1988 Formula 311.
My Kenwood stereo works perfectly when my Perko switch is in "ALL". When I switch to battery 1 (deep cycle) the stereo starts to cycle on and off?
Any places to start troubleshooting?
Could a low battery output (about 10.5 volts) do this?

Yes.
10.5 volts is no bueno.

Here is the state of charge for a 12 volt battery

Wow, is that for deepcycle specific or all 12V batteries (including cranking battery).
I guess I skipped that day in my (very few) electrical engineering courses...THANKS!!!
Ps-courtesy lights all seem to work okay? (probably very low amp draw, right?)

Battery can also have a dead cell in it. Each battery cell is 2.1V. 12.6 - 2.1 = 10.5V

All 12volt batteries.

Psst: real world beats classes once again….:wink

F14,

I'd say you've got one dead-ass battery, and, when you have your switch to "both", it's sucking the good one dry.

Thanks. Brad.
(937)545-8991

I topped of the water in both Interstate batteries (crank and deep) and put the deep cycle on trickle charger. Within an hour or so the voltage is rising and the stereo issues resolved. I will top off the charge and watch the draw/drop. I have located a replacement battery and will pull the trigger if I have any voltage issues.
THANKS AGAIN ALL AT OSO!
Ps-I have surely learned much more from "doing" than I did from reading/studying (but then again...I did not apply myself or get good grades).