I have a project that calls for 3x18650 cells. Should I use 3 separate BMS circuits, or one? Why?

Thanks so much

Joe

 

Unless all three cells are "perfectly identical", or all in parallel, they should be managed separately for the best reliability.

 

There are ICs with multiple input channels to perform multiple battery management with a single IC. These would be your optimal solution, as the internal reference voltage will be identical for all batteries.

 

If they're in series, you should use a 3S protection board that keeps them balanced as well as providing undervoltage (over-discharge), overcurrent (charge and discharge), and overvoltage (charging) protection. Those are commonly advertised as BMS, but a real BMS has more smarts and can report voltages and stuff back to a microcontroller. You still need a separate lithium-ion charger (like a module that accepts DC input, or a wall-wart/brick style charger). The "BMS" protects against catastrophic overcharging, but doesn't manage normal charging.