Cross polarization of battery:

[speeder]

So what would actually happen if someone hooked jumper cables up backwards? As in, positive to negative and vice versa? I’ve never done it but a friend did recently and we can’t seem to find any damage from this goof-up. He was kind of expecting melted wires or something but everything seems fine.

Now we are both wondering what the damage is supposed to be?

[908/930]

Did he try to start it? How modern is the car? Most electronics likely have a diode in the circuit so it would not turn on with wrong polarity, Im thinking that a starter will just spin the wrong direction providing the solenoid is able to move the plunger in the right direction. Fuel pump could be another one spinning backwards, if the relay will turn on. I just tried reversing the polarity on a DC relay and it will operate that way.

[VINMAN]

Actually did this 3 weeks ago on my jeep. Didn't seem to have e any effect, other than still not starting.... But caught it real quick.

.

[flatbutt]

Originally Posted by speeder

So what would actually happen if someone hooked jumper cables up backwards? As in, positive to negative and vice versa? I’ve never done it but a friend did recently and we can’t seem to find any damage from this goof-up. He was kind of expecting melted wires or something but everything seems fine.

Now we are both wondering what the damage is supposed to be?

As in one end is crossed and the other end is correct? I'd guess that you wouldn't have a complete circuit?

[Arizona_928]

I've seen it blow giant amp fuses.

[gregpark]

I remember hearing of a couple guys jumping a car backasswards like that and a battery blew up. Both guys were drunk and one was injured pretty bad.

[sc_rufctr]

Most modern cars have a big fuse that will blow if you connect the jump leads to the wrong terminals.

If you have two AA batteries joined in series (+ to -) what happens if you short the ends?

[JackDidley]

Golf carts often have several batteries hooked in series. 4 12 volt batteries in series = 48 volts. They do not blow up, just being hooked up that way. Battery gasses and a spark can blow up badly.

[Ayles]

Had this happen a while back helping someone jump start their car in the grocery store parking lot. Typically I would inspect both ends but this person seemed confident it on correctly.

Basically the jumper cables got really hot, I noticed pretty quick and pulled the clamps. I got it set back up right and it worked. Did not seem to cause any damage other than a little melted plastic on the cables.

[GH85Carrera]

My wife & I recently walked out of PetsMart with our dog's food and saw a chick that was likely mid 20s with the hood of her car up. She was wiggling the battery cables and they were shot and not clamping properly.

I puled the trusty El Camino over to her car and used my jumper cables but I told her I would hook them up. The positive side of her battery was hard to tell if it was positive, but the negative side had a cable running right to the ground point, so I knew it was negative. It was not some old British car so I did not worry about positive earth. Anyway I hooked it it to my car, and her car started. I strongly recommended she drive the few blocks up the road to the local auto parts store, and ask them to check her cables and the battery. She was easily cute enough that she would have a parts guy willing to help. I did not mention that observation out loud as my wife was standing there, and the chick did not need to get creeped out by some old geezer leering that she was right purdy. I did tell her she will be stranded again real soon if not on the next start if she does not get it fixed.

The entire story is just to say I personally made sure the connections were correct. I then got home and cleaned the clamps of my jumper cables and the corrosion and crap from her battery was still on there. She had a dirty battery.

I have seen the aftermath of a blown battery for hooking it up backwards. It was not something I wanted to see first hand.

[masraum]

Originally Posted by GH85Carrera

I recently walked out of PetsMart and saw a chick that was likely mid 20s with the hood of her car up. She was wiggling the battery cables.

I puled the trusty El Camino over to her car and used my jumper cable but I told her I would hook them up.

Then got home and cleaned my jumper cable and the corrosion and crap from her battery was still on there. She had a dirty battery.

I'm sure she did.

I feel like I've seen this story somewhere before. Maybe in a magazine.

[GH85Carrera]

Originally Posted by masraum

I'm sure she did.

I feel like I've seen this story somewhere before. Maybe in a magazine.

Only if I was still in my 20s and single. As a very happy old married dude, I would not even consider her anymore than I would have had a chance with her. I can't even fathom cheating on my wife. And my definition of cheating is doing anything at all that I would not tell my wife I did or was thinking of doing.

[aschen]

if both batteries are fully charged and healthy it will definitely be problematic as 2000 amps or more try to flow through the cables and clamps.

Since one of the batteries was presumably dead or near dead it may have had a high effective internal resistance saving him from drama. Definately not something you want to do of course.

[speeder]

Originally Posted by aschen

if both batteries are fully charged and healthy it will definitely be problematic as 2000 amps or more try to flow through the cables and clamps.

Since one of the batteries was presumably dead or near dead it may have had a high effective internal resistance saving him from drama. Definately not something you want to do of course.

Thanks. The thing is, the battery being charged was not dead, just a little down on charge. And the charger vehicle was a late model F-150 with a large alternator hooked to a tiny battery, barely bigger than a MC battery. Zero damage.

[Gretch]

Originally Posted by Arizona_928

I've seen it blow giant amp fuses.

and diodes............ it the blink of an eye.

[vash]

the spot where you ground the clamp gets really hot super fast.

so i've heard. maybe this should go in Tabs' greatest failure thread. i was lucky i didnt fry a fuse.

[vash]

and there is cussing. lots of cussing.

[T77911S]

Originally Posted by sc_rufctr

Most modern cars have a big fuse that will blow if you connect the jump leads to the wrong terminals.

If you have two AA batteries joined in series (+ to -) what happens if you short the ends?

its like shorting a battery, but a 3v battery instead of a 1.5v. you would get max current flow that the battery can supply.
fuses only blow when too much current.
car fuses aren't in series with the two batteries that are connected backwards. IF the fuse blew it would be because current flowed through it

thing with doing it with jumping a car, one battery is usually dead.
yes I have done it. no damage. even connected my battery charger up backwards one day. kinda messed up the charger but it still sharges.

im sure he saw arcs and sparks. again, its just like putting a wrench across the terminals. worst thing that could happen is the battery explode.

[porsche930dude]

I recall two instances where it caused damage. On an old mini that was positive ground. He said it fried everything but i dont know what exactly. And on my neighbors ford tractor that is 6v positive ground it melted the voltage regulator. Other than that Im pretty sure iv done it before and just had sparks and hot wires. I hook up my battery charger backwards quite often and it just trips something inside till i unhook it. But i dont think cars have anything like that to protect them

[sc_rufctr]

I bought one of these about 12 months ago and I've used it three times. It lives in the "boot" (trunk) of my daily.

- Honestly it's one of the best things I've ever spent money on. Highly recommended.