I wouldn't use both at the same time. I would unhook the leads to the on-board until I removed the manual charger. I would think you run a good chance of ruining the on-board doing what you mentioned.
If you have a 2 bank, 10 amp charger (5 amps per bank) and want to charge your batteries quicker every once in a while, can you hook up a manual 10 amp charger in addition to the on board charger? will it charge at 15 amps then?
I wouldn't use both at the same time. I would unhook the leads to the on-board until I removed the manual charger. I would think you run a good chance of ruining the on-board doing what you mentioned.
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satdoc1 LIKED above post
Duck, I'm not for sure but I'm thinking your going to have a situation where one charger is sensing voltage from the other charger and reduce the amps it would provide. Thus still not getting full charge. Then like Mrdux said your running the risk of damaging one or both of your chargers.
On a side note, sell a few more shirts and get a bigger charger
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BigRiverMarine, KDAVID1 LIKED above post