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I have been to the car store four times and each time they tell me that the battery is good. A few of the cold-cranking amps (maybe at 50 or so, I don't remember the exact amount) are gone otherwise it just needs a good charge. Despite that information, my car's battery is found low enough that I can't start the car way too often ('98 Chevy Malibu). The battery is around 3 or so years old. I can charge it and things work for a while. One battery test they did also tested the alternator and that seemed to be okay.

The last time I was there, they suggested maybe something like a radio or computer is starting to suck battery when it isn't supposed to. I have a multi-meter. Where should I start to diagnose this myself so I don't have to pay an arm-and-a-leg for diagnosis. The only accessory I use is an iPod/iPhone radio transmitter. I realize it does use some power and doesn't help, but I have been using that for at least 3 years now and never had problems before. Any other ideas?

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up vote 4 down vote accepted

Can you disconnect the battery without upsetting the car's electrics? (You should be able to on a car of that age, but some newer cars or cars with fancy alarms can get upset)

If so, remove one terminal and connect the multimeter, set to ammeter mode, in series with it. DO NOT attempt to start the car like this (best to leave the keys well clear of the ignition), as that will destroy the multimeter. The meter should tell you if there is any current being drawn. If there is a draw, remove fuses one by one until it goes away - start with the likely suspects such as the radio.

If you can't disconnect the battery, the same test can be done at the fusebox - simply remove the fuse from the suspect circuit, and touch the probes of the meter to the contacts in its place - if the meter shows a current draw, you have your suspect.

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That makes sense. The radio does have the security code on it so it does draw some minimal power to begin with, but I have the code to unlock it. Should I disconnect the positive or negative? –  Mike Wills Feb 8 '12 at 14:06
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It is generally best to disconnect the negative, as this removes the risk of a loose positive cable touching any part of the (negative) bodyshell. –  Nick C Feb 8 '12 at 14:40
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As a follow up, despite what the car shop was telling me. I replaced the battery anyway and haven't had any problems since. So as I suspected back when it first happened, the battery was starting to go bad. –  Mike Wills May 11 '12 at 15:12
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