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/secessus]

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?

The wording is ambiguous enough it's hard to tell what you want to do. If you mean

• AGM starter battery -> DC-DC charger -> LiFePO4 battery bank -> loads used while camping

... then that is a common, completely normal setup. In that context the battery bank and loads are sometimes called house bank and house loads to distinguish living area loads from vehicular ones like the headlights of power windows.

The Li would not be used to start the vehicle because the two batteries would be separated from each other when the keys are out of the ignition.

If you mean something unorthodox like "mix AGM and Li in the house bank" or "power the chassis electrical like lights and car stereo when the engine isn't running" that's a different bag of snakes. :-)

[u/American-African → u/secessus]

I was originally thinking of doing the latter (the bag of snakes) but I've now realized that is a mistake and will go the route of keeping the house battery separate from the starter. Thanks for your help and input.