I’m working on a 6kw electric scooter, building my own 12s lipo pack for it running a pair of 6s ones in series through a fatboy slim sparky, but 12s bms’ are rare as hens teeth for some reason, i’ll only be using it for charging, so i had the idea to run a 6s bms to each packs balance port and wire them in series, but i dont know if the bms’ will have a heart attack in parallel, should i just charge the batteries separately?
what do you mean 12s bms are rare? literally 95%+ builders with 12s batteries use 12s bms. get yourself red daly bms from aliexpress (16-17$) and use it for charge only. yeah and 12s (50.4v) charger
i was looking at an aliexpress one but i cbf to deal with a 30 day wait, i couldn’t find any cheap charge only ones local to me, and i already have a 25v charger setup and dont wanna spring for a 50.4v charger too, it’s just a convenience thing, i’ll get the aliexpress one if i have to, i’d just rather not bc i’m lazy and stupid.
okay you have a point. to answer your question, you can install 6s bms on each 6s battery, connect them in parallel and charge them through one charging port. you can charge each battery separately too. just take care when connecting them in parallel that they are both on same voltage.
after charging, you can connect them in series and use them as 12s.
btw I have bestech 12s bms, both charge only and with charge/discharge protection and e switch and can ship worldwide.
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Yeah the bestech ones are amazing but I don’t have the cash or the space in my scooter for one, it’s a very cramped build as is, I wanted to integrate 100w USB c charging but there’s not enough room so I’ll have a little box that turns my USB c power supply to 25v cc cv to charge the battery (as for why, because I’m an idiot with far too much ambition and the battery is kinda small and i take a 65w usb c power supply everywhere anyway to charge my laptop). Thanks for answering my question man, really helpful 
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Was that your thread on another forum?
Usb C charging is great idea actually! Hope it will work well!