Battery Management System (BMS)
Power
The guardian circuit for lithium packs: stops over-charge, over-discharge, over-current and shorts, and balances cells so one weak cell doesn't wreck the pack. Never build a lithium pack without one.
What it looks like
Battery Management System (BMS)
A typical power — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Cell count (S rating)
SMust exactly match the series cell count — 3S board for 3 cells.
Continuous current
AThe load current it can pass; peaks handled briefly.
Overcharge / overdischarge thresholds
V~4.25 V and ~2.5–3.0 V per cell for Li-ion.
Balance current
mAHow fast it can equalise cells — passive boards manage tens of mA.
Markings
S count and current rating printed on the board; pads labelled B−, B1, B2…, P−/P+ (pack vs protected output).
Standard values
1S 18650 protection strips, 3S 20 A and 4S 30 A boards for power tools/e-bikes-lite.
How to choose
Match S count and chemistry (Li-ion vs LiFePO₄ have different thresholds!), current ≥ real continuous load, and prefer boards with balancing and temperature sensing for anything beyond 1S.
Pinout & package
B− to pack negative, sense wires to each cell junction, load/charger on P−/P+. Wire order matters — connect sense leads lowest first.
Example circuits
- 3S 18650 pack for a cordless drill conversion
- TP4056+protection charging a single-cell project
- Smart BMS on a DIY powerwall
Common failures
Blown FETs from over-current (pack reads 0 V at output but cells are fine), unbalanced packs when balance leads were miswired, and boards with wrong chemistry thresholds quietly murdering LiFePO₄ cells.
How to test
Cell voltages at the sense connector should match direct cell measurements; P− to B− should show continuity when healthy. Output 0 V with good cells = tripped or dead BMS — check for a charger-attach reset.
Substitutes
A same-S, same-chemistry board of equal or higher current. For experiments, a bench supply with current limit — but never leave lithium unattended without protection.