How do bms's work?

I think this is a good topic for the community to understand better. Having just built my first pack with a bms, there’s somethings which are unclear to me, and I’m sure there’s other people who feel the same way.

What I do know:

  • The Battery Management System’s job is to balance cells while charging and prevent any parallel groups of cells from being over discharged.
  • This balancing is done through a balancing cable, which is connected to the negative end terminal of the battery, then every positive in the battery, including the positive end terminal.
  • You can discharge around the bms while still charging through the bms by splitting the battery negative input (usually marked B- on the bms) before the bms, essentially putting the bms and esc’s in parallel. This allows you to draw more amps while discharging than the bms supports. Since lower amp bms’s are smaller and cheaper, you can save money and space doing this, but risk over discharging a group of cells in parallel.

How does the bms balance? My understanding is that as the cells are charged, when a cell is fully charged, through the balance cable, the bms discharges the cell. The cell is essentially being charged and discharged at the same time, in equal proportion to keep it at 4.2v, while the rest of the cells not at 4.2 are charged. Is this view correct?

The other part I’m confused about is there’s only negative terminals on the bms (besides the balance port). The negative of the charging port (input), the negative of the battery (input) and the negative discharging (output). It’s not like a power switch where it runs both negatives and positives through the pcb before it is outputed. Why is this so? How does this work?

Anybody with knowledge wish to shed some light on bms’s.

Any other questions? Let put together all the info we have on bms’s so that we can turn this topic into a topic full of all the info anyone would need to know about bms’s.


AFAIK BMS charges cells to a set voltage (about 4.15v depending on bms) then continues charging with a smaller balance current until it reaches 4.2v. it doesn’t actually start balancing until it hits that ceiling.

So if a cell becomes largely unbalanced, it won’t be able to fix it? I just tried to balance charge what used to be a space cell before I tore it apart and soldered it up in the shape I needed and added another p. The problem is, they didn’t balance. My problem could be that some of the cells I used were at 3.6, others at 4.1. Since I didn’t have a way of charging the new cells, I figured it would balance out when I charged them, but they didn’t.

Are the series cells not balancing or the parallel? Once you hook up batteries in parallel they’ll passively balance out and conform to a voltage.

The series cells are not balancing. I checked my balance plug that goes into the bms, all the voltages are in the correct order. If I go cell by cell, I get 4.1, 4.1, 4.1, 4.2, 4.2, 4.2, 4.2, 3.6, 3.6, 3.6. Not even the 4.1’s balanced. I looked up dozens of diagrams, and confirmed I hooked them up correctly. Does it matter if they are hooked up to vescs or not? Cause right now, they aren’t hooked to anything other than the bms and charging port.

So they’re all hooked up to to a 10s BMS right? If so, plug in the charger and leave it charging there. Don’t unplug to check until the charging light goes green which indicates a full balanced charge. Right now your pack is at 39.9v. It’ll continue to balance until your pack is at 42v, it just might take a bit of time since the balance current is lower than the normal charge current.

The thing is, it went green… Does it still balance once it goes green?

hm are you using a 42v charger? Chargers are pretty simple and if it’s a 42v charger it should only go green when it hits 42v. I’d get a multimeter to read the voltage of the charger when it’s plugged in.

Let me read it, but yes, its 42 volt charger. Came with my space cell.

42.10 volts coming from the charger. Should I just leave it plugged in? I’m afraid on over charging some cells if it’s not working right for some reason.

It should only green once your pack has balanced out to 42v as well, which means something is broken or not reading correctly. Since it only turns green at 42v this means green = balanced. Your current series voltage is 39.9.

You won’t overcharge the cells unless you weren’t on a BMS. each balance lead is set to 4.2v so no current will flow to cells that have balanced out to 4.2v

my multi meter says 40.1 volts (I left off some decimals)

Just leave it charging. The whole point of the BMS is so you can do that.

Come back after around 10 minutes and test the cells to see if they balanced way up to 4.2v or not. Balance charging takes awhile.

Right, try leaving it plugged in for an hour or so and check voltages again.

So i just checked again. all cells increased. 4.216 now is my max cell, up from 4.205

Cool, low cells too? If everything’s hooked up right, just keep it on the charger until all cells balance

yes, low cells also went up. I want those 4.2 guys to stop charging though…

they will, the BMS protects them from overcharging.

I know it should, but I’m worried there might be something wrong with the bms. It was a working bms and pack when I ripped it apart.