1996 Pontiac Bonneville battery does not charge.

My car does not start. I can jump uit and it will run fine but as soon as I stop the car it will not start again. as soon as I stop it won't go again.

I had the battery checked and nothing was wrong, they said the alternator was good. what I need to do now is get a battery drain test, or so I'm told. How much should it cost for a drain test, also how can I go about doing my own test if there prices are too high.

Thanks,