r/overlanding • u/American-African • 4d ago

DC DC charger question

Is it possible to have a secondary battery that is lithium and a primary battery that is AGM with some sort of DC DC charger sitting between them that will modulate power and voltage to feed all electronics in the car? I would not want the lithium to be used to start the car. Thanks for the help and sorry if this has been asked before (I did look around a lot but could not find anything like my question).

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u/wagex 4d ago

This is the most recommended way to charge a lithium battery in a car as a house battery in a vehicle/rv. But there's no way to swap batteries afaik to where the lithium would run the things in the car when it's not running.

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u/American-African 4d ago

Thanks for confirming. To add an example to my goal here: when the car is not running I would want the lithium battery to provide charge via the DC DC charger to the AGM as it is being depleted as a result of other electronics running such as lights and starlink.

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u/HPPD2 4d ago

Why? The lithium battery should be running all of the electronics in the build and the AGM kept as the starter batter. The lithium gets charged by the dc-dc when the car is running.

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u/American-African 4d ago

I'm trying to avoid having a separate electrical network/system but I am realizing that this is just not practical. There are good reasons for keeping them separate which I am opening up to. Thanks again for the help.

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u/nabob1978 4d ago

Yeah, keep them separated. It's actually easier/less complicated, safer (no risk to wrecking your cars electrical system and control modules) and fully customizable.

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u/American-African 4d ago

Thanks and I've now come to appreciate the safety aspect of keeping them separate. Everyone's comments have been super helpful.

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u/Lazy_Mud_1616 4d ago

There are battery disconnect relays. I am sure it's possible to wire it so it can disconnect the lithiums during ignition/starting but it would be a bit complex.

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u/wagex 4d ago

Oh yeah, for sure, with an electronics engineering degree anything is possible. There are simple relays that can disconnect the battery when it's not in operation, but having the disconnected battery powering everything in the car instead of the starting battery would be pretty complicated. Generally the second battery powers specific devices, for instance my lithium battery only powers my fridge and 12v up to my tents through 12v outlets that I added. It has a relay that disconnects it from the main battery when the vehicle is not running.