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.
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.
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.)