Any chance you have an inverter nearby & if so could it be that sounding a low voltage alarm? I could well be wrong, but wonder if the battery voltage is low turning off the generator from standby mode *might* be just enough to allow the voltage to rise enough to stop an inverter beeping???
First thing I did was check the manual... as you mentioned, nothing in there referring to beep.
I like your idea of testing the 12v battery that is used to start the generator from standby.
That battery is connected to a battery charger, so it should always be fully charged. But maybe after 10 years of sitting on trickle charge, the battery is not holding charge.
Once you have charged the 10 year old battery. Let it sit for 24 hours to stabilise & then check voltage. With extreme luck it may be ok.
The original starter battery in my car I replaced after 4 years. I used it as a test battery since. The current started battery died after 5 years. I ordered a new battery but it took awhile to turn up so I put back the old battery in the meantime.
I had to do this all with one hand as I had badly stuffed my wrist bending a bit of stainless by hand. Will put the new one in when I am able.
The 9 year old battery originally used for 4 years. 80AH 700 CCA still functions at 568 CCA.
I can self jumpstart with 4 x 26AH batteries (1040 amps 5 seconds or 360 amps for 5 minutes) Had to do this once with the dead starter battery.
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