Electronics How much current from a car battery?

This is kind of a strange question but... does anyone know how much current can I pull from a standard car battery for a breif period (30s to 60s) without causing damage? I would think 150 to 200A wouldn't be a problem.

 

Well check the manual of your car to see how much current the alternator sucks out when starting, maybe half it (for extended drain) and that would probably be about right...

 

it depends on the battery and the wiring, your estimate is close. The highest load I've come across is an electric winch, they're good for a few hundred amps or more.

 

Its not for use in a car so I don't need to worry about the alternator. But I looked into some manufacturer web sites and it looks like I can expect no less than 500A @ 0°C (most are closer to 700A), so 500A @ room temp should be no problem.

edit/ these numers are for 30s /edit

 

Now we've found out a little about this whole thing, what the hell are you using 500A for?

 

Well actually going to need a whole lot more than that. I'm in the high voltage industry and I want to look into building whats called a "high current injection" test set. The idea of it is to pump a whole lot of current through a breaker to either test its calibration/functionality or to test the output characteristics of the current transformers (CT's for short). In a typical installation the output of the CT's will fall from linearity under high/fault current conditions due to stray flux. When i do a retro-fit, replace the brains of a circuit breaker with a different (usually more advanced) unit, it would be nice to ensure that the sensors will operate properly up to the availible fault current. This fault current could be upwards of 35kA to 50kA.

Now for the car batteries... The commercially available units use a medium voltage input (usually 600V) and use a variable tap transformer to dial up the current. These units are BIG and HEAVY and can only supply about 10kA. I want a more portable unit so i'll use car batteries (quite a few paralled) to give me a power supply that can give enough output power short term. To control the current I'll use something like a class d amplifier. I'm not sure how feasable or cost effective this project will be so I'm going to start small. A singe car battery should give enough wattage to make a unit that supplies about 3kA.

 

A special WARNING.

Don't mess with car batteries, they explode and you can get badly hurt.
Just in case, I'm not talking about this without knowing, I'm a car mechanic.

 

I think you will find that everyday garden variety car batteries are not going to take that kind of service for very long. I would suspect that at the very least you would need a small forklift battery set. Heavy sustained currents will cause plate damage due to internal heating. The voltage on a typical car battery will fall to about 9 or 10 volts at maximum load due to internal resistance. To hold a steady voltage at high levels requires a specialty battery that is usually very large and heavy, due to construction designs for that type of current. Forklift batteries can deliver that kind of power without damage, since they are designed to do this all day everytime you start to move the forklift. Obviously they are way larger/heavier than what you want to use though.

Another possibility is large capacitor banks. The biggest determining factor is how long you need to maintain the current for at those types of levels.
If it is for short duration, capacitor banks are the usual way it is done. The banks are charged and then avalanched into the load with very large "hockey-puck" type SCR's. Most of the time large currents are only passed for less than 1 or so seconds, so capacitors are the way to go. This way is not simple or cheap either though.

Another problem is controlling the current in a repeatable fashion. This will involve serious power engineering. At high current levels into basically a dead short, your wiring and the internal resistance of the power source will be the limiting factor to what power levels are delivered to the "load."

I have worked on power supplies for galvanizing tanks in factories and these systems can deliver well over 20,000 amps at 5 to 18 Volts DC, powered from 600VAC three phase. The controls on these supplies are very complicated with failsafe and fall back protection since problems at these power levels go seriously wrong seriously fast, with messy ( expensive and dangerous ) results. I have detonated a few batteries, ruptured capacitors and made toxic dust out of some 300 dollar puck type SCR's, all while working on or repairing industrial equipment of this nature.

Not to put you off of an interesting project, and sorry if it seemed like a lecture but having been in a related field, I thought I would point out that there is quite a bit of engineering involved in this type of endeavor.