Join Date: Jan 2017
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Re: Charging outboard battery onboard
Presumably the charge source is not active while you are connecting / disconnecting the batteries to be paralleled.
Depending on the CAR of the chemistries involved, connecting directly a very low SoC thus low voltage pack, to one at a very high SoC / voltage, could lead to a very sudden flow of hundreds of amps.
The usual advice, is to make sure the two banks are at a very similar voltage before connecting them together.
A DCDC charger in between accomplishes many things of value, and handling that issue safely is one of them.
A BMS may or may not include over-current protection, and even if it does that should only be used for failsafe/backup, rare-to-never accidents
not day to day operations.