Cell Balancing

Cell balancing is the process of reducing differences in charge level between cells or cell groups in a battery pack.

In an EV battery, all cells do not age or behave in exactly the same way. Small differences in capacity, internal resistance, temperature, and self-discharge can cause some cells to reach a higher or lower State of Charge (SOC) than others. Over time, these differences can reduce usable battery capacity and limit charging or discharge performance.

In the example above, the top group of cells reaches full charge before the others. That means the battery cannot continue charging at full pack level without overcharging the most full cells. Balancing helps correct this by reducing the charge of the highest cells or transferring energy so the lower cells can catch up.

This allows the battery pack to use more of its total capacity and helps prevent one weak or overfull cell group from limiting the entire battery.

Why cell balancing is needed

An EV battery pack contains many cells connected in series and parallel. Even when the cells are produced to tight tolerances, they are never perfectly identical.

Over time, differences can appear because of:

• Small manufacturing variations
• Temperature differences across the pack
• Uneven ageing
• Different internal resistance
• Slight differences in self-discharge

Because the cells are connected together, the whole battery pack is limited by the cells that reach their voltage limits first. If one cell group fills before the others during charging, charging must slow down or stop. If one cell group empties first during driving, discharge power may need to be reduced even if the rest of the battery still has energy available.

This is why balancing matters: it helps keep the pack aligned and prevents a small mismatch from reducing usable energy and performance.

What cell balancing does

The Battery Management System (BMS) monitors cell or module voltages across the battery pack.

If some cells or cell groups are at a higher charge level than others, the BMS can initiate balancing to reduce the difference. The goal is not to make every cell mathematically identical at every moment, but to keep them close enough that the battery can operate safely and efficiently.

Balancing helps with:

• Maximizing usable battery capacity
• Supporting full charging performance
• Preventing overvoltage in individual cells
• Reducing the risk that one weak cell limits the whole pack
• Maintaining consistent long-term battery behavior

Passive balancing

The most common balancing method in EVs is passive balancing.

In passive balancing, the BMS discharges the highest-voltage cells or cell groups through resistors. This removes a small amount of energy as heat so the lower-voltage cells can catch up.

Passive balancing is relatively simple, low-cost, and widely used. Its main disadvantage is that the extra energy is not reused; it is lost as heat.

In practice, passive balancing is often most noticeable near the top of the charge window, where voltage differences between cells are easier to detect and correct.

Active balancing

Some battery systems use active balancing.

Instead of burning excess energy as heat, active balancing transfers energy from higher-charge cells to lower-charge cells or back into the pack. This can improve efficiency and may help in very large or high-performance battery systems.

However, active balancing is more complex and more expensive than passive balancing, which is why passive balancing remains more common in production EVs.

When balancing happens

Balancing does not necessarily happen all the time.

In many EVs, balancing is most active:

• Near high states of charge
• During or after charging
• When the vehicle is parked
• When the BMS has enough time to measure and correct cell differences

This is one reason some EVs benefit from occasionally being charged to a high SOC, especially if the manufacturer recommends it. Reaching the upper part of the charge window can make it easier for the BMS to evaluate and balance the cells properly.

That does not mean drivers should keep the battery at 100% unnecessarily. It simply means that occasional full or near-full charging may help the BMS maintain a more accurate view of the pack and improve balance in some systems.

Cell balancing and battery ageing

Cell balancing does not stop battery degradation, but it helps the battery age more gracefully.

Without balancing, differences between cells can grow over time. That can reduce usable energy, make charging behavior less consistent, and increase the chance that one cell group reaches its limits earlier than the rest of the pack.

A well-managed battery pack uses balancing together with thermal management, buffers, and conservative voltage limits to maintain performance and durability over many years.

Summary

Cell balancing is an important part of EV battery management.

Because battery cells never remain perfectly identical over time, the BMS must monitor differences between cells and correct them when needed. This helps the battery pack use more of its available energy, charge more consistently, and reduce the risk that one mismatched cell group limits the whole battery.

Most EVs use passive balancing, while more advanced systems may use active balancing. In both cases, the goal is the same: to keep the battery pack operating as evenly, safely, and efficiently as possible.