Capt.,
Many thanks, again, for the pertinent battery info—it clears up my nagging muddle. I noticed that when the unit's power cord is plugged in, the disconnect "Use" light is always on and can't be turned off.
With regard to the electrical schematic, if it comes with the unit, then I must have it with my paperwork—but at this point I don't need it.
Regarding the second battery, I'm lost without pictures and I just can't picture it in my mind with words.
I'll read your instructions again in front of the house battery compartment.