Short answer: The process of equalizing voltage across all cells in a battery pack so they charge and discharge evenly for maximum capacity and longevity.
Explanation
An EV battery pack contains thousands of individual cells connected together. Over time and through use, these cells develop slight differences in their voltage levels. Cell balancing is the process where the battery management system (BMS) equalizes these voltages so that every cell contributes equally to the pack's overall capacity.
There are two types of cell balancing. Passive balancing dissipates excess energy from higher-voltage cells as heat, bringing them down to match the lowest cell. Active balancing transfers energy from higher cells to lower ones, which is more efficient but more complex and expensive. Most EVs use passive balancing.
Cell balancing is one reason manufacturers recommend charging LFP batteries to 100% periodically. At a full state of charge, the BMS can most accurately measure individual cell voltages and perform balancing. If cells become significantly unbalanced, the car may show inaccurate SoC readings or have slightly reduced usable capacity until balancing is completed.
Where you'll see this
- In vehicle specifications
Common confusion
Cell balancing is not something you do manually. The BMS handles it automatically. For LFP batteries, simply charging to 100% occasionally gives the BMS the opportunity to balance cells properly.
Example
If your LFP battery car shows erratic SoC readings (jumping from 30% to 25% suddenly), charging to 100% and leaving it plugged in for a few hours allows the BMS to rebalance the cells and restore accurate readings.
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