View Full Version : Shorting a Car Battery: Help Me Understand


Bull
07-10-2010, 09:50 AM
So, what happens when you short out a car battery? I mean, inside the battery what happens? Let me try and condense what could be a long-winded story into a few lines.

1. A friend comes over to help me rewire the rat's nest job that someone did on the mini-starter in my GTO years ago.

2. During the process, he accidentally touches the copper end of a wire that is connected to the starter to the negative terminal of my battery. A spark show ensues, and a small burn mark on the top of the battery case is created.

3. We finish rewiring the starter circuit and try to start the car. It turns over once, then stops. We spend the next several hours checking our wiring, the fuse box, the neutral safety switch, the starter etc. to see what's wrong. Battery is showing 12.5 volts at this time, but there's some weirdness going on. Not even a click at the starter, headlights wont come on, radio wont come on. Only one interior light under dash works when door is opened. If the door is left open the voltage at the starter is only 9, but if the door is closed it is 12 etc.

4. I finally get the idea that maybe the short fried the battery. He obstinately maintains that this is not possible. We take my battery charger, set it to 50amp engine start, and connect it to the battery cable leads after removing them from the battery. The engine turns over right away.

So, my guess is that the battery got killed by the short. But, I can't explain to him what might have happened inside and he's being kind of stubborn about accepting that batteries can short out on the inside. The fact that the battery shows 12.5 volts is also curious to us, but we are hack amateurs so lots of things are curious to us.

This ended up being longer than I wanted, sorry. But before I go to the auto store to get my brand new battery replaced, I'd like to know what happened :headscrat

larry_g
07-10-2010, 10:09 AM
First of all does the battery show voltage while under load? Will it run the head lights? What may have happened is that one of the tye bars between the cells melted or burned. It could have enough path to carry the voltage your measuring but not carry current to a load. Where exactly is this 'burn mark' you mention? If it is out on the top of the battery this could be the location of the burned tye bar.

lg
no neat sig line

Moose-LandTran
07-10-2010, 10:09 AM
I'm very upset that you've done this to my precious GTO.





Otherwise, i dunno. Except when i was in college someone left a wrench across battery terminals and it blew up and sprayed everything with battery acid.

Bull
07-10-2010, 10:16 AM
First of all does the battery show voltage while under load? Will it run the head lights? What may have happened is that one of the tye bars between the cells melted or burned. It could have enough path to carry the voltage your measuring but not carry current to a load. Where exactly is this 'burn mark' you mention? If it is out on the top of the battery this could be the location of the burned tye bar.


I cannot recall if, among the various tests we conducted in the wee hours of the morning, one was to watch the voltage while someone turned the key and tried to start the car. I honestly cannot recall. The burn mark is on top of the case, right near the negative post. The headlights would not come on at all, not even dimly, when the battery was in the vehicle. However, the operate fine when the battery is eliminated and the cable leads are connected to my battery charger; they are nice and bright, as are the tail lights, and the engine will turn over.

I'm very upset that you've done this to my precious GTO.


Otherwise, i dunno. Except when i was in college someone left a wrench across battery terminals and it blew up and sprayed everything with battery acid.

I know, Moose. When this happened I tried to explain to him that my buddy across the sea, the real owner of the car,was going to be very upset.

When the battery shorted, and then afterward during the tests, I was worried about the acid situation. I believe there was a thread on here fairly recently about batteries exploding. I had glasses on when near the battery, sure, but I just kept saying to myself: "My face,my face; what about my beautiful face?" :spit:

walrus
07-10-2010, 10:30 AM
Isn't 12.5 low for a charged battery? not much low but low

Bull
07-10-2010, 10:32 AM
Isn't 12.5 low for a charged battery? not much low but low

I believe you are correct. I bought the battery from the store maybe a month ago. Started the car once or twice. But otherwise it just sleeps in the garage, with the negative cable disconnected.

bmwpower
07-10-2010, 10:32 AM
Yep battery is shot. Like Larry said it can't handle the load. I can wire a couple small batteries together to get 12v on a meter but they won't start your car.

Moose-LandTran
07-10-2010, 10:32 AM
I know, Moose. When this happened I tried to explain to him that my buddy across the sea, the real owner of the car,was going to be very upset.

Honest mistake, shit happens, i'll let it slide. :lol:

When the battery shorted, and then afterward during the tests, I was worried about the acid situation.

The only thing i know (Someone else can confirm if this is true or not.) is that when you short them out you overload cells and burn them out. So one cell might produce 12.5v, but little or no current. Like 8 AA batteries will produce 12v but very little current. This may've been the case.

I believe there was a thread on here fairly recently about batteries exploding. I had glasses on when near the battery, sure, but I just kept saying to myself: "My face,my face; what about my beautiful face?" :spit:

Battery acid is nasty stuff. If you wash it off instantly there's little chance it'll do much/any damage. Get it in your eyes and you may be in trouble.

Where i used to work we had big (4'x4') old battery containers, they had about 1" of water/acid mix that had leaked out of batteries sitting in the bottom. One day i put a battery in and some splashed on me. After a few hours i had pink spots on my blue t-shirt and later that night it looked like a fish net.

Battery acid won't make my face any worse.

Moose-LandTran
07-10-2010, 10:34 AM
Isn't 12.5 low for a charged battery? not much low but low

I think 12.5v is ok for a battery. When running you should be showing about 13.9-14v.

RPH
07-10-2010, 10:46 AM
The plates ( anode and cathode) inside the battery warp under this extreme load. This causes them to short out and no longer can handle the load. Battery is shot. Simple test is to turn on headlights and try to start the car. Headlights dim or go out battery dead. If they stay bright starter circuit problem.

p_mori7
07-10-2010, 10:50 AM
Voltage is one thing, but if it can't carry any Amps, then none of the heavy load stuff is gonna work (Headlights, Heater motor, starter motor). The short by the starter cable (essentially directly connected to the positive lug of the same battery) caused something to fry under the top case of the battery. Perhaps a tie-bar, or more likely one of the soldered connections under the negative post. Did the cable touch momentarily, or did it sit on the post for a while ? Was the ground clamp still connected to the post ? An average car battery can supply about 500 Amps (albeit for a short period of time), that's more than a lot of welding machines...

66HertzClone
07-10-2010, 11:28 AM
Did you keep the fusible link wire in your new harness? If I remember correctly it is or was located in the harness near the starter and protects everything my melting if a short happens by melting and opening the circuit. It is the first couple of inches of the wire other than the battery cable attached to the large terminal stud on the starter solenoid.

bazzateer
07-10-2010, 12:53 PM
My brother got a face full of battery acid helping to jump start a neighbour's car. Brother put the cables on his battery correctly, neighbour put black to positive and red to negative- my brother's battery exploded in his face.

So yes, the battery can be buggered!

Kurn
07-10-2010, 01:03 PM
The battery also is a ballast so don't run the car for more than a minute or two connected to just the charger.I also second the bad battery.Betcha the tie bar is toast.If ya don't need it for a core you could cut the top off the battery to see what really happened.

OccupantRJ
07-10-2010, 02:15 PM
If you ever hear a battery making a high pitch squeeling sound, best to get your azz out of Dodge, right then! Do not pass go, do not collect money, just GO! I had one making that noise on one of my golf carts, and when it blew, it lifted the seat and banged my head on the overhead canopy. My daughter called the sound to my attention as she was riding it, and I had her get off so I could check it out. Pushed the pedal, high pitch noise, then BLAM!, one of the batteries completely disintegrated, like a bomb went off. There was no piece larger than your hand left of that battery. The seat shielded us both from the acid and shrapnel.

sdowney717
07-10-2010, 03:25 PM
the plates break or melt inside where they meet the bus bars which connect to the outer terminals.

tonydanzah
07-10-2010, 04:41 PM
How old was the battery before this happened? Most likely the battery was on its way and he helped put it out of its misery.

Moose-LandTran
07-10-2010, 04:51 PM
How old was the battery before this happened? Most likely the battery was on its way and he helped put it out of its misery.

This ended up being longer than I wanted, sorry. But before I go to the auto store to get my brand new battery replaced, I'd like to know what happened :headscrat

Come on, Terry, read the whole thread! :lol:

tonydanzah
07-10-2010, 04:56 PM
I think 12.5v is ok for a battery. When running you should be showing about 13.9-14v.

12.65 is ideal 12.5 is a little low

doug.j
07-10-2010, 05:03 PM
The battery probably has high internal resistance caused by high current from the temporary short. Sometimes the contact under the post/side terminal goes bad. Voltage is being dropped across the internal resistance when placed under load. That battery is shot.

Moose-LandTran
07-10-2010, 05:10 PM
12.65 is ideal 12.5 is a little low

*sings*

Touch my terminals... Terry Daniels..

Torque1st
07-10-2010, 06:32 PM
The battery is gone, internal damage. With a burn mark you will be lucky if the parts store warranties the battery. Just bite the bullet and go in and buy another one. If the parts store "offers" to warranty the battery take them up on their offer. Otherwise it is really not a battery defect but "abuse" that killed the battery.

BTW, a short like that would not have harmed a battery back in the 60's but now everything is built with a minimal amount of material.

caper
07-10-2010, 07:28 PM
BTW, a short like that would not have harmed a battery back in the 60's but now everything is built with a minimal amount of material.

Yeah,things aren't as tough as they used to be.I can remember when I was young and stupid taking a spare battery and flipping it upside down to stick the terminals to a dead one in the car because we didn't have booster cables.Used to keep the extra one in the trunk,never seemed to harm it.

nissan_crawler
07-10-2010, 07:42 PM
Did you keep the fusible link wire in your new harness? If I remember correctly it is or was located in the harness near the starter and protects everything my melting if a short happens by melting and opening the circuit. It is the first couple of inches of the wire other than the battery cable attached to the large terminal stud on the starter solenoid.

If this was the case, it wouldn't work with the charger, either.

kenners
07-10-2010, 09:46 PM
From the sounds of it, you drained the battery.
Put a 2 or so amp charger on it for a day or two, or a 10 amp or so for a day. Make sure there is water covering the plates if it has caps.
The reason dome light works is it draws very little amperage. Same with the headlights not working on account they have high amperage draw.
Normal charging circuit on a car is 13.6 to 14.2 volts.

mkirkpatrick
07-10-2010, 10:44 PM
The battery is dead, replace it and feel blessed it didn't cause a fire or blow up in your face. I had two batteries over the years that shorted out and caught fire.

scofo
07-10-2010, 11:19 PM
Clean the cable ends and if ness. do voltage drop testing.

Professur
07-11-2010, 08:45 PM
Ah, the ever under estimated car battery... source of so much joy, and so many tears.


The plates inside a car battery are not solid (anymore). They're like a waffle, with a paste buttered into the voids. That's how they're able to supply so much power all at once. What you've done if blow out the voids. The plates are still intact, that's where your 12.5V is coming from... but your amp/hrs has dropped to almost nil. What you've got now is roughly the equivalent of 8AA in series ... lots of volts (pressure), but no amps (flow).

I recall well one day way back when. Dad was out helping a neighbour working on his old Charger. I was standing back a ways. I don't recall exactly how it happened, but a 3/4" wrench dropped across the battery terminals. The car was running at the time, and somehow the battery didn't blow up... but there was a flash of blue light ... and to this day there's still an empty slot in dad's tool box where that wrench used to be. We never found hide nor hair of it. Dad never did replace it.

Torque1st
07-11-2010, 08:53 PM
People also forget that under the right conditions (lots of sweaty arms) a car battery can kill you.

The car makers are planning 36V batteries which gives a 42V system instead of the 12/14V we use today.

38Chevy454
07-11-2010, 10:39 PM
People also forget that under the right conditions (lots of sweaty arms) a car battery can kill you.

The car makers are planning 36V batteries which gives a 42V system instead of the 12/14V we use today.

This is because they can use smaller wire sizes. Weight and material costs.