how to not destroy an alternator when charging LI batteries.

FWC wrote:
In many cases there will be no issue with the alternator. I have had various LiFePO4 setups over the past 10 year, and never had an issue with overloading the alternator. The wiring, breakers and ACR between the alternator and batteries provides plenty of resistance to limit the current to the battery bank.

In specific cases with higher voltage alternators and very large gauge wiring, there could be an issue, but it is easy enough to try a direct connection and see how much current flows before adding a DC-DC charger.

CharlesinGA wrote:
Do it right and install a DC to DC charger between the vehicle charging system and the Lithium batteries, and your problems are solved. that is what they are designed for. Lithium connected directly to a alternator charging system will fry the alternator.

The video linked to in the first post is a very good one and while one way (usually on boats) of controlling the charge, the DC/DC works well without messing with hugely expensive alternators custom fitted to a vehicle. New vehicles have computer controlled alternators and none of this plays well with lithium in the house battery box.

Charles

I've put over 6,000 trouble free miles on a standard MB Bosch 220A smart (internal fan, internal regulator) alternator charging 2 X 100Ah Relion RB100-LT batteries, with no additional hardware to do anything in between the charging source and the lithium targets. In varied ambient temperatures and at varied engine RPMs, at varied SoC, including some idling.