2000 jeep Cherokee.... open the door and my interior lights look a little dim. Put the key in and turn it to start, and nothing, lights go out, no dash lights, no buzzer, no click, nothing. Auxiliary volt meter I've got hooked up says 7.9. Disconnected the battery for a couple of minutes, then reconnected them, and the thing starts up with no problem. Auxiliary volt meter now says 14 and some change while running, drops to 12 and some change after cutting it off. It will start and run fine for awhile, then it'll do it again out the blue. Any suggestions are greatly appreciated.

 

Over time, moisture gets through the insulation of the battery cables and they corrode under the insulation where you can't see it. It usually manifests itself as "I twisted the terminals to get a better connection and it worked". In reality, the cable was flexed enough to get good-enough continuity again to allow the starter to work. If your battery cables haven't been replaced, that's where I'd start...

 

Have the battery load tested
clean up your connections
Do a voltage drop test

All free diagnostics

 

I checked the battery with a multi meter and it was fine. Going to clean the connections tomorrow. Now, assume I'm an idiot, what's a voltage drop test...lol

 

Checking the voltage alone gives you limited information on your battery's health- a so called load test gives a good indication if your battery can handle what you ask it to do during starting. Have it done at one of your partstores for free (assuming you're in the US, which I am not)

Look up youtubes on voltage drop testings
"Ericthecarguy" channel has a few videos which are basic and very clear to understand. Explains it a lot easier than I can type. Happy to help if unclear.