"12-something" is not helpful. It must be measured to at least one decimal place. A fully-charged battery will read 12.6 volts. A good but discharged battery will read 12.2 volts. If you find somewhere around 11 volts, it has a shorted cell and must be replaced.
The place to start is by charging the battery at a slow rate for an hour, turn the headlights, (or any other load), on for about ten seconds, then remeasure the voltage. If it's 12.6 volts, suspect the charging system. To test that, the first part of the test simply involves measuring battery voltage again with the engine running. You must find between 13.75 and 14.75 volts. If it is low, the main suspects are the generator and voltage regulator. At that point the next measurement of value is at the large bolted-on output wire on the back of the generator. You should find full battery voltage there with the engine off. If you find 0 volts with the engine off, and much higher voltage than battery voltage with the engine running, look for a break in that wire between there and the battery positive terminal. Most vehicles now have a large fuse bolted into the under-hood fuse box for that circuit.
Saturday, June 28th, 2014 AT 10:51 PM