Using a BMS

A project log for Portable Battery Pack for Light Electric Vehicle

Versatility. Just enough energy for 1 day's commute.

Samir DupontSamir Dupont 06/05/2017 at 05:230 Comments

One thing I assumed and learnt that not so easy - beyond 6S it's very hard to find a fast enough charger that also balances.

Most EV setups will require a BMS and it's a very bad idea to think you don't need one. First months will be fine, but once the first cell fails, you're in for a world of flames. You better have a way to detect that in time to retire the defective cell/pack.

So it might be ok to prototype without a BMS, but put in your plans to get one. It's not that hard to install even if you barely know what you're doing :

If you're still thinking you can get away without it, consider the following scenario :

Most stealth LiPo failures come from cells that due to age have a << capacity than the rest of the pack. You're charging and that one will charge "just fine", until it reaches 100% of its reduced capacity and the charger doesn't see that, keeps charging the pack. Voila - our cell goes over 4.2v and becomes a fire starter.

A similar one but in the discharge scenario - you're riding your bike and the reduced capacity cell gets to 3.7v much faster than its neighbors. Your monitoring (whatever it is, unless it's per cell) won't catch it, you'll keep riding and guess what? Depleting LiPos under 3v also ends up in flames.