Lithium Battery install ?

You already have a solar charge controller set up for Li battery charging?

That’s good for the solar but you can’t use a solar charge controller to charge from one battery to another. Solar modules are current sources, meaning that they will provide a limited amount of current (depending on how much sun they’re getting). They will provide current at a range of voltages, so the charge controller can decide what the output voltage limit will be for the battery type you’re using.

Batteries are the opposite, they provide a nearly unlimited amount of current at a fixed voltage. A solar charge controller is unable to change that voltage. If the battery you’re charging is at a lower voltage than the source battery the charge controller will try to supply all the current available, which is a lot, so it will either blow a fuse or burn up. If the battery being charged is at a higher voltage than the source battery the controller will supply no current at all.

Dc to dc converters can charge one battery from another by stepping the input voltage up or down to what you have set the output voltage to, while also limiting the current. But if you try do that youll wind up discharging your supply battery unless you shut off the dc dc converter to protect it.

I haven't converted my trailer to Li yet but I've designed and installed Li battery systems for other applications. Li batteries have a very different charge profile from lead acid batteries. They can catch fire if overcharged. So in general you should not try to use equipment set up for charging lead acid batteries to charge Li batteries. Following that safety rule: