Either the battery is bad or it's drawing a load when the car is off (like when you leave the lights on.)

Next time you park it for the night disconnect the battery. In the morning reconnect the battery and try to start the car. If it starts something is drawing current overnight and killing the battery, if it doesn't start buy a new battery because it's not holding it's charge.

 

So you are having to rev the car to 2k to get the alternator to work?

 

1965fastback:
I don't know. He just said to run it at 2k while he tested it.

Oxnard:
Good idea. I'll try that out.
It had no problem starting up this morning, but I'll test that theory while I can.

BA Mustang:
Voltage regulator?
I'm assuming it wasn't tested.

 

Depending on electrical demand, the alternator may not be able to keep up when the engine is only idling. So 2000 RPMs is a more "realistic" driving condition. Plus, it gave the him a chance to verify if the voltage regulator was doing its job.

Oxnard - good suggestion about the "phantom load". Could be a bad door switch keeping a light on, or even the clock in the radio could be enough to drain a bad battery. Since his car runs when jumped, that indicates that his charging system is working. I've still got my two cents that the battery is not able to hold a (decent) charge.

 

Well, as far as I can tell, no lights stay on... The radio's the original AM radio (10 channels, whoooh!). xD So no clock.
I'll give it a couple days and see what happens.

 

Magiusavvail,
It takes a pretty good load to drain a battery overnight to the point where it won't cank a starter. (which is a possibility) As a electrical engineer and ex auto electrician, It smells like a battery with a short in it however. Physically, take the battery out and have the parts people put it in the load machine and test it. If it passes, leave it with them overnight without any further charging and have them retest it again the next day. This should find an internal battery short. if it passes again, you need to have a qualifed electrician determine if you car has an electrical drain.