What Size BMS Do You Need? A Battery Builder's Sizing Guide
What Size BMS Do You Need? A Battery Builder's Sizing Guide
Two numbers decide your BMS, and only two: how many cells in series, and how many amps the pack has to deliver. Get the first one wrong and nothing works. Get the second one wrong and it works fine right up until it doesn't.
This guide covers what the ratings mean, how to size from your actual load, and the specification most builders never check until it bites them.
What a BMS actually does
Despite the name, most boards sold as a BMS are protection devices rather than management systems. A typical board watches for four conditions and disconnects the pack when it sees any of them:
- Over-voltage — stops charging above roughly 4.2V per cell
- Under-voltage — cuts the load around 2.5–3.0V per cell
- Over-current — trips above its rated discharge
- Short circuit — near-instant cutoff on a dead short
Better boards add balancing: during charge they bleed off the cells that reach full first, letting the laggards catch up. It works slowly — typically 30 to 80 milliamps — so it corrects gradual drift over many cycles rather than fixing an already-mismatched pack. Balancing is not a substitute for starting with matched cells.
You will also see boards labelled PCM or PCB. For a single cell that is the correct term, because with one cell there is nothing to balance against.
Step 1 — Match the series count exactly
The S number is how many cells sit in series. It sets the pack voltage, and it is not negotiable. A 3S board on a 4S pack does not run at reduced performance — the balance taps have nowhere to land and every voltage threshold is wrong.
Parallel count doesn't enter into it. A 5S2P pack and a 5S4P pack both take a 5S board. Parallel cells raise capacity and current capability, not voltage.
| Series | Nominal | Full charge | Typical use |
|---|---|---|---|
| 1S | 3.6 V | 4.2 V | Power banks, torches, single-cell projects |
| 2S | 7.2 V | 8.4 V | RC, small tools, LED and hobby builds |
| 3S | 10.8 V | 12.6 V | Drones, 12V replacements |
| 4S | 14.4 V | 16.8 V | Larger RC, portable power |
| 5S | 18 V | 21.0 V | Almost every 18V / 20V MAX tool pack |
| 6S | 21.6 V | 25.2 V | Hilti-style 22V tool packs |
| 8S+ | 28.8 V | 33.6 V | E-bikes, scooters, solar storage |
Some boards cover a range and are configured on install — a 3S/4S or 3S–8S board is set for your pack rather than fixed at the factory. Useful if you build a variety of packs; unnecessary if you build the same thing repeatedly.
Step 2 — Size the current from the load
Work from what the pack has to deliver, not from its capacity. A 20Ah pack tells you nothing about current draw.
- Find continuous draw: watts ÷ pack nominal volts. A 300W motor on a 7.2V pack pulls about 42A.
- Add 25–30% margin.
- Choose a board whose continuous discharge rating exceeds that figure.
Peak matters less than builders assume. Most boards tolerate brief surges well above their continuous rating, and a two-second stall current is not what heats the MOSFETs. But if your system sits near peak for minutes at a time, that is your continuous figure.
The specification nobody checks: charge current
Almost every board carries two current ratings, and the charge rating is usually far lower than the discharge rating — a board rated 30A discharge may only accept 10A charge.
This rarely matters for hobby charging at 1–2A. It matters a great deal if you are using a fast charger. Exceeding the charge rating is a common way to kill a board that was correctly sized for discharge.
Balance leads and the NTC
A multi-cell board connects to every junction in the series chain, starting at pack negative and stepping up one cell at a time. Connect them in order, lowest first, and verify each tap with a multimeter before seating the connector. Reversed or out-of-order balance leads destroy the board instantly, and it is the single most common way a first build ends early.
Many boards also carry an NTC — a temperature sensor that sits against the cells. If you are rebuilding a power tool pack, this is not optional: the charger reads pack temperature and will refuse to charge if it sees nothing there.
Single-cell protection (1S)
For power banks, torches and single-cell projects, a small PCM soldered to the cell handles over-charge, over-discharge and short protection. Buying in a multipack makes sense because these are consumed one per project.
Two-cell packs (2S)
2S is where most hobby builds land, and where sizing actually requires a decision. Three current levels cover nearly everything:
Up to about 10A — LED projects, small electronics, light-duty tools.
Up to about 20A, with balancing — the sensible default for a 2S pack you intend to keep for years. The balance function is what keeps the two cells tracking together over hundreds of cycles.
Up to about 30A — high-drain 2S builds where the load approaches what the cells themselves can deliver.
Building a 2S pack from scratch? Both of our step-by-step guides use these boards and walk the wiring: 2S1P 7.4V 21700 and 2S2P 7.4V 21700.
Power tool packs (5S and 6S)
Here the advice changes. Almost every 18V and 20V MAX tool pack is 5S, and 22V packs are 6S — but sourcing a loose board is the harder path, because a tool pack also needs the right housing, terminal block, NTC placement and nickel layout to work with its charger.
A matched rebuild kit solves all of that at once, and the BMS inside it is already the correct series count and current rating for the tool.
Kits are available for Milwaukee M18, Makita LXT, Ryobi, RIDGID, Bosch-style and Hilti-style B22 platforms. If you want the reasoning behind why the industry standardised on this layout, see why every 18V brand uses 5S2P.
Large packs (8S and above)
E-bike, scooter and storage packs move into a different class of board — higher current, heavier MOSFETs, often a metal heat spreader, and sometimes Bluetooth monitoring.
At this current level the wiring matters as much as the board. Main leads and connectors have to carry the same current the BMS is rated for — see our XT connector sizing guide for matching those to your load.
Common mistakes
| Mistake | What happens |
|---|---|
| Wrong series count | Pack won't charge or won't run — thresholds don't match the chemistry |
| Sizing from capacity instead of current | Board trips under normal load, or overheats |
| Ignoring the charge rating | Board dies on a fast charger despite correct discharge sizing |
| Balance leads out of order | Board destroyed instantly on connection |
| No NTC on a tool pack | Charger refuses the pack entirely |
| Expecting balancing to fix mismatched cells | Drift returns within weeks — balancing corrects milliamps, not a bad cell |
If your BMS isn't behaving
A pack that reads full but won't run, or a charger that refuses to start, is usually a protection latch rather than a dead board. Our 2S BMS troubleshooting guide works through the diagnostic sequence step by step.
Safety
- Verify every balance tap with a multimeter before seating the connector
- Never bypass a BMS to test a pack, even briefly
- Work on a non-conductive surface with no loose metal within reach
- A board that runs hot in normal use is undersized — replace it rather than tolerating it
- Do not fit a protection board to cells that are dented, swollen or leaking
Related reading
- How to choose the right lithium-ion battery pack
- What nickel strip size do you need?
- How to tell whether a used cell is still good
- Why rebuilding tool batteries beats buying new
Browse the full range in our BMS and protection boards collection.
Current ratings vary by manufacturer and operating conditions. Figures above are general guidance for selection, not a substitute for the datasheet of the specific board you are using. Lithium battery packs can short, vent, or catch fire if wired incorrectly — verify polarity and balance-lead order before connecting anything.
