In my 1990 300zx my battery wont hold a charge. i have a brand new battery in it. (less than 1 month old)…. i went out the other morning and it did not have enough charge to start my car. though my car jumped just fine. i drove my car down the road and back, it took probably ten minutes or so and i parked my car and turned it off. about ten minutes later i went out to start it and it would again did not have enough charge to start. so i jumped it again, and while it was running i turned on my cd player, lights, rear defrost and heater. no change in the car performance was noticeable while idling. while these were on i unplugged the negative battery terminal to test the alternator. i let it run with the negative cable unplugged for about 20 seconds and no change was noticed. the car ran just fine and the electronics were going just fine. any ideas? did i maybe not let the car charge the battery long enough the first time on my 10 minute drive? any ideas?