Battery short-ciruit current rating

[AMink]

I am stumped! I've looked at a lot of sites, but can't find the answer. How do you determine the short-circuit current rating of a 12v deep cycle battery? My owners manual for my inverter says to get a fuse or circuit breaker that can "withstand the short-circuit current that the batteries can produce. " No mention of how to figure it out! Seeing as how I don't want to burn anything up (or down!), I thought I'd ask the SMART ONES on this site. Thanks!

 

[rbattelle]

Gotta be thousands of amps...



-Ryan

 

[klenger]

Fuse the wire for the max current draw of the inverter plus some extra. It doesn't matter what the capacity of the battery is. The fuse needs to be sized to prevent the wire from burning up under a direct short of the wire.

 

[EMD Diesel Power]

Gotta be thousands of amps...



-Ryan

:-laf :-laf :-laf Are you nuts? are you saying that you could weld ship steel with a standard 12V battery?



Most standard welders put out 200-300 amps at most... . Locomotive traction motors at full,buried needle in the redzone, melt everything in sight, power are taking in at least 1500 amps. and thats per motor. Usually 4 or 6 motors on a unit.





sorry... couldnt resist. just had to razz.



Klenger is right... . fusible it for the max amp draw on the system (the inverter) plus some extra. Fusible link blows and the system is protected. Only way the battery is going to blow is if it gets shorted terminal to terminal. Dont ask how I know..... but I will tell you that I blew up a locomotive starting battery once (74V kind) Plates shorted as I went to crank... . needless to say it didnt start til the next shift.

 

[AMink]

Don't think I'll be welding with it, at least not on purpose! Thanks for the replies. So, if the biggest current draw I'll see is 10 or 11 amp, fuse for 15? 10amps x 120volt = 1200 watts, still under inverter rating. The whole thing looked so simple in the box :-laf

 

[KBennett]

Don't think I'll be welding with it, at least not on purpose! Thanks for the replies. So, if the biggest current draw I'll see is 10 or 11 amp, fuse for 15? 10amps x 120volt = 1200 watts, still under inverter rating. The whole thing looked so simple in the box :-laf

You have to do the math for the input and the output side of the inverter.

Input Current = 1200 watts/ 12VDC = 100 amps

Output Current = 1200 watts/ 120VAC = 10 amps



Just make sure the wire you are using will handle the current that the fuse is rated for, otherwise the wire will act like a fuse



Good info on batteries including Amp Hour ratings: Battery Info

 

[tractorface]

Short circuit ratings (interruption rating)

The short circuity rating refers to the amount of current available during a short circuit. They are not referring to the fuse size as you are referring to it. For instance a 20A fuse may have a short circuit withstand rating of 10,000A, which means the fuse can safely interrupt current up to 10,000A during a fault condition. If you have a source that has the potential for a 20,000A fault, a fuse rated 10,000A may not "open", and instead weld shut. In this instance you would have no protection. All that being said if I am not mistaken most fuses have a withstand ratings far in excess of what you would need for your battery. The problem would come in if you were using circuit breakers.

Remember Volts/Resistance=current. A short circuit yields extremely low resistance say . 00097 ohms for the sake of argument. That is 12,371A of current from a 12 volt source assuming the 12v source is capable of that. To be honest I am not sure how to figure out what your battery is capable of, just wanted to clear the air a bit.

Hope this helps a little.



Take a look at the circuit breakers in your house, I know Square D breakers have the Short circuit rating visible on the breaker. Many times Homeowners mistakenly think they have a 10,000A breaker (which they somehow decide must mean 10Amps. I have seen this first hand many times :-laf

 

[EMD Diesel Power]

The short circuity rating refers to the amount of current available during a short circuit. They are not referring to the fuse size as you are referring to it. For instance a 20A fuse may have a short circuit withstand rating of 10,000A, which means the fuse can safely interrupt current up to 10,000A during a fault condition. If you have a source that has the potential for a 20,000A fault, a fuse rated 10,000A may not "open", and instead weld shut. In this instance you would have no protection. All that being said if I am not mistaken most fuses have a withstand ratings far in excess of what you would need for your battery. The problem would come in if you were using circuit breakers.

Remember Volts/Resistance=current. A short circuit yields extremely low resistance say . 00097 ohms for the sake of argument. That is 12,371A of current from a 12 volt source assuming the 12v source is capable of that. To be honest I am not sure how to figure out what your battery is capable of, just wanted to clear the air a bit.

Hope this helps a little.



Take a look at the circuit breakers in your house, I know Square D breakers have the Short circuit rating visible on the breaker. Many times Homeowners mistakenly think they have a 10,000A breaker (which they somehow decide must mean 10Amps. I have seen this first hand many times :-laf

Doh!!! I completely forgot about this. :-laf :-laf :-laf There is a reason why TDR is here.

 

[KBennett]

If you have a source that has the potential for a 20,000A fault, a fuse rated 10,000A may not "open", and instead weld shut. In this instance you would have no protection

You can get a circuit breaker to weld shut but a fuse will not. A fuse will typically act faster than a breaker and completely melt or vaporize when an overcurrent condition occurs. A typical DC circuit breaker used in vehicles will not last through a short circuit but are really good for overloads.

I have killed a few DC breakers in my past

 

[rbattelle]

:-laf :-laf :-laf Are you nuts? are you saying that you could weld ship steel with a standard 12V battery?

I thought he meant if you took a big cable or piece of metal and connected it right between the (+) and (-) battery posts. Wouldn't that result in a massive current spike?



-Ryan

 

[AMink]

Xantrex's reply

I emailed Xantrex about my question. Here is the response.

I wish that they would remove that passage from the book. If you use a Class T fuse, then you don't have to worry about the short circuit current. Basically this is talking about the fact that you need to make sure that if the fuse blows, that it will still withstand the possible short of the battery and not arc and continue providing current to the short circuit.



Just use #2 AWG with a 200A Class T fuse and don't go over 5 feet in length. These fuses can be purchased from us at http://www.xantrex.com/productcart/pc/viewPrd.asp?idcategory=27&idproduct=93 or from just about any good RV and Marine store.

Much simpler! Of course, by the time I get a fuse, a fuse block, new wire, and the connecters, I'll have more money in the hookup than the inverter. By the way, the Class T fuse is rated at 20,000 amp interupt. Most other "big" marine fuses that were at the same store were rated at 2000 to 6000 amp interupt. Once again, not sure what all that means, but I'm learning!