You missed my point. A three-year-old battery can crank the engine just fine for starting but may no longer be able to dampen voltage spikes. It is REAL important on GM cars to replace even a two-year-old battery if the generator needs to be replaced. As I mentioned, other car brands don't have nearly as much trouble, but an older battery and / or loose or dirty cable connections can still cause the symptoms you described.
You need to measure battery voltage to the tenth of a volt. If you really find 13.0 volts with the engine not running, that suggests it is over-charged, and that is also a result of much of the lead being flaked off the plates. That leaves you with just a fraction of the battery it originally was. With little lead left on the plates, all the charging current has less place to go, so the plates and acid heat up and voltage goes up. That speeds up the eventual failure due to one of the cells becoming shorted when the fallen lead builds up in the bottom and shorts it.
My recommendation is to at least try a new battery to eliminate that as the culprit before you spend money on potentially unneeded service. If the problem is still there, THEN have your mechanic look further.
Thursday, March 10th, 2016 AT 6:18 PM