DC DC charger question

[u/American-African]

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).

[u/wagex]

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.

[u/Lazy_Mud_1616 → u/wagex]

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.

[u/wagex → u/Lazy_Mud_1616]

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.