Cell Balancing

Cell balancing corrects differences in charge level between the series-connected cell groups in an EV battery. It helps prevent one group from ending charging or discharge too early, but it cannot restore lost capacity or repair a degraded cell.

What cell balancing actually balances

The name cell balancing can suggest that the Battery Management System (BMS) measures and adjusts every physical cell separately. That is not always how an EV battery is constructed.

Cells connected in parallel share the same terminal voltage and are normally monitored as one group. In a 108s4p battery, for example, 108 groups are connected in series and each group contains four parallel cells. The pack has 432 physical cells, but it will normally have 108 voltage-monitoring and balancing channels.

Throughout this article, cell group means one monitored position in the series chain. It may be one large cell or several parallel cells.

Every group connected in series carries the same pack current. If 50 A flows through the battery, the same 50 A passes through every series position. The groups can nevertheless reach different states of charge because their capacities, resistance, temperature, self-discharge, and ageing are not perfectly identical.

Balancing is the BMS function that reduces these differences in charge level. The wider monitoring, estimation, and protection system is explained in Battery Management System.

Why cell groups drift apart

Cells are selected and matched during battery production, but no two remain exactly alike throughout their service life. Small differences accumulate over repeated charging, driving, and parking periods.

The main causes include:

Manufacturing and initial-capacity variation
Different rates of ageing and capacity loss
Differences in internal resistance
Uneven temperature and cooling
Small variations in self-discharge
Leakage through sensing or balancing electronics
A weak cell within a parallel group
Differences in busbars, terminals, or other electrical connections
Voltage-sensor offset or a sensing-wire fault

These causes do not all represent the same problem.

State-of-charge imbalance

A state-of-charge imbalance means that two groups contain different amounts of charge relative to their own usable capacities. If two otherwise identical 100 Ah groups contain 80 Ah and 78 Ah, their states of charge are 80% and 78%.

This is the mismatch balancing is intended to correct.

Capacity imbalance

A capacity imbalance exists when one group can store less charge than the others. A degraded 90 Ah group and a healthy 100 Ah group can both be at 100% SOC, but the degraded group contains less charge and will normally reach empty first.

Balancing can align the two groups at a chosen point in the operating window. It cannot restore the missing 10 Ah. The lowest-capacity group can therefore continue to limit usable pack energy after balancing.

Resistance and temperature differences

A high-resistance group shows a larger voltage rise during charging and a larger voltage drop during acceleration. A cold group can also sag more under load than a warm group at the same SOC.

These temporary voltage differences are not necessarily charge differences. Balancing purely from instantaneous voltage while current is high can remove charge from the wrong group.

Self-discharge

A group with higher self-discharge gradually falls behind while the vehicle is parked. Balancing may repeatedly bring it back into alignment, but persistent drift can indicate a cell defect, contamination, leakage through monitoring hardware, or another fault that needs diagnosis.

Series-connected cells naturally diverge because of differences in temperature, impedance, loading, and self-discharge, while passive balancing is generally sufficient for small SOC differences between closely matched cells. (Analog Devices)

How one group limits the battery

Every monitored group must remain inside its permitted voltage and temperature range. The pack average cannot override the limit of an individual group.

During charging

Charging is restricted by the group that reaches its upper voltage limit first. Imagine four groups near the end of a charge:

Group A reaches its upper limit.
Group B is at 98% SOC.
Group C is at 97% SOC.
Group D is at 99% SOC.

Continuing at the same current would overcharge Group A, even though the other groups still have room. The BMS must reduce charging current, balance selected groups, stop charging, or combine these actions. Some capacity in Groups B and C remains unused.

During discharge

Discharge is restricted by the group that reaches its lower limit first. The stronger groups may still contain energy, but the vehicle cannot keep drawing unrestricted current without over-discharging the limiting group.

The BMS normally reduces available drive power before the group reaches its absolute lower limit. If necessary, it eventually prevents further traction discharge.

The group that limits charging is not always the group that limits discharge. A higher-resistance group can rise furthest during charging and sag furthest under acceleration, while a lower-capacity group can reach the end of its usable charge first. This is why a voltage spread needs context rather than a single threshold.

Passive and active balancing both aim to recover usable energy that would otherwise be stranded by SOC mismatch. Neither method changes the physical capacity of the cells. (Analog Devices)

How the BMS identifies an imbalance

Cell-monitoring electronics measure every series-group voltage. The BMS then evaluates those readings together with:

Pack current
Cell and module temperatures
Voltage response under charge and discharge
Voltage recovery after current stops
Estimated group SOC
Learned capacity and resistance
Previous balancing activity
Self-discharge while parked
Sensor and communication diagnostics

The difference between the highest and lowest measured group voltage is often called the cell-voltage delta or voltage spread. It is useful, but it is not a complete measure of balance.

Voltage is easiest to interpret when current is low, temperatures are similar, and temporary polarisation and resistance effects have settled. During fast charging or hard acceleration, the measured spread can be dominated by resistance rather than SOC.

A group that appears highest while charging and lowest while discharging may have higher resistance, not excess stored charge. The BMS must distinguish among:

A real SOC difference
A capacity difference
A resistance-related voltage difference
A temperature effect
Abnormal self-discharge
A sensor, sensing-wire, or connection fault

Balancing decisions made from voltage alone at high current can worsen the mismatch. Technical balancing guidance therefore places emphasis on current, resistance effects, and conditions near the end of charging where current has fallen. (Texas Instruments)

Why LFP needs special care

Lithium iron phosphate (LFP) cells have a comparatively flat open-circuit-voltage curve through much of their SOC range. A meaningful change in stored charge can produce only a small voltage difference, while temperature, hysteresis, sensor error, and recent current flow can produce differences of similar size.

Voltage becomes more informative in the steeper parts of the LFP curve, but an accurate BMS still needs precise current integration, temperature compensation, a chemistry-specific model, and suitable reference conditions. Analog Devices’ LFP fuel-gauging guidance describes the flat voltage curve and hysteresis as central estimation challenges. (Analog Devices)

This does not make LFP impossible to balance. It means that a simple rule such as “bleed the highest voltage” is less reliable through the flat middle of the operating range.

Passive balancing

Passive balancing removes a small amount of charge from selected higher-SOC groups. A transistor connects a resistor across the group, and the diverted electrical energy becomes heat.

During charging, bleeding a higher group lets the lower groups catch up while the pack continues to receive energy. Some systems can also discharge selected groups while the vehicle is parked or after charging has stopped.

Passive balancing is common because the circuit is compact, relatively inexpensive, and easy to integrate into cell-monitoring hardware. Its main limitations are:

The removed energy is discarded as heat.
Balancing current is small compared with charging or traction current.
Large differences can require many hours.
Resistor temperature can limit how many channels operate at once.
It cannot move energy into a lower group during discharge.

Balancing current and time

Automotive balancing hardware commonly operates in the milliampere range. NXP’s MC33775A automotive cell controller, for example, supports passive balancing up to 300 mA per channel and includes time, voltage, and temperature controls. That is a component capability, not a value that applies to every EV. (NXP)

A simple time estimate is:

Balancing time = charge difference ÷ balancing current

A 1% SOC difference in a 100 Ah group represents 1 Ah. At a continuous balancing current of 0.1 A, removing that difference would take about 10 hours.

Real balancing can take longer because the BMS may pause it for voltage measurements, restrict adjacent channels, reduce duty cycle as components warm, stop outside a permitted temperature range, or let the vehicle enter a lower-power state. Current automotive monitor designs include these timer and thermal controls because the bleed resistors and switching devices need protection. (Texas Instruments)

Passive balancing is therefore well suited to correcting gradual drift between otherwise similar groups. It is not a rapid repair process for a severely mismatched battery.

Active balancing

Active balancing transfers charge through power-electronic circuits instead of deliberately dissipating all of it in bleed resistors. Depending on the topology, energy can move:

From a higher group to a lower group
Between a group and a larger section of the pack
Between modules or pack sections
From the pack into a low group

The transfer circuit may use capacitors, inductors, transformers, switching matrices, or bidirectional DC-DC converters.

Active balancing can operate at higher current and can redistribute energy during charge, discharge, or rest. During discharge, for example, it can feed energy from stronger groups into a group approaching its lower limit, delaying the point at which that group ends the pack’s usable discharge.

The method is not lossless. Conductors, switches, magnetic components, and control electronics consume energy and generate heat. It also adds:

Cost and component count
Space and weight
Control complexity
Electromagnetic-interference considerations
More diagnostic and functional-safety requirements
Additional possible failure modes

Active balancing can make more of the existing pack energy usable when group capacities have diverged, but it does not restore a weak group’s original capacity. The same group may again become the limiting one during the next cycle. Analog Devices describes the central distinction clearly: passive balancing dissipates excess charge, while active balancing redistributes it. (Analog Devices)

Active balancing is not automatically the better design for every EV. Passive balancing can be sufficient when cells are closely matched and the BMS corrects small drift regularly. Active balancing becomes more attractive when the usable-energy benefit justifies its added hardware and validation burden. Reviews of EV balancing architectures reach the same broad trade-off: active systems offer more flexible energy transfer, while passive systems retain advantages in cost and simplicity. (Energies)

When balancing happens

There is no universal balancing schedule. Depending on the battery and BMS strategy, balancing can occur:

During AC charging
Near the end of charging
After charging has stopped
While the vehicle remains connected
During selected parked or driving conditions
Whenever voltage, current, and temperature meet the programmed criteria

Many systems perform much of their balancing near a high SOC. Charging current is lower, the BMS can identify which group reaches the upper limit first, and the connected vehicle may have enough time to make a slow correction.

This is often called top balancing, but other strategies can align groups near the lower end or use model-based balancing across a wider operating range. The chosen method depends on cell chemistry, pack design, operating window, and the manufacturer’s priorities.

Charging to displayed 100% does not prove that balancing occurred. The BMS may also require suitable temperature, a minimum voltage or estimated-SOC difference, low current, additional connected time, or another manufacturer-specific condition.

Balancing and SOC calibration are different

Balancing changes the distribution of charge between series-connected groups. SOC calibration changes the BMS estimate of how full the battery is.

A high or low reference point can help the BMS recalibrate SOC, and a long charge may also provide time for balancing, but the two processes remain separate. A vehicle can update its displayed percentage without moving meaningful charge between groups. It can also balance groups without producing a visible change on the dashboard.

Drivers should therefore follow the charging guidance for their exact vehicle rather than using a generic “charge to 100% for balancing” rule. Chemistry, BMS strategy, software, storage time, and daily range needs all affect the appropriate routine. Charging performance and high-SOC taper are covered separately in Battery Charging and Charging Performance.

What balancing cannot fix

Cell balancing cannot:

Restore capacity lost through degradation
Reverse permanently increased resistance
Repair an internal short circuit
Stop abnormal self-discharge
Repair a damaged busbar, terminal, or sensing wire
Correct inadequate cooling
Fix an inaccurate voltage or temperature sensor
Make every group age at the same rate
Recover lithium or active material already lost inside a cell

If one group repeatedly moves out of alignment soon after balancing, the battery may have an underlying capacity, resistance, self-discharge, temperature, connection, or sensing problem. Balancing can temporarily align the group, but persistent imbalance requires diagnosis.

Most EVs do not show balancing activity directly. Possible indirect symptoms include:

Charging tapering or stopping earlier than expected
Reduced usable energy
A sudden correction in displayed SOC
A faster SOC drop near empty
Reduced drive power at low SOC
A large reported voltage spread
The vehicle remaining awake after charging
A battery or drivetrain warning

None of these symptoms proves imbalance. Temperature, normal charging taper, SOC-estimation error, charger limits, general degradation, cooling faults, and software strategy can produce similar behaviour.

A reliable diagnosis compares group voltage, temperature, resistance, self-discharge, and history under controlled conditions. One voltage-delta screenshot or one charging session is not enough. Permanent capacity loss and battery-health testing are explained in Battery Degradation.

Cell balancing is most effective when it quietly corrects small differences before the driver notices them. Once a group has become weak enough to limit the pack repeatedly, balancing can manage the consequence but cannot remove the cause.