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This is probably a dumb question or simple answer, but I'm failing to wrap my head around the best practice.

I'm using a complete Tesla M3 pack with the internal 50A 12v (programmable) DC-DC converter in a build. I have an auxilliary battery which is a 40Ah lithium battery with internal BMS, fully charged at 14.4V. The lithium battery spec calls for a max charge rate of 10A.

I was assuming to charge this battery, whenever the vehicle is on, I would program the Tesla DC-DC converter to 14.4 or 14.6v, and the internal BMS of the lithium battery should take care of charging itself. However, it's not clear if these internal BMS's limit current as needed to charge.

Let's say the lithium battery is completely dead, and the 50A DC-DC converter is enabled. Will it then attempt to charge beyond the maximum charge current given the converter can maintain a constant voltage with up to a 50A load?

E.g. Do I need an onboard DC charger to manage charging the battery, or just hook it right into the DC-DC?
 

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Why are you using a lithium battery vs a deep cycle lead acid?

Take a deep cycle to full discharge and you can charge it. Take a lithium to full discharge and it's garbage.

Then there's the cold weather performance and charging issues that lithium has.

A BMS balances cells and reports on conditions - it's not a charger, though it can control one.

If you're using an Optima car battery - different animal....it's designed to charge off a voltage source.
 

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I think I'd go with an AGM or sealed lead-acid for 12V duties. They're just less finicky. You don't need much juice to "jump start" the DC/DC converter, so the battery could be the size of your fist and be more than enough.

I have a Battery Tender Jr hard-wired to my 12V battery, and it gets triggered when I plug the car in to charge by a special. This ensures my 12V doesn't die from sitting too long.
 
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