Battery or alternator?

Sounds like the alternator quit charging. Get a Battery Tender and let the battery get a slow, full charge. If you don't have the tools and/or know how to remove and reinstall a new alternator yourself, a full battery should be okay to get you to a close shop safely...during the day, no A/C, no stereo...just engine!

The battery starts the vehicle, the alternator keeps it running. If the alternator fails, the battery will be used but will slowly drain. If the battery is left to sit, it's possible that it will recover enough energy to give a quick boost and start a vehicle but then it would quickly die again.

I'd _always_ first check the battery terminals and connections before doing anything else. Make sure they are _very_ clean and tight. Next, I'd charge the battery complety, start the engine, and then disconnect the battery so that you just using the alternator. See what happens then.

Thanks to all who replied. Will move on these tips.

Thanks!

You don't say whether you have opened the hood and taken a look. As another poster suggested, look for corroded battery terminals. If in doubt clean them, but also look at the alternator. Is the belt slipping? Is the alternator frozen? Are there any loose or broken wires on the alternator?

It could be as simple as cleaning a terminal, replacing a wire that has fallen off, or replacing a belt. Of course if the alternator is frozen, and doesn't turn, you will have to replace it, but the diagnoses will be simple. If you have a battery charger, recharge the battery. If it takes a charge, then it is probably the alternator, if not it's the battery.

If you want to mess up the ECM. follow this advice, other wise go get it checked somewhere with the proper tools. That's 1970's and earlier advice, and back then it was fine, it wasn't accurate, as most vehicles would run fine as long as there was 10 volts, and 10 volts wont charge the battery. Today you can cook all kinds of things doing this. Even before fuel injection this was a dangerous thing, Fords for instance would pop the ignition module most of the time the instant the cable came off., if it stayed running sometimes as soon as it was reconnected. The battery today acts as a surge protector of sorts. You pull the cable off a battery that is pulling a major load from the alternator, and the voltage spikes before the voltage regulator can clamp the voltage down. That's one of the reasons it is strongly advised that when jump starting a vehicle you shut the vehicle that is doing the jump off before hooking up cables, and then restart and try to start the second vehicle. Wither the vehicle being boosted starts or doesn't, shut the vehicle giving the jump off before disconnecting cables.

Whitelightning

Disconnecting the battery isn't reccomended on newer vehicles. It can spike the alternator voltage way up, and fry things.

Better to check at the battery with a voltmeter. With engine off,

12.0 to 12.5 volts or so is normal. While the vehicle is running. 13.5 to 14 volts is normal.

If the running voltage is below 13.5, you have an alternator problem.