Pre-charging before top balance

[Tulex]

Watched Will's video on top balancing. He mentioned that if the batteries are not fully charged, you can charge in groups of 4 in series with a regular charger first.
My 304ah batteries all came in at 3.291v, so I would need a month.....
What is a safe amp setting for charging with a regular charger, and to what V would I want to charge them?

 

[meetyg]

This should only be done with a BMS in place. Please do not attempt this otherwise.
If you have a 4s BMS with proper overvoltage settings, then you can charge up to 14.4v-14.6v. As for amps, well since these are large cells, you can theoretically charge up to 0.5C safely (150 amps). I doubt you have a charger that large... So go with whatever your charger can pump out. But, please use appropriate wire guage for that amperage. Stay away from those cheap alligator clips. They can't handle alot of amps, and will heat up (not good!).

Also, use a proper constant current / constant voltage charger. The ones for lead acid batteries are not meant for Lithium, as they have a different charging profile.

A desktop power supply should work well, just be sure to be in CC mode.

 

[ericfx1984]

I do it all the time... And I don't get worried about whether I have a BMS for 4S or not

But there are plenty of cheap BMSs you can get on Amazon or eBay for less than $20 a piece

Usually when I do this I will charge it and monitor the individual cell voltage with a voltmeter checking it from time to time

Cuz I get closer and closer to 100% state of charge I will eventually stop usually when I see one of the cells (or more likely all) are Right around 3.4 volts

 

[DIYrich]

I attached the bms; hooked it up to the inverter; let the inverter charge it at 0.1c.