I just noticed yesterday that every once in a while my battery charge meter will suddenly shoot up to 18V (outside of the area marked "NORMAL"), but only stays there for 1-2 seconds then jumps back down to the middle. I am afraid that it might stay there too long and fry something.

I understand that a normal car/truck battery has a very low impedance so if an alternator is charging it at a normal voltage of, say, 13.6V then if the alternator's regulator suddenly cuts out I can't imagine that the alternator would be able to yank up the batter voltage all the way to 18V. However, I could be dead wrong. Or maybe the battery is internally defective and is opening up (I've always read that is is very bad to run a car with only the alternator running w/no battery hooked up).

Help please! Where do I start on diagnosing this problem? Does this seem like a battery problem or is it more likely the charging system?

Thanks

The alternator control system, within the PCM, changes the voltage generated at the alternator (through the field windings), in accordance with driving conditions. Therefore, the voltage regulator and the PCM work together to manage the voltage generated by the alternator. I would begin by getting your alternator and battery tested. Most auto part stores will do this for free. Erratic alternator voltage readings can indicate problems with the voltage regulator or that the alternator needs servicing; e.g. replacing brushes and cleaning slip rings. Personally, I enjoy rebuilding alternators and would replace brushes, bearings, voltage regulator, and test diodes. Before removing or working around the alternator, disconnect the negative battery terminal.

Thanks TXgreasemonkey. Do you trust Sears Auto Centers? There's one near my house and they have that drive-up battery service where they load-test your battery and check the alternator output current. What you think? Or would I be better off with a local mechanic/garage?

Sears should be fine. Be sure to clean battery and cable connections first. Many alternators burn out, when they have difficulty charging a battery due to corrosion.