Recharging a dead battery...

Dear Car Talk

Dear Car Talk | Aug 01, 1990

Dear Tom and Ray:

I've received so many different answers to this question that I decided to ask the experts. When a car battery dies due to cold weather or leaving the lights on, how many miles should the car be driven, and at what speed, in order to bring the battery back up to full power?

TOM: If you run your battery to where it's stone cold dead (ie to where you need a jump start to get it going again), you could partially recharge it by driving at highway speed for 20 to 30 miles. That assumes, of course, that you're not using the lights, the air conditioning, the electric seats, the microwave oven, etc. A drive like that should give you at least enough power to start the car again. (If it doesn't, jump it again, mumble a few unkind words about us, and try driving 40 or 50 miles).

RAY: But the truth is, Barbara, it's not a good idea to charge the battery by driving the car. What that does is put an undue load on the alternator. The alternator does a fine job of recharging the battery under normal day to day circumstances, but it's really not designed to charge a completely dead battery.

TOM: The best thing to do if your battery is dead is to pay a repair shop $10 to put it on a charger for several hours. And unless you know why it went dead (for ex??ample, you left the lights on during the entire Toledo Mud Hens game, which went into extra innings) have him do a complete diagnostic test of the charging and starting system to make sure it won't just die again.

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