> What readings do I need to take?
Amps and voltage output from the alt, ideally able to view while connecting jumper cables and adjusting engine rpm. Be able to maintain a higher than idle rpm, and keep an eye on engine temps. Real ideal would be the ability to do all this while driving.
Deplete House battery to 50% SoC.
Begin with engine running, House isolated from Starter batt, but jumpers ready to combine.
Note charging voltage and amps (M) at idle. Increase rpm, note M at different speeds.
Combine, still idling, and note M changes.
Ideally measure temp (infrared?) at the alt too.
Increase rpm, note M at different speeds. Volts should rise a little, and amps by a fair bit.
Set a rate with decent amp, no more than 3/4 of alt's rating, and not high enough rpm to cause the engine to overheat.
Within 30 minutes, maybe as early as 5-10, the alt may get hot enough that output drops, ideally just amps, volts holding constant or continuing to rise.
If voltage drops sharply as a result of the alt getting hot, there is no charging going on.
If volts continue to stay high, note M when that peaks, transition from Bulk to Absorb, House bank now at maybe 65-80% SoC.
After that you should see V holding and Amps falling. To get from here to 100% Full will take another 3+ hours, as Amps get tiny it becomes obvious why this should only be done when you're planning to drive anyway, same level of output as a small solar panel.
If all goes well, it's definitely worth getting robust permanent wiring to the House bank location and something to isolate Starter from House loads (different topic).
If not, and the alt keeps dropping out more often than charging, consider going to a DCDC charger.
Otherwise, it becomes a judgment call.
Remember your third alternative, carrying a small genny.
