Originally Posted by seattleboatguy
I intended to connect my alternator to the AGM start battery. It was interesting to learn that some people connect their alternators to the lithium house battery.
Mine is direct connect to the LFP house bank. The alternators are externally regulated, and set to appropriate LFP values, just as you would set your inverter/charger, or stand-alone charger. That way the LFP gets the benefit of full alternator output to the extent it's available, and regulation follows the needs of LFP, not AGM.
Then the DC-DC chargers take care of the AGMs with voltages and charge cycles optimum to their needs.
A lot of people leave their alternators on the start batteries then use a DC-DC charger to charge the LFP. Personally I think that's backwards.
I think many do it because they don't have or want to install an external regulator, but I believe that's a false economy because a DC-DC charger is going to cost just as much, and often becomes a bottle neck between the alternator and LFP bank because alternators typically have more capacity than the DC-DC charger. It's also done because people don't want to overload their alternator, but every external regulator I know of has the ability to limit alternator output if required.
With an external regulator you get all the requisite control over the alternator and LFP charging, utilize the full capacity of the alternator, and use a lower capacity/cost DC-DC charger on the start batteries where demands are modest.