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Short Circuit

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I was wondering, how fast does a short circuit discharge a battery? I know it depends on the battery but just theoretically? My guess is if lets say a 1.5V standard AA battery discharges in 20 hours running at spec then running a short circuit it would discharge in minutes? I know the wires and the batteries get very hot if you short circuit them so that probably ?increases? the resistance thereby creating some sort of non short circuit?
 
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Sceadwian

Banned
It wouldn't even last minute if it were a true short circuit. The heat would become so high the chemistry would fail and likely leak explode or possibly catch fire. This is an okay area to be curious about but not a good one to experiment with, all you're doing is wasting batteries and risking fire. What is your reason for asking this question?
 
Just trying to understand current, how batteries work, conductance of materials etc... basically trying to to figure out electronics. I just remember creating a short circuit when I was a kid and remember it getting very hot.... wanted to build on the experience. (Thankfully nothing blew up back then).
 
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peufeu

New Member
For your AA, the short circuit current will be : 1.5V / (total circuit resistance)
With total circuit resistance = battery internal resistance + wires + contacts, etc

In a short circuit most of the power will be dissipated in the element having the highest resistance (P = RI²). Usually, it is the battery itself.

An alkaline battery will overheat and leak.
A lithium/ion or LiPo battery will overheat and explode / catch fire.
A car lead/acid battery usually has such a small internal resistance (and huge thermal mass) that whatever short circuits it will probably melt first (even a wrench).
 
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Awesome thanks... you've tempted me into trying to do the wrench trick... don't know whether I want to waste a perfectly good car battery though... some day maybe.
 

Sceadwian

Banned
Atomic_Sheep, you should immediatly stop tinkering in such a matters as you clearly don't understand the danger. If you short circuit a car battery with a wrench the electrolyte will boil in a few seconds and explosively vent. Unless you want to get sprayed with super hot Sulfuric acid and end up dead or in the hospital I wouldn't recommend it...
 

Sceadwian

Banned
Worse, it's at the temperature of steam which is over 200 degrees Fahrenheit... Scared for life, or if you're right next to the stuff when it vents
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
I want to take you here and the topic is "Arc flash". **broken link removed** Pay particular attention to the EC&M article.

Hopefully, you will see that the threat is life threatening. Every power source has an internal resistance which will limit short circuit currrent. Take the wrench/car battery story and lets say that the wrench has a 1 ohm resistance including contact resistance. And the battery has a cold cranking amp rating of 400 and delivers 12 volts. The internal resistance of the battery is ~0.030 ohms.

Power = I^2*R and if I assume a large contact resistance of ~1 ohm and neglecting the 0.030 ohms, 160,000 Watts could be produced in a very short amount of time. This is not something you want to play with.

Furthermore metalic objects can weld to the terminals or can go flying into you.
 
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JimB

Super Moderator
Most Helpful Member
Take the wrench/car battery story and lets say that the wrench has a 1 ohm resistance including contact resistance. And the battery has a cold cranking amp rating of 400 and delivers 12 volts. The internal resistance of the battery is ~0.030 ohms.

Power = I^2*R and if I assume a large contact resistance of ~1 ohm and neglecting the 0.030 ohms, 160,000 Watts could be produced in a very short amount of time.

Whilst I am fully in agreement with the spirit of your post, the electrical theory and mathematics leave a lot to be desired.

If the wrench has a resistance of 1 ohm and is connected across a 12v car battery, the current will be
I = V/R = 12/1 = 12 amps.

the power dissipated in the wrench will be
W = I²R = 12² x 1 =144 watts

However, a more likely scenario is that the wrench resistance is much less than 1ohm, maybe 0.01 ohm.

Also, the battery, if it has a CCA rating of 400 amps, its internal resistance will be much less than 0.03 ohm.
You cant just calculate R = V/I = 12/400 = 0.03, where is the voltage for the starter motor?
When the 400amps are flowing there will be 9 or 10 volts across the load (the starter motor).

JimB
 

peufeu

New Member
> you've tempted me into trying to do the wrench trick

Yeah, but don't.

But you can try with a small electrical wire (like 0.5 mm²), this should be safe (but you might get burned skin, or a some molten metal in your eye, if you do something stupid)
 
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