The 12v accessory pin from the 7-way would need to be connected to the battery in order to charge it so you'd first need to make sure it was actually wired that way since not every manufacturer does. That being said the trickle charge coming from the 7-way wouldn't be anything to worry about since it wouldn't send a high enough voltage to fully charge a lithium battery.
If you were wanting to make sure that the trailer battery was being charged while on the road then you could use a DC... view full answer...
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