Series batteries and charging

[infrago]

Hello all,

I have quite a specific application, where I need a high voltage supply (85v-120v) that is capable of outputting up to 2A continuous.

I currently own a 13S2P, 21700 pack, with an average cell capacity of 4300mAh.

Is it a bad idea to take this pack and create two 13S1P packs, each with an independent BMS (I would purchase a new one). I would then charge each pack with a 48V mains charger at the same time, while the batteries are connected in series. The reason for this is that 96V packs seem to be very expensive / and also not suitable for 1S applications (very high output current). Also, I would be relying on each BMS to cutoff at the correct voltage - which I'm a little cautious about.

I guess I have three areas of questions:

1. How will the balancing of the packs work out during discharge, with two separate BMSs? Would the slight imbalance cause problems? How big would this problem be?
2. Can I even connect two chargers effectively in series? Would they reference mains ground somehow? (bad news!)
3. Is there a better way of doing this?

BMS: https://www.aliexpress.com/item/1005003162150119.html?spm=a2g0o.store_pc_groupList.8148356.7.42ae20d9drGIZP
Charger: https://www.aliexpress.com/item/1005003136109249.html?spm=a2g0o.store_pc_groupList.8148356.31.130a4cfbwJ3uRY

Many thanks in advance

 

[JackFlorey]

I would then charge each pack with a 48V mains charger at the same time, while the batteries are connected in series. . . .

IF you connect each (isolated) charger to each battery at the same time, charge both at the same time, and let both terminate via the charger's terminate feature - then you can do this. Be sure to not ground the return of either charger to anything, because the grounds will be "floating" for this application. Also ensure that the ground pin on the AC plug is not connected to the - output of the charger (usually it isn't.)

Also, I would be relying on each BMS to cutoff at the correct voltage

Never do that. BMSes are not designed to terminate charge at the right voltage - they are designed to prevent battery fires. You will quickly damage your battery if you just keep charging to (say) 60 volts, hoping that the BMS will cut out before anything bad happens. You are also now in a position where a single failure will burn your house down. (Another ES member lost his garage to such a fire, even though he was charging more safely.)

If you want to use a power supply instead of a dedicated charger, then get one that can be trimmed to the correct voltage. With series batteries you have to ensure (manually) that both are at the same state of charge, so either get a monitor to watch currents (and terminate them both at the same current) or charge them long enough that you are ensured that both are at 100% state of charge. Again ensure that both have independent returns.

Charger: https://www.aliexpress.com/item/1005003 ... 4cfbwJ3uRY

This is a dedicated charger, set to the correct voltage, so you should have no problems with it.