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Car battery AMPs question

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kenw232

New Member
I'm trying to magnetise a steel rod. So I need a large DC current through a coil.

So I bought a small car battery (actually ski-do battery or whatever). It was rated at 235 Cold Cranking Amps and is obviously 12.5V. When I hook up my coil directly to the battery and essentially short the battery I get pretty much nothing. If I measure the AMPs its 5A. The wires get warm but nothing at all happens.

Isn't it suppose to be extremely high amperage? why only 5A? I would assume it should blow the fuse in my multimeter (which is 10A max).

I had the battery checked and its fine.
 

RODALCO

Well-Known Member
You need heavy gauge wires to do this job

A starter motor has a resistance of 0.05 ohms or even less
U = I*R 12 = I*0.05 therefore I = 240 Amps.

Try a coil with 2 turns of 16mm² wire.

Be carefull not to spark at the accu terminals.
 

RCinFLA

Well-Known Member
The smaller the battery the higher the internal resistance.

The current limiting is the battery's Rs, wire resistance, and contacting resistance all adding up.

Best way is to wrap a few turns around the bar and just flash it with a quick swipe from a good sized battery. It only takes a short jolt to do it. Be careful though. If you weld the wire to the battery terminal the battery may explode.

Real magnetizers do this by charging a big capacitor and dumping it to a coil. This controls things better.
 

neo_star

New Member
I'm trying to magnetise a steel rod. So I need a large DC current through a coil.

So I bought a small car battery (actually ski-do battery or whatever). It was rated at 235 Cold Cranking Amps and is obviously 12.5V. When I hook up my coil directly to the battery and essentially short the battery I get pretty much nothing. If I measure the AMPs its 5A. The wires get warm but nothing at all happens.

Isn't it suppose to be extremely high amperage? why only 5A? I would assume it should blow the fuse in my multimeter (which is 10A max).

I had the battery checked and its fine.
The first thing u must know is that car batteries cannot supply or handle higher amperage. This is because the car batteries are essentially used for starting a car, driving headlamps and when the car is on road the internal alternator connected to the engine provides the necessary electricity for driving the head lamp and other electrical devices and in fact the battery also charges when ur electrical demand is low.

This one of the main reason why car/automobile batteries are not essentially used for running a ups(uninterrupted power supply) in ur home.

The second most important thing is that u r not supposed to short circuit a battery and especially the kind of battery u used. The reason u got 5 amp is that every battery has a limit to the amount of current that it can supply. If u had short circuited the battery long enough the electrodes in the battery would have melted and the battery permanently damaged.
 

MikeMl

Well-Known Member
Most Helpful Member
I'm trying to magnetise a steel rod. So I need a large DC current through a coil.

So I bought a small car battery (actually ski-do battery or whatever). It was rated at 235 Cold Cranking Amps and is obviously 12.5V. When I hook up my coil directly to the battery and essentially short the battery I get pretty much nothing. If I measure the AMPs its 5A. The wires get warm but nothing at all happens.

Isn't it suppose to be extremely high amperage? why only 5A? I would assume it should blow the fuse in my multimeter (which is 10A max).

I had the battery checked and its fine.
Assuming that your battery is ok, then the resistance of your coil must be R=E/I = 12/5 = 2.4Ω.

It matters not what the battery is capable of delivering during cranking. The current that will flow through your coil is determined only by Ohm's Law.
 

agis68

New Member
Usually a car battery of 12V has 25 or 27 and sometimes 32 and 45 AmpH....and works in maximum of 10A. But this when battery is in idle condition. When a car get starts the battery supports the engine and the multiplier rich the 3000V to ignite the car engine.
Your battery is maybe supportive type and not the main batter which has the above features as i told. An other idea is to build a very simple alternator to keep charging the battery while it works. Look on Google....
Permanent Magnet Alternator
How to Make an Alternator | eHow.com
Just what is an Alternator??
Coils and Magnets, How to build your Alternator / Generator
 

smanches

New Member
Wow, this thread has some of the most diverse answers I have ever seen. Of all I think MikeMi is correct. The coil you made has too high of a resistance.

On a side note, and what some others have been trying to say, don't use a car battery for high current if you have loose connections. Lead acid batteries discharge Hydrogen under heavy load, and if you happen to make a spark when that's happening, boom.

Make good connections to the battery with appropriate connectors and put a high current switch some distance away.
 

tcmtech

Banned
Most Helpful Member
Heres what you need to know.
Your battery is rated at 235 cold cranking amps. That means its designed to put out 235 amps at or above a specific temperature. Usually 32 degrees F but some times lower. What that means is it can provide a 235 amp current while maintaining 12 volts at the terminals. It will supply more amps than 235 but its output voltage will drop off rapidly. Also this output capacity is only for a short period typically 10 -20 seconds at best.

Ideally you would want a heavy piece of wire to be able to carry that current.
12 gauge copper can take short bursts of over 200 amps for a few seconds. Approximately 25 feet of 12 gauge will have a current draw of around 200 amps at 12 volts, less as it heats up. 14 gauge would be around 17 feet. And 16 gauge would be around 12 feet.

That should work well enough for magnetizing since it only takes a second to get iron magnetized if its the type that can be magnetized. Many types cant or at least wont hold their charge very long.

As far as connecting I would recommend using a heavy relay like whats used for vehicle starters or one of those old fashioned starter buttons they had on tractors. Your local auto parts store should have both for around $12.;)They can take the very high currents without problems. Plus getting your system down to a simple push button or other type of switch will make it safer and easier to work with.

200+ amps going through an iron core inductor (electromagnet) will produce a very high voltage and current inductive kickback when the circuit is broken. :eek:

Use solid and adequate sized connectors and wire if possible too.
 

Diver300

Well-Known Member
Most Helpful Member
Your battery is rated at 235 cold cranking amps. That means its designed to put out 235 amps at or above a specific temperature. Usually 32 degrees F but some times lower. What that means is it can provide a 235 amp current while maintaining 12 volts at the terminals.
Cranking is much harder on the battery than that. I can't find for sure what the test voltage is, but I think that it is 7.2 V. That is why anything that isn't needed for engine starting in a car shuts down while cranking.

(The rest of tcmtech's post is excellent advice)
 

tcmtech

Banned
Most Helpful Member
I have few friends who work at auto service centers and my information came from the operators manuals for their battery testers. Plus I also have an old service station battery charger that has the built in testing system. It shows a good 12 volt battery as being over 11 volts ,volt meter green zone, at the end of the amp load test. 10 - 11 volts are the yellow zone and below 10 volts is the red zone. However mine uses a fixed load of 350 amps. Small batteries typically show lower cranking voltages than bigger battery's because of that.
My 300 cranking amp garden tractor battery still shows around 10 - 11 volts while having the 350 amp load put on it but only for about 4 seconds then it drops off rapidly.

There other bigger old style true usage testers for commercial batteries I have seen that state a good battery as holding a minimum of 11.8 volts at the terminals while drawing their rated cranking capacity. They use the massive carbon pile or wire wound resistance load systems to be able actually draw that level of current.

However yes I have seen many that have different numbers as well. And different battery manufactures some times rate their battery's different also.

I have seen the new hand held digital ones pass bad battery's several times though and now dont trust them. They only spike the battery for a few milliseconds of heavy load. To me that does not realistically simulate real life cranking conditions of a larger engine in very cold weather. The old true test units would do the full load current test for 5 - 10 seconds when I have seen them used and thats also how long I do my tests for.

7.2 volts would never crank of anything that I am aware of that uses a 12 volt system.

put a good volt meter on your vehicle and pull the distributor wire off. Then read the actual battery terminal voltage while someone cranks it over. If your battery is new it will stay over 11 volts while you do it.
 
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