Cell balancing
Passive bleed, active transfer, and why one weak cell caps the pack.
You'll learn
- Explain cell balancing at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real pack
Overview
A series string is only as usable as its weakest cell: charging stops when the first cell hits maximum, discharging stops when the first hits minimum.
In depth
Passive
Active
Passive vs active balancing
- Bleed excess through a resistorMechanismShuttle charge between cells
- LowCostHigh
- Slow (tens of mA)SpeedFast (amps)
- All excess becomes heatWaste heatMostly conserved
- Most passenger carsTypical useHeavy duty, second-life packs
Balancing takes time
Passive balancing may only move a few tens of milliamps. A badly drifted pack can take many long charge sessions — not one — to come back into balance.
Key takeaways
Remember this
- The weakest cell caps both charge and discharge.
- Passive balancing bleeds; active balancing transfers.
- Balancing works slowly and mostly near full charge.
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