Recently my cutlass has been killing its battery while driving and not being able to start back up. Recently it seemed like the battery has been getting weaker and weaker (so i thought) So i decided to get everything tested. Took alternator to autozone, tested, pass 3 times. Took battery to autozone and it passed as well. I didnt believe them with the battery so i decided to drop the newer battery from my truck in the car. Drove it around and all was good. so i drove it to the gas station to get gas, when it was done filling i went to start the car and it didnt have enough power to crank the starter. So that leads me to believe my battery WAS actually good. Now being that my alternator passed the test as well, does anyone have any idea to what this could be?

 

Have you taken any voltage tests at the battery terminals with the car running? It may be some thing as simple as a bad return wire from the alternator to the battery. It could be a bad or corroded battery terminal or ground. With the engine running you should have around 14v across the battery.

 

Yep, we (and you) need voltage tests done to be able to pinpoint the problem.
Any voltmeter will work - don't need anything fancy...

 

Try to provide the voltage readings across the battery with the engine off, cranking, running at idle and then with a load on the system such as high beams, blower motor on high at a high idle. If the readings are low try running a temporary ground jumper wire to the engine block.

Measure the voltage at the output wire of the alternator to see if it is different than the reading across the battery.

Testing the battery and alternator off the car is good but it does not test the system in operation, only the components independent of one another. The belt could be slipping, a symptom of this is that the pulley where it is slipping will get VERY hot. It also does not test the wiring and connections.

Good call by trying another battery that was known to be good, that is a good way of ruling things out.