How to charge a 72v battery?

[CUDAcores89]

Question says it all, how do I charge a 72v battery I am building?

I am planning to build a 72v battery out of some samsung 20Q cells from power tool batteries that I got a really nice deal on locally, Only $1 per cell. Only issue now is how to charge it?

Currently I run a 52v battery with no BMS. The battery pack is actually two 7S packs electrically. I use my icharger 208B for charging up the pack after I have split it into two 7S sub-packs and discharge it by putting the packs together into a 14S battery. I use those 8S battery buzzers on the balance leads during my ride to warn me when the pack get's too low.

With 72 volts, I can't do that since that would be two 10S battery packs. I don't currently own a 10S lipo charger so that won't work.

I could also split the pack into a four 5S battery packs. But that means I will have a ton of balance leads and connectors coming out of my pack that I want to avoid.

I could also just install a single large connector and install an 8S + 6S + 6S balance leads on the pack and charge through the balance leads using my icharger and my two imax b6, but that would be very slow.

I could also just hook up a 20S BMS for charging and run the battery BMSless for discharging.

What has the community done? Charging 72v DIY ebike batteries starts to get tricky, and I would like to see what others have done.

 

[dustNbone]

I normally just bulk charge my 16S pack with a 5A charger, and occasionally balance it with an iMax B6. I've attached a 4S and 2x 6S balance leads for this, but it rarely takes longer than 15 minutes for each set to balance if I bulk charge it first.

As long as the pack stays reasonably balanced, something like this should work fine. Not letting the pack go below 30% or so seems to help.

 

[Voltron]

I lean towards splitting it into 10s packs and bulk charging with occasional manual balancing. 36v chargers are cheap and plentiful these days...