How to tell the difference between a bad battery and a bad alternator.

Dear Car Talk

Dear Car Talk | Nov 01, 2009

Dear Tom and Ray:

How does one tell the difference between a bad battery and a bad alternator? My car will not crank, but has some power (the dome lights, panel lights and power locks work). I've had it on a trickle charge for about a week. That has worked in the past when I've had a dead battery, but it still won't start. Thanks for your help. -- Cynthia

RAY: Well, it's possible that the trickle charge isn't working because your battery is dead and will no longer hold a sufficient charge.

TOM: But I wouldn't rush out and buy a Diehard just yet, Cynthia, because that's far from the only possibility.

RAY: Your battery could already BE completely charged. The car may not be cranking because you have a bad starter, a bad neutral safety switch, a bad clutch interlock (if it's a stick shift) or even a bad connection from the battery to the starter.

TOM: So here's what you do: Get someone to give you a jump-start. If you can get the car started with a jump, that suggests the battery is no good. Once you get it started, you can drive it to a local garage and ask them to test the battery for you. That will give you a definitive answer.

RAY: If you can't jump-start the car, then you can be relatively sure that it's a bad starter, or one of the other things we mentioned.

TOM: But if you've been charging it for a week and it still won't start, that puts the alternator way down on the list of things I'd suspect. The alternator's job is to charge the battery. And if the trickle charger can't do it with the help of your power company, I think the alternator may be off the hook. Good luck, Cynthia.

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