4S 40 A BMS board
Balances cells and cuts off on over-current, over-voltage and under-voltage. Non-negotiable on a hand-built pack — the failure mode without one is thermal runaway.
The role it plays
Power sets the ceiling on everything else: peak torque (voltage × current), runtime (watt-hours ÷ average draw) and safety. Size the bus voltage to your actuators, size the pack to your worst-case current, and put protection upstream of the pack — before, not after, the first puff of smoke.
Typical use: Custom Li-ion packs for robots.
What the numbers actually mean
The headline figure is 4S (variants for 3S–7S) (configurations), with 40 A as the next most load-bearing number (continuous current). At $10 in the power tier it sits cheap enough to prototype with and replace without regret, which is the correct way to learn a new subsystem.
How it compares
Same subsystem, closest alternatives. The primary spec column is usually the one that decides it.
| Part | Price | Primary spec | Second spec |
|---|---|---|---|
| 4S 40 A BMS boardBattery management | $10 | 4S (variants for 3S–7S) | 40 A |
| Samsung INR18650-30Q18650 Li-ion cell | $5 | 3000 mAh | 3.6 V |
| Molicel P42A (21700)21700 Li-ion cell | $8 | 4200 mAh | 3.6 V |
| LiPo 4S 5000 mAh 50CLiPo pack | $55 | 4S (14.8 V nominal, 16.8 V full) | 5000 mAh |
| Pololu D24V50F5 buck regulatorDC-DC step-down | $20 | 6 – 38 V | 5 V fixed |
Where it shows up
It is a component-level choice rather than a defining part of a named platform — pick it for the subsystem it serves, not to match a reference build.