i have a 2002 Honda Civic EX,automatic.165000 miles.couple month the factory alternator was not charging right,and was replaced.also got me a new battery.all was fine until couple days when starting the car i noticed like low power on the battery.i charged the battery and tested the alternator the output with a cold car was 14.50v and 14.35v with a warm car.turned the radio heater etc and the output from alternator is good.unplugged the cable from battery terminal and the battery had 12.70v.2 hours later drove the card not using any radio,AC or light.parked the car,an hour later turned the car on and did not had enough juice to run the car.i did a test to see if there was any thing draining the battery(with car off removed negative cable from battery terminal.got the multi-meter on AMP test between the negative cable and negative terminal and did not show any power draining).like i said alternator tested good and the battery is 2 1/2 month old from Napa..any idea,could it be a faulty battery?

 

Here's a good video to help diagnose.

 

Check for poor ground at the alternator bracket.