Onan gen, solar charge voltage

westend wrote:
13.8 V for the solar voltage limit is too low.

Probably but it really depends on whether he has a smart controller or just a voltage limiter.

13.8 V for the solar voltage limit is too low. You don't state the type, amount, and capacity of your batteries but a typical arrangement of two 12 V batteries will not be fully charged in a day of typical use with the 13.8 V limit. FWIW, my programmable controller is set at 14.6 V for daily bulk and absorption charging and the float is set at 13.6V. The controller charge is temperature compensated so voltage will swing off those numbers in the seasons.

Your generator problem can be one of a number of things. I'd suggest to search the Forum for that starting problem as it has been addressed many times.

Regarding question 2, the generator needs 12V power in to operate; the battery provide excitation power for the field (and to run the generator controls). If it's not getting enough power or detects any of various other problems, it shuts down. Many of the fault checks are disabled while the start switch is held in.

If this is a motorhome, you probably could have started the main engine so that the alternator charges the house battery and whatever battery isolator system you have engages, and then started the generator. Once it is running and the converter providing 12V power, it likely would have enough voltage to keep going and operating (and you could then shut down the main engine).