BMS needed?

Hi all,

I have been using three 12V lithium ion batteries wired in series for my son's (converted) electric mini bike. They are simply wired in series and I use a standard 36V lithium ion charger to charge the three batteries. It worked for a couple years but now they are unbalanced. I believe that I need to have some sort of BMS to make sure that they are charged evenly. I want to avoid charging each 12V battery individually. Any suggestions? What do I need to buy? Thanks for your help!!

The kit which I have is similar to this one:
https://www.ebay.com/itm/36V-800W-Scooter-Motor-Brushed-Speed-Controller-Throttle-Twist-Grip-Electric-Bik/272780453227?epid=2236584696&hash=item3f82fb656b:g:dFwAAOSwmkpZea5a

marqtte

What about the cells in each individual 12 battery? Do those have a bms built into the battery? If not that is not vary safe... If they do and its just the 3 12v packs as a whole that are unbalanced just charge them up fully independently and then wire them back up in series

The 12V batteries have a bms built into them. They were purchased from clean republic and are very similar to these.
https://www.electric-bike-kit.com/12v-lithium-lifepo4-replacement.aspx

Thanks for your suggestion but I want to avoid charging them independently as that is time consuming. Isn't there a way to evenly charge the three with a 36V charger?

One bms to control 3 bms, it’s becoming a logistical nightmare. You would be best served getting one 36v bms to service the whole pack without the original 3. Also if one of the original 3 bms gets a fault you basically overload the other 2 left, either way it’s not a great scenario. Seeing as it took 2 years to get out of balance most just rebalance the 12v packs once every 8-10 charges, but the fact you already have 3 bms with the above problem you would be best serviced with a 36v bms. I have a load from torn apart hover board packs you are welcome to if that helps but I am unsure of the amperage they allow, there’s lots of people here testing and probing these packs so they can give you a better idea.

If you charge them independently to a high enough voltage and for long enough, you can restore the balance. Once they are balanced, you can go back to the 36v charger. If you have to balance charge them once a year, that's not too bad.

Most BMS circuits have very limited balancing currents and can't balance at all if the charger voltage is too low. So the charge voltage has to be high enough to allow balancing, and it will take a long time if they are significantly out of balance. If there is any documentation for the batteries, it may indicate the charging voltage. For 4s LiFePO4, it should be around 14.6V, but maybe a bit higher.

This information is very helpful. Thanks for the replies!! I will likely rewire so I can more easily charge each 12V battery every so often and continue to do a normal 36V charge. I will also keep a close eye on the batteries when charging. Thanks!!!