My battery light on 2003 Discovery recently has been coming when I start up engine. However, it usually turns off after I rev up the engine to about 3500 rpm's.

So the light goes away as soon as I drive.

I took her to AZ (Autozone) to check the battery and the alternator and both are good. Or at least that is what they said.

Any Ideas...?

 

Do you live in a cold environment, and how old is the battery?

I'm asking because I had a battery with a bad cell that tested fine when warm, but had low voltage when cold, around 0 deg. Celsius.

With the alternator running, even at idle unless the battery was really depleted, voltage should be around 14 volts. With a full charge, without the engine running, and with the battery unhooked, it should read at least 13 volts. A bad battery's voltage will drop significantly as soon as a load is applied, even a small load like the AC fan.

In cold climates like here, a car battery is expected to last around four years.

Also smart chargers are relatively inexpensive nowadays, and they have nice features such as desulfating and testing.

 

I do. but it wasn't that cold...

 

check the connection on the back of the alt. especially the smaller wire

 

I'm going to throw my bet on the battery...