batteries drop from 13v to 8v randomly

[GrahamHO]

Check and clean all your battery and earth connections. Clean and Vaseline the battery posts. If the voltage goes up and down and up again it's most likely a poor connection somewhere. If it stays down it just might be a faulty battery. A faulty connection also affects the charge rate. I had the same problem on my RAV 4 the other day. It would sometimes start and sometimes not. The earth cable on the battery had worked loose as it was touching the bodywork.

[Terra Nova]

Originally Posted by skipmac

...the charger shows fully charged but the batteries have 8 volts. But you turn the charger off and back on and it charges the batteries back to 13V...

No it doesn't.

You can be certain the charger is not causing the batteries to go from 8v to 13v, just by switching it off and on.

Likely a loose ground.

[skipmac]

Originally Posted by savoir

You are thinking of AGM batteries.

Oh yes. 100% correct. Sorry to the OP. I had a brain fart and got Gel and AGM confused. But in my defense, the both have three letters and a G in the name.

That seems to be happening more often these days. Must be time for a vacation.