Dear Tom and Ray:
We had a dead battery and called for a jump start. The service agent could not jump-start the battery with his cables. He then proceeded to turn another battery over and lay it on top of my battery. Is this a common practice, and is this a safe practice? Could this have caused any damage to the electrical system? He left all the accessories on, and my headlights blew out and the alternator had to be replaced.
-- Susan
Tom: Can it cause damage? Apparently, yes!
Ray: It shouldn't, if it's done correctly (leaving out, for a moment, the issue of spilled battery acid). When you jump-start a car, essentially you're touching the terminals of two batteries together. You're just doing it via cables. So if you remove the cables, it should work exactly the same way, electrically speaking.
Tom: Unless you touch the wrong terminals together.
Ray: Bingo.
Tom: And that's what this guy did. He hooked up the batteries backward. They were in series -- like when you line up several batteries inside the tube of a flashlight. So instead of 12 volts, he sent 24 volts through your electrical system.
Ray: Some accessories can handle that. For instance, if your windshield wipers had been on, the motor would just have pushed them to wipe so fast that the raindrops wouldn't have known what hit them.
Tom: Any accessories that were turned off would be protected, as would any components that are protected by fuses or fusible links.
Ray: But some things just can't handle 24 volts. Your headlights, for example, probably blew immediately. And if you'd had other lights on, like the taillights, they probably would've blown, too. Or at least suffered some damage that would've shortened their lives.
Tom: And the alternator can't take 24 volts because the diode bridge gets burned out.
Ray: But there won't be any hidden damage. Anything that was harmed will be apparent to you because it doesn't work now, or is on fire.
Tom: But you might notice that everyone on your car radio is now talking twice as fast.
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk Web site.)
5 Comments
By Phillip Stroud on December 13, 2009 7:47 AM
I would love to know how you can produce 24 volts without hooking the batteries together in series, which can not be accomplished by simply touching the terminals of the batteries together. At best you would have a parallel connection for a total of 12 volts at worst you would have short circuited the two batteries producing a giant arc or possible battery explosion.
By Dick Wilhelm on December 13, 2009 9:41 AM
I agree with Phillip. Imposible to create 24 V with a parallel connection. What likely happened was that terminals were connected opposite of what they should have been. With the one battery dead, the polarity to the car was reversed. This would possibly damage the diodes on the alternator. Unless the headlights are some new high tech type, reversed polarity should not damage them.
By Gabriele on December 13, 2009 3:58 PM
Ray and Tony hanno ragione-
I suspect the genius must have used the cables to connect the dead battery's positive to the fresh battery negative, and the fresh battery's positive to the dead car's ground, thus producing 24v and doing the damage-
The dangerous job of connecting the batteries together was after the did was done-
Darwin at work, though this time it not eliminate the defect form the gene pool....
By Dick Wilhelm on December 14, 2009 9:52 AM
Gabriele,
Your method also will not produce 24 volts. The only way to do that would be to disconnect one terminal of the battery and connect the new battery in "series" between the disconnected cable and the battery terminal. I still think that the dead battery was a failed battery with open connections internal to the battery. The jumping battery was connected reverse of what it should be thus taking out the alternator diodes.
You should probably check your detail before you invoke Darwin.
By chuck on December 21, 2009 7:38 AM
We knew these guys were funny....but we also assumed they were somewhat knowlegeable. They wrongfully said the batteries were in series. C'mon guys, get out your books....maybe "DC Electricity For Dummies".