Occasionally my battery goes dead very suddenly.. I can charge it for a bit, then it starts fine & will hold a charge for quite a while..days, even weeks. ( I don't drive it very often, so thought it was going dead from just sitting)
I used my small multi-meter to check the voltage after charging and it would not settle down to read 12 volts or so... 4.1 volts, 2 volts, 6 volts, meter just bouncing around. Took it to Auto Zone and their meter did the same thing and wouldn't run the system test. When I got homebelfore I took the cables off the battery and it measured 12.5 volts. Put the cables back on and it wouldn't measure correctly, like some other alternating feedback thru the cables.
Also, as for the title of this post: Engine running, turn off the key. Engine stops, panel gauges (fuel gauge, etc) drop to zero, then 10 to 15 seconds later bounce back on!?! But at a false reading.. fuel gauge needle pops up to about 3/4, not the actual level. I can stand outside the van, look in the window and seen the fuel gauge showing 3/4 full, click the lock button on the remote & the gauge zeros out, only to pop back up in a few seconds.. I don't know where to look & I've called the two local auto electric places for advice & I basically get "can't diagnose it over the phone, bring it in"...
Just another thought: when I first noticed this my driver side window wouldn't open so I was looking into that, couldn't get the last connector off to remove the door panel so I gave up & put it all back together. Window started working!! Then when I tuned the key off, the needles in the gauges started flipping up & down erratically AND the motor wouldn't start - the battery had gone flat! Charged it for about an hour & all was fine - except the gauges coming on after turning the key off... I'm Lost except I'm thinking some kind of a short - in the door maybe? (I don't know if it was doing this before or if thats just when I noticed it)
One other point: While it won't measure correct voltage AT the battery, when the engine is running, the panel meter is showing right around 14 volts so I know it's charging.