The last post from the OP seems to indicate he has side mount battery connections which are famous for making bad connection to the battery when corrosion sets up in them.
It does sound like a corrosion problem is the most likely explanation for the inability to measure a steady battery voltage. A thorough cleaning of the battery posts, side and top, and the connectors which attach to the battery top is in order. Either that or replace those connectors and wires with new ones. Then you should be able to get a good connection with your volt meter and hopefully to the rest of the car.
If there’s no other symptoms than the meter reading, there may in fact be nothing wrong. Just a meter measurement problem.
Very True…I HATE Side post battery connections…They should be done away with…then again I hate all battery connections come to think of it. I have issues…
Blackbird
Faulty battery connections are one of the most common problems and easiest to fix on vehicles. If you perform the little effort it takes to maintain them, there will be no problem with them.
Even batteries in flashlights. I don’t think I’ve ever had a flashlight that I didn’t have to knock sometimes to get it to work.