cell of the multi-cell battery have reached a steady state.

20. The method of any one of claims 1 to 19, wherein the multi-cell battery undergoes multiple charge-discharge cycles during the balancing of each unbalanced cell according to its estimated balancing time.

21 . The method of any one of claims 1 to 20, wherein the battery module comprises a capacity of between 45 to 55 Ampere-hours.

22. The method of any one of claims 1 to 21 , wherein the battery module is configured to provide a power output of between 450 to 550 kW.

23. The method of any one of claims 1 to 22, wherein the battery module is configured to provide a discharge current of between 450A to 550A.

24. A system for balancing a multi-cell battery, comprising:

a balancing circuit connected in parallel to each cell of the multi-cell battery; and

one or more processors configured to:

determine whether any of the cells of the multi-cell battery meet an unbalanced condition;

identify one or more of the cells that are determined to meet the unbalanced condition as unbalanced cells;

for each unbalanced cell, estimate a balancing time being an estimated time for balancing the unbalanced cell; and

control the balancing circuit to balance each unbalanced cell according to its estimated balancing time.

25. The system of claim 24, wherein the one or more processors are further configured to:

update one or more estimated balancing times; and

control the balancing circuit to balance one or more unbalanced cells according to the one or more updated estimated balancing times.

26. The method of claim 25, wherein the one or more processors are further configured to determine whether to carry out the updating based on one or more balancing criteria.

27. The method of claim 26, wherein the one or more balancing criteria comprise one or more of: a period of time elapsed since the balancing of each unbalanced cell according to its estimated balancing time; and a number of charge-discharge cycles undergone by the multi-cell battery since the balancing of each unbalanced cell according to its estimated balancing time.

28. The system of any one of claims 24 to 27, wherein the one or more processors are further configured, after controlling the balancing circuit to balance each unbalanced cell, to further control the balancing circuit to balance one or more cells of the multi-cell battery when voltages measured across each eel! of the multi-ceil battery have reached a steady state.

29. A non-transitory computer-readable medium, having machine-readable Instructions stored thereon, the instructions configured when read by a machine to cause the steps of any one of claims 1 to 23 to be carried out.