BMS Cutoff Voltage Calculator | BatteryPackCalc

How to Use the BMS Cutoff Voltage Calculator

Pick your cell chemistry, enter the series cell count (S), and type the continuous design current your pack must carry. Each chemistry preset carries its own per-cell thresholds: NMC protects at 4.25 V over-voltage and 3.00 V under-voltage, LFP (LiFePO4) at 3.70 V and 2.50 V, and LTO at 2.90 V and 1.80 V. Those single-cell limits are what your BMS actually compares each cell against, and choosing the wrong chemistry is one of the most common causes of premature cutoffs or, worse, overcharging.

The pack-level cutoffs scale with the series count. Pack over-voltage protection (OVP) is S × the per-cell OVP and pack under-voltage protection (UVP) is S × the per-cell UVP. A 13S NMC pack therefore trips OVP at 13 × 4.25 = 55.25 V and UVP at 13 × 3.00 = 39.0 V, which is why a '48 V' nominal e-bike pack actually swings between roughly 39 V empty and 54.6 V full. Set your charger's maximum voltage at or just below the pack OVP so the BMS protection stays a true last line of defence rather than a routine trip.

Over-current protection (OCP) is sized with headroom above your steady draw: the calculator sets it to 1.3 × the design current, so a 30 A continuous design yields a 39 A cutoff that tolerates brief hill-climb or acceleration spikes without nuisance tripping. The S count you select must match your physical series string exactly — a 13S BMS on a 14S pack leaves one group unmonitored and unbalanced, a serious fire risk. Choose a BMS whose continuous rating exceeds your design current with margin, because a unit labelled for 30 A may only sustain that in free air and will derate inside a sealed, warm enclosure.