EV battery cell balancing: passive and active methods

Cell balancing adjusts the charge held by series-connected cell groups so that differences between them do not unnecessarily restrict the battery’s usable energy. Passive systems remove charge from selected groups; active systems transfer energy between parts of the pack.

Why charge differences limit a pack

A cell group is one monitored position in the battery’s series chain: either a single cell or several cells connected in parallel. The battery pack chapter explains how these connections build pack voltage and capacity.

The battery management system (BMS) must keep each group within its operating limits. During charging, one group can reach its upper voltage limit while others still have room. During discharge, one can reach its lower limit while others still contain energy. Either situation restricts what the pack can deliver. Balancing helps recover the usable energy lost to differences in charge level. Analog Devices — Maximize the Run Time in Automotive Battery Stacks Even as Cells Age

Three distinctions explain what balancing can achieve:

• State-of-charge (SOC) mismatch means groups hold different amounts of charge relative to their capacities.
• Capacity mismatch means one group can store less charge than another.
• A measured voltage difference can also reflect resistance, temperature or measurement error.

Unequal self-discharge and small differences in electrical loads on individual groups can gradually create a charge mismatch. Differences in initial capacity and ageing change how quickly each group moves through its SOC range. Analog Devices — Maximize the Run Time in Automotive Battery Stacks Even as Cells Age

Consider a simplified pair of groups with capacities of 90 Ah and 100 Ah. Both can start fully charged, but after each supplies 90 Ah, the smaller group is empty while the larger still holds 10 Ah. Aligning them at the top of the charge window cannot restore the smaller group’s missing capacity. This illustration ignores other limits that could stop discharge earlier. Analog Devices — Maximize the Run Time in Automotive Battery Stacks Even as Cells Age

Passive balancing

A passive balancing circuit connects a resistor across a selected group through an electronic switch. Current through this branch converts electrical energy into heat. The BMS controls which branches operate and for how long. Analog Devices — Passive Battery Cell Balancing

During charging, current entering that series position divides between the cells and the balancing branch:

Net charging current into the cells = pack charging current − bleed current

For example, if the pack receives 1 A and a selected branch draws 0.1 A, that group’s cells receive 0.9 A. An otherwise identical group with its balancing branch switched off receives the full 1 A and gains charge faster. These are illustrative currents, assuming other small electrical loads are negligible.

The same branch can keep removing charge after the charger stops, with the selected group supplying the resistor current itself. The Orion Jr. 2 implementation described below supports this operation while the BMS remains powered in CHARGE mode. Ewert Energy Systems — Orion Jr. 2 BMS operation manual

A small bleed current only slightly reduces the current entering a group during high-current charging. Charging still has to slow or stop when a group reaches its limit. Balancing therefore works alongside charge control rather than replacing it.

The circuit is relatively simple, and it can correct gradual charge drift between groups with closely matched capacities. Analog Devices — Maximize the Run Time in Automotive Battery Stacks Even as Cells Age Its trade-off is that the removed energy becomes heat. Passive balancing also has no transfer path to supply a low group from the remaining groups during discharge. Analog Devices — Passive Battery Cell Balancing

Balancing current and time

The useful starting point is the amount of charge that must be removed, measured in ampere-hours (Ah):

Balancing time (h) = charge difference (Ah) ÷ average balancing current (A)

For two otherwise identical 100 Ah groups, a difference of one percentage point in SOC represents 1 Ah. Removing that difference at a continuous 0.1 A takes 10 hours. If that branch operates for only half the elapsed time at the same current, its average current is 0.05 A and the correction takes 20 hours. Both are simplified calculations assuming no additional drift and an unchanged target group.

NXP’s MC33775A automotive cell controller supports passive balancing up to 300 mA per channel. Its controls include individual timers and voltage thresholds. It automatically pauses balancing during measurements and interrupts it if the balancing resistors overheat. The 300 mA figure is a component limit; the installed circuit and software determine the current and operating time in a vehicle. NXP — MC33775A product short data sheet, revision 2

Actual current also depends on cell voltage and resistance in the balancing path. TI’s BQ7961x-Q1 design guidance explains how to select the resistors and account for temperature constraints; its automatic mode alternates between odd and even channels. Texas Instruments — BQ7961x-Q1 automotive BMS design recommendations, revision B Measurement pauses, thermal interruptions and channel sequencing reduce the time a branch can spend balancing. They explain why average current can be lower than the current while the branch is on; the 50% operating time in the 20-hour example is an illustrative assumption.

Active balancing

Active balancing uses a power-conversion circuit to move energy between groups or between a group and a larger part of the pack. The circuit may temporarily store energy in a capacitor or magnetic component before delivering it elsewhere. Its layout determines which groups can exchange energy. Ashraf et al. — Review of Cell-Balancing Schemes for Electric Vehicle Battery Management Systems

A design that supports transfer during discharge can feed a group approaching its lower limit from groups that still contain energy. This can extend usable pack discharge even when group capacities differ. During charging, a suitable circuit can move energy away from a group approaching its upper limit. Analog Devices — Active Battery Cell Balancing

Transfer current and available balancing time constrain that benefit, as ADI notes. Analog Devices — Active Battery Cell Balancing A practical consequence is that a transfer circuit may supply too little energy to keep a limiting group above its lower limit during a high-power demand. Redistributing energy improves pack utilisation; it leaves each group’s storage capacity unchanged.

Switching and energy conversion incur losses, and additional hardware increases cost, size and control complexity. Ashraf et al. — Review of Cell-Balancing Schemes for Electric Vehicle Battery Management Systems The engineering choice is whether the recovered energy and balancing speed justify those additions for the intended pack.

How voltage readings guide balancing

The difference between the highest and lowest measured group voltage is often called the voltage spread or cell-voltage delta. Its meaning changes with current, temperature and SOC.

A group with higher internal resistance can show a higher voltage during charging and a lower voltage during discharge, even without an SOC mismatch. Removing charge merely because it has the highest voltage under charging load can make the charge mismatch worse. Lower current and settled conditions reduce these effects, making voltage easier to interpret. Texas Instruments — Battery Cell Balancing: What to Balance and How

Temperature differences also affect voltage behaviour. A sensor offset or sensing-wire fault can produce an apparent difference without creating a physical charge imbalance. The battery management system chapter explains how sensing faults are distinguished from cell faults.

Lithium iron phosphate (LFP) adds a particular challenge: its voltage changes little through much of the SOC range. Different charge levels can therefore produce similar voltages. Hysteresis, where voltage depends on the cell’s recent charging or discharging history, further complicates the comparison. The steeper parts of the voltage curve provide more information about charge differences. Analog Devices — Considerations for Fuel Gauging Lithium-Iron-Phosphate Batteries

Voltage, current measurements and battery models contribute to SOC estimation. Balancing physically changes the charge held by selected groups; SOC calibration corrects the BMS estimate. A change in the displayed percentage alone does not demonstrate that charge has been redistributed.

When balancing runs and what drivers can infer

The control strategy determines when balancing is enabled and when it pauses. Conditions can include group voltage, the measured difference between groups, temperature and the vehicle’s operating state.

Top balancing aligns groups near the upper end of the charge window; bottom balancing aligns them near the lower end. These terms describe the alignment point. Passive and active describe the mechanism used to change charge distribution. Groups with different capacities can be aligned at one end and diverge as the pack moves towards the other.

One documented implementation is the Orion Jr. 2, a BMS for packs up to 48 V nominal in light vehicles and stationary applications. Its 2018 manual describes adjustable balancing thresholds and recommends top balancing. Middle or bottom balancing can require an external controller to signal when balancing should run. Ewert Energy Systems — Orion Jr. 2 BMS operation manual

Displayed 100% marks the top of the driver-facing charge window, rather than every cell’s absolute full-charge endpoint, as explained in battery buffers and the operating window. It does not reveal whether the required balancing conditions were met or how long balancing ran. Follow the charging guidance for the exact vehicle instead of adopting a universal full-charge or deep-discharge routine. The battery charging chapter explains the separate factors behind charging speed and taper.

If a group repeatedly moves out of alignment, diagnosis needs to establish whether the cause is charge drift, reduced capacity, resistance, temperature or a sensing problem. A voltage-spread reading is most useful alongside the conditions and history that produced it. Permanent capacity loss is covered in battery degradation.