Car jump-start question

My friend recently left her lights on and presumably needed a jump. She called roadside assistance but the jump didn’t seem to work. The car was completely dead.

She had it towed to the Hyundai dealer and they said that the whole electrical system had been fried, presumably due to the fact that the cables were reversed when we tried to jump it.

She got the car back but the cigarrette lighter and radio still is out (apparently they just didn’t check that).

I just want confirmation here that this whole scenario is plausible, since it cost her a lot.

Moe – do you recall if the connection from the good battery’s to your friend’s battery was, indeed, Positive-to-Positive and Negative-to-Negative?

Got ahead of myself … should read “Negative-to-Metal-Grounding” (like a metal part of the dead car’s engine.


Actually, before the roadside assistance guy came, another guy came to help with a jump. I don’t believe he went negative to ground, but rather connected it directly with his negative terminal.

Yes, it’s plausible. If the cables were indeed reversed during the jump attempt, many electrical components could have been ruined.

I wouldn’t expect this to damage the lighter, thought I won’t say it’s impossible. It might just be a faulty fuse. It certainly could have damaged the radio.

Connecting the negative jumper cable to the dead car’s ground rather than to its negative battery terminal is a safety procedure, to avoid sparks near the battery. It’s not a factor in electrical damage.

Positive to positive, negative to negative will work too, but it’s safer to connect the negative of the good battery to the ground of the car with the dead battery.
And yep, connecting the batteries wrong, such as reversing the terminal connections, can messed up the electrical system.

I’ve successfully jumped cars “positive to positive, negative to negative” myself many times. However, after reading the link above, I didn’t want to somehow steer Moe wrong.

I guess I’m not clear on the mechanism by which reversing the leads on the battery would have trashed the electrical system. What would happen is that you would get many, many amps running directly through the jumper cables, any intervening wires and both batteries would get extremely hot (and quite possibly go boom). But I’d expect that the net voltage seen by anything hooked up to the battery would be zero (unless the original battery was totally dead. And any reasonably engineered automobile circuit should be diode protected against being hooked up in reverse (although that doesn’t mean that they are). So my guess would be that, at worst, a wire or fusible link blew.


Net voltage couldn’t be zero unless both batteries had the same charge, in which case it’s unlikely jumper cables would be used at all (if they were, though, I still believe damage could occur).

Many electronic components are polarity-sensitive, and I have to assume it’s either impossible or unfeasible to make them invulnerable to damage from reversed polarity. Sometimes it only takes a momentary spike to fry something.

I’m sorry I can’t explain the mechanism, but I can tell you that in the real world damage does occur from reversed polarity.

I had a customer one time who replaced his own battery.
Being the just slightly dumber than a bag of hair he connected the blue battery cable to the red + on the battery, and the red cable to the big blue - on the battery. (this would have the same effect as hooking a set of jumpers up backward)
the damage included

[li]alternator was toast (diodes physicaly blown)[/li][li]Ignion control unit blown[/li][li]Hall pick up in distributor blown[/li][li]Fuel injection control unit was DOA[/li][li]radio no worky any more[/li][li]A couple of relays went buh bye[/li][li]a couple of fuses were blown[/li][/ul]
it took a couple of Kilo bucks to put his right. BTW it was covered under his comprehensive car insurance.

On most batteries, the posts are to one side; this should be difficult (though not impossible…no matter how foolproof you make things, they keep building better fools) to do. Your bag-o-hair aspirant must have pulled out all the stops in his efforts to visually inspect his intestinal tract.


Blue, huh? On my car (and I’d assumed on most cars), the negative lead is black.

Anyhoo, I just like to hum to myself “Red-dead, red-live, black-live, black-ground” when connecting cables.

I fried my Honda Civic’s electrical system crossing the jumper cables.

Cost about $175 to replace a ground and a relay.

He may have put the battery in backwards. Then the cables would just have to stretch across the width of the battery instead of the length, which is more commonly possibly.

Yep, it cost her about $800 so far, and that’s not including the lighter and radio.
Ummm… “covered under his comprehensive car insurance” you say??

I will of course recommend that my friend inquire about this with her own insurance, but just so I have a better idea, is this type of thing commonly covered? Without altering the facts exactly, is there any way I should perhaps, er… present the facts to the insurance company? (I’ll of course recommend that she read the policy closely first).

On my car, the cables just barely reach the posts. There’s no way (without modification) they could reach across the battery. I would assume that car manufacturers would do this intentionally, but perhaps not.

I once designed a tranny bracket that was almost but not quite symmetrical with an offset hole that was intended to match up with a small alignment stud in order to prevent misassembly. They were having some problems with the bracket out on the shop floor, so what did they do? Grind off the stud and then bore out the slightly misaligned holes. Never mind that they had this same exact problem on every single unit that came down the line; manufacturing didn’t get around to mentioning the problem until the trannys were (sometimes) coming loose in Inspection. :rolleyes:

It wasn’t entirely the assemblers’ fault; the bracket was, unbeknownst to me, assembled to the tranny first then to the chassis rather than the other way around as had been discussed during preproduction, and was being incorrrectly assembled to the tranny, owing to a process instruction sheet that documented the wrong way to assemble it, 'cause the first guy to put it together did it that way. What a mess.


FWIW, you don’t necessarily have to stretch cables to get them backwards on a battery. If you’re buying a new battery, and inadvertently choose one with opposing orientation to the one being replaced, it’ll hook up nicely. It’ll send everything to hell, but will be easy to hook up.

Note: Does not apply to Federation batteries.

Actually he bought a non-standard size battery. Rather than being rectangular, it was square, and the terminals were almost in the center. Basically it could hav been placed in the battery tray in any one of four directions. Of course only two of those directions would allow the cables to attach, and Mr. Murphy made damn sure he got it wrong.