In this scenario you would use the Buck Boost to charge your Lithium Battery from the start battery, you would set it up to only run when the engine is running and you would program it to charge at whatever voltage your battery manufacturer recommends. You can limit the current flow in the programming meun, this is to avoid overtaxing your alternator. The maximum current limit with this model is 50 Amps. Its a very elegant solution to a problem that many people have to wrestle with. Unfortunately it is a bit expensive, but when you see the level of sophistication and the programming options you will understand why it is priced the way it is.