Size a BMS
Computes continuous, surge and charge current from the MINIMUM battery voltage, not the nominal one. Current peaks at low state of charge — which is exactly when the surge load occurs. Calculating from nominal voltage comes out 22 % too low and yields a BMS that trips at the critical moment.
- Continuous current of 133.3 A exceeds the BMS limit of 120 A. The BMS will cut out under load. Choose a larger BMS.
BMS_CONTINUOUS_INSUFFICIENT - The BMS has no low-temperature charge cut-off. With LiFePO4 in winter use, the first sunshine after a frosty night destroys the battery. A BMS with cut-off or a heater is mandatory.
BMS_NO_LOW_TEMP_CUTOFF
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
Safety-relevant calculation. This is an estimate based on the stated assumptions. The final design must be checked by a qualified electrician against the rules that apply where you are.
Data as of: 2026-06-15 · The technical review of this safety-relevant calculator is still outstanding.
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Required continuous current | P_cont / (U_bat,min * eta) | 133.33 A | exact |
| Required surge current | P_peak / (U_bat,min * eta_peak) | 352.94 A | exact |
| Using nominal voltage | P_cont / (U_nom * eta) | 104.17 A | exact |
| Recommended BMS size | next rating >= I_cont,req | 150 A | exact |
- Formula
I_cont = P_cont / (U_bat,min · η) · I_charge = P_charge / U_absorb- Valid for
- Currents from the END-OF-DISCHARGE voltage (not nominal): 1,200 W → 133 A instead of 104 A — the nominal-voltage figure stands beside it as comparison. The minimum temperature is checked against the site TMY minimum (cross-check outside the cold check — it engages precisely on too-mild entries) but not set automatically. Peak-window check only with the BMS’s datasheet values (switch); balancing rule of measure: one per mille of cell capacity.
- Not covered
- Cell-internal protection parameters (over/undervoltage per cell — datasheet duty), BMS MOSFET dissipation and cooling, communication-BMS features, extreme years below the TMY minimum, the actual temperature at the installation spot (annual profile and lock days: calculator 39).
- Data sources
- Manufacturer datasheets (Victron, Fronius, BYD) and IEC 61427-1, aggregated · retrieved 2026-06-15
Frequently asked questions
Which voltage do I use when sizing a BMS for current?
The minimum battery voltage — the discharge cut-off — not the nominal voltage. In the reference case, a 12.8 V system with a 10.0 V cut-off needs about 133 A for a 1,200 W load; sizing from nominal voltage gives only 104 A, 22 percent too low, and yields a BMS that trips exactly when a surge hits a nearly empty battery. The charge current, by contrast, is calculated with the charge-end voltage in the denominator.
Why does a low-temperature charge cut-off matter so much?
Inexpensive BMS units often lack a low-temperature charge lockout, and lithium cells must not be charged below the lockout temperature. Without it, a correctly sized system still destroys its battery in the first winter — at the first sunshine after a frosty night. The calculator checks for this and raises its most important warning.
Is the BMS enough as the only disconnect in my system?
No — without a separate main switch or fuse the BMS becomes a single point of failure, and the calculator warns about exactly that. Voltage sag from internal resistance during surges, cell imbalance, ageing and the thermal behaviour of the BMS itself are not modelled, and the design must be verified by a qualified professional.
How large is the error when calculating with nominal voltage?
Substantial: in the reference case (12.8 V system, 10.0 V cutoff, 1,200 W continuous, η = 0.90) the BMS needs 1,200/(10.0 · 0.90) = 133.3 A — nominal-voltage maths yields only 104.2 A, i.e. 22% too little. The result would be a 120 A BMS that shuts down at the worst possible moment: empty battery under peak load, when the current is highest.
Why is the low-temperature charge cutoff the most important question of this calculator?
Because LiFePO₄ must not be charged below 0 °C — metallic lithium plates out, permanently damaging the cell and potentially causing shorts. Cheap BMS units often lack this cutoff. A perfectly sized system then destroys its battery in the first winter: frosty night, morning sun, the charge controller charges — into the frozen cell. That is why this indicator has its own shape in the chart and the warning is critical (red, not dismissible).
Why do I need a main switch and fuse in addition to the BMS?
The BMS is a single point of failure: its MOSFET switch can weld shut (then nothing disconnects any more), and it does not protect against every fault pattern — a short before the BMS bypasses it entirely. A slow main fuse close to the positive terminal and a load-break-rated main switch are the second, BMS-independent layer. The calculator reports their absence as its own notice.
Does the calculator check my minimum temperature against the site?
Yes — precisely when the entry is too mild. If you enter +5 °C because the battery “won't get that cold anyway”, the cross-check answers: Berlin's typical annual minimum is −7.9 °C, so the charge-lock question still arises there. The cold check itself would stay silent at +5 °C — which is why the cross-check runs outside it. You still set the temperature.
Why does the peak-window check require a switch?
Because an always-sent default would count as a datasheet value to the engine — the same error that cost almost an hour of runtime in the Peukert calculator. Without your BMS peak data the window check is skipped by name (“peak window unknown”) instead of testing against an invented 250 A. With the switch and datasheet values the calculator checks both current AND duration of the inrush against the window.
What does calculating with nominal voltage actually cost?
22 percent too little BMS: 1,200 W over (10.0 V × 0.9) is 133 A — with the nominal 12.8 V only 104 A results, and you buy a 120 A BMS that cuts out under full load on a low battery. Exactly then, at low state of charge, current is highest and peak load most likely. Both values therefore stand side by side on the page.
Why a separate disconnect when the BMS already cuts out?
Because the BMS is a semiconductor device with a single failure path: if the MOSFET welds through, nothing disconnects any more — and the BMS itself can be neither serviced nor replaced under fault while current flows. A mechanical disconnect or fuse beside it is not redundancy for its own sake but the precondition for being able to act at all in a fault.
How fast does balancing need to be?
As a rule of measure: balancing current should be at least one per mille of cell capacity — 0.1 A per 100 Ah. Below that, equalising one percent of capacity difference takes longer than a charge cycle and the cells drift apart faster than the BMS catches them. The calculator computes the hours per percent; cheap BMS units at 30 to 50 mA regularly fail here.