BESS cell imbalance
Growing voltage spread between cells in a battery string.
Cell imbalance is a widening voltage spread between individual cells in a battery string. A spread above ~50 mV at 50% SoC signals a weak or failing cell, limits the usable range of the whole string, and is an early warning of thermal-runaway risk if left unbalanced.
Symptoms
- Max-minus-min cell voltage spread climbing over weeks.
- A string hitting its voltage limits before reaching target SoC.
- One or more cells consistently lagging the pack on charge/discharge.
SCADA signatures
- Max cell voltage spread trending toward / past the 50 mV threshold at 50% SoC.
- >2% of cells in a string deviating >50 mV.
- The spread growing monotonically rather than fluctuating with load.
Root cause
Manufacturing variance amplified by ageing, a developing internal short, or uneven thermal exposure across the pack. The BMS balances passively, but when imbalance outruns balancing it indicates a genuinely weak cell.
Financial impact
Imbalance throttles the usable energy of an entire string to its weakest cell and is a leading indicator of cell failure. Catching it early means a balance cycle or single-cell swap instead of a string-level thermal event.
How NuraVolt detects it
NuraVolt trends the max cell-voltage spread and projects days until it crosses the 50 mV threshold, flagging when the BMS can no longer keep up. It correlates the imbalance with temperature to distinguish a thermal cause from an intrinsic cell defect.
Frequently asked questions
See also
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