my friend has a car that , while running the battery loses its charge , shes had her altenator tested a few times but says its good , i told her have the starter tested , she hasnt yet , what would be the cause of this problem ?

 

Check all the wiring first. Frade lines. Dirty/bad conections. It's the cheapest start to the problem.

 

If the alternator checked out it should either be the battery or the wires running to and from the battery.

 

Agreed about the wiring, verify that is all good and clean. Corroded cables create all kinds of electrical issues.

Also, the battery itself my be weak. If you pull the battery, most parts stores can "bench test" the battery.

They can also bench test the alternator (instead of testing it in the car). Most the time, testing it in the car is good enough, but I have seen a case where having it bench tested proved an alt was in fact bad.