Voltmeter? Battery should be 12.5 0r so and should be 13+ when runnimg. Leave your battery un hooked for a while and see if it holds a charge. 12.87 is what my meter reads on a good battery. It should be the same, hours later.
Don't disconnect battery while engine is running on the newer vehicles. That's a no no. Old ones it's okay to do that, new ones not. By old ones I mean before fuel injection.