That's not the problem though. It's putting out 16V, which I suspect
will fry the little U1 battery.
RWL

You mean that with the engine running you measure the voltage across
the battery and it reads 16 volts?

Yup. With the engine off, the battery reads something like 13.6V
freshly charged, and once the engine's running the voltage across the
battery terminals is 16+V

I phoned Onan today and have an inquiry in regarding what I should be
seeing. The secretary said the tech who knows that information was
gone for the day, but she'd get back to me tomorrow.

RWL