BFL, if you have a DC-DC charger between a Li battery and the alternator, you don’t have to worry about what the alternator output voltage is. The DC-DC will only output what the battery it is charging needs, and can handle a fairly wide range of voltage from the alternator. If the alternator voltage gets too high or too low, the DC-DC charger will drop out.
Most DC-DC chargers are connected to the starting battery, not the alternator. When I was shopping for mine, I only ran across one DC-DC charger that connected directly to an alternator, and it had to be a specific type of alternator. It was a marine unit, and very expensive so I didn’t look into it very long.
The Li charging profile on the Redarc charger I’m using will output a max of 14.5 volts, and float of 13.6. I have AGM, so I’m using a different profile.
Here’s a picture of the Turn On/Off Thresholds for the Redarc. The triggers are switchable between standard (for fixed voltage alternators) and low voltage (for variable voltage alternators).
Edit: not sure why I wrote “your Li battery”. I know you’re not using that type. My point was, as long as your DC-DC charger supports your battery charging requirements, you shouldn’t worry about what the alternator voltage is doing.
:):)
