batteries drop from 13v to 8v randomly

[jeepdoesrubicon]

Lots of good advice here. i would also get a bigger charger. It clearly is the charger overheating. Do maintenance on the batteries, but replace the charger. I have a 160 amp alternator that has no trouble keeping the batteries afloat until they fail!

P

[hamburking]

Turn everything off, then check the voltage of each battery with a digital voltmeter right at the posts. Hopefully this will show you the bad battery. However, it is just as likely a bad connection. This is harder to find. I've had cables broken inside the sheathing that looked fine, but would work only intermittently. Have someone watch the voltage while you jiggle each battery cable, watching to see if you can induce the voltage loss.

And finally, maybe its something like the motor for your fridge or bilge pump coming on, giving a huge drop in voltage because of a big power draw? Do you have anything like that?

Good luck. I'm searching for my own wiring gremlin myself.

[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.