Best place is to keep your engine problems right here.
Using the DIC, a fully charged battery at Key: ON should show 12.4 to 12.6 volts. During cranking it will drop a bit, then, as the engine starts and comes up to fast idle speed the voltage should ramp up to 14.2 - 14.8 and stay there for a while. As you drive and the battery approaches full charge the amperage (you can't see that) will ramp down and voltage should stay at 14.2 or somewhat higher - the voltage regulator ALWAYS maintains a slightly high system voltage and varies amperage to that required to run the car and maintain full battery charge.
Engine OFF, using a VOM, battery disconnected from anything (cables off) it should read 12.6 volts.
If the battery is more than 5 years old it's on its last legs. Group 79, sealed vented, at least 800 CCA.
If a "12 volt" automotive battery reads 12 volts or less it's essentially dead - there's some surface voltage on the plates but there's no cranking ability. Google "automotive lead acid storage battery theory" or somethig close.