BMS sizing calculator
Enter your pack and your load. You get the voltage window and the continuous and peak current a BMS has to handle — with the arithmetic shown, so you can check it.
This gives you the requirement, not a model number
The calculator works out what your pack needs. It deliberately does not name a product: our catalogue specifications are still being verified against manufacturer documentation, and a wrong current rating is not a mistake a customer can catch. Matching a requirement to a board is done by a person — send us the numbers and we will answer with the verified figures.
Calculate
Recommended BMS continuous rating
75 A
Voltage window
40.0–58.4 V
Peak rating to look for
150 A / 3000 ms
- 16S LiFePO4 (LFP): 3.2 V/cell × 16 = 51.2 V nominal
- 3000 W ÷ 51.2 V nominal = 58.6 A
- Pack voltage window: 40.0 V (cutoff) to 58.4 V (full charge)
- 58.6 A × 1.25 margin = 73.2 A → round up to 75 A
- Inductive load (motor / inverter / compressor): peak ≈ 2× continuous → 150 A for about 3000 ms
Need the figures checked against a real board? Send these numbers to a technician
How the numbers are derived
Three steps, each of which you can redo by hand. Nothing here depends on a product datasheet — it is cell chemistry and Ohm's law.
- Pack voltage. Nominal voltage is the per-cell nominal voltage times the series count. LiFePO4 is 3.2 V nominal per cell, so 16S is 51.2 V. The usable window runs from the cutoff (2.5 V/cell) to full charge (3.65 V/cell), which for 16S is 40.0 V to 58.4 V.
- Load current. Continuous watts divided by nominal volts. 3000 W ÷ 51.2 V = 58.6 A. Using nominal rather than full-charge voltage is the conservative choice, because current is highest when voltage sags.
- Margin and peak. Multiply by the margin, then round up to the next common rating. Peak is 1.5× continuous for resistive loads and 2× for inductive ones, and it only means something with a duration attached.
Pack voltage reference
The same series count means different voltages depending on chemistry. This is the most common sizing mistake: a "48 V" pack is 16S in LiFePO4 but 13S in NMC.
| Configuration | Nominal | Full charge | Cutoff |
|---|---|---|---|
| 4S 12 V | 12.8 V | 14.6 V | 10.0 V |
| 8S 24 V | 25.6 V | 29.2 V | 20.0 V |
| 16S 48 V | 51.2 V | 58.4 V | 40.0 V |
| 13S 48 V (NMC) | 48.1 V | 54.6 V | 39.0 V |
Continuous current by load
Required BMS continuous rating, already including a 25% margin and rounded up to the next 5 A.
| Load | 12 V | 24 V | 48 V | 48 V (NMC) |
|---|---|---|---|---|
| 500 W | 50 A | 25 A | 15 A | 15 A |
| 1000 W | 100 A | 50 A | 25 A | 30 A |
| 2000 W | 200 A | 100 A | 50 A | 55 A |
| 3000 W | 295 A | 150 A | 75 A | 80 A |
| 5000 W | 490 A | 245 A | 125 A | 130 A |
Common questions
How do I work out what size BMS my battery pack needs?
Start from the load, not the battery. Divide the continuous load in watts by the pack nominal voltage to get the current in amps, then add a margin — 25% is typical for resistive loads, 50–100% for motors and inverters because of inrush current. The BMS continuous rating must be at least that figure. Also check that the BMS series range covers your pack: a 16S LiFePO4 pack is 51.2 V nominal and charges to 58.4 V, and the BMS must be rated for at least that voltage.
What is the difference between continuous current and peak current on a BMS?
Continuous current is what the BMS can carry indefinitely without overheating. Peak current is a short burst — typically a few seconds — for motor startup or inverter surge. Peak current is only meaningful together with its duration: "300 A peak" with no time given cannot be compared between products. When sizing, the continuous rating is the number that matters day to day; the peak rating only needs to cover the startup window.
Does a 48 V pack need a different BMS from a 12 V pack?
Yes. A BMS is built for a series range, and both the voltage withstand and the cell-count sensing differ. A 4S LiFePO4 pack (12.8 V nominal, 14.6 V full charge) and a 16S pack (51.2 V nominal, 58.4 V full charge) need boards rated for those series counts. Connecting a board outside its rated series range is a safety issue, not a configuration detail — the balance leads and the overvoltage thresholds will be wrong.
Why does this calculator not recommend a specific model?
Because the specification data for our catalogue is still being verified. Recommending a model means asserting its series range and current rating, and those figures are not yet confirmed against the manufacturer documentation. The calculator therefore gives you the requirement — voltage window and current — which is physics and can be checked by anyone. Matching that requirement to a specific board is done by a person once the figures are verified.
Next step
The calculator tells you the requirement. To turn it into a board — with verified series range, current rating, protocol support and port type — talk to a person. Include the chemistry, series count and continuous current above and you will get a specific answer rather than a catalogue link.