12V Battery load tester

[Gregory]

I am looking for a circuit ,12V, 40Amp load tester for a car battery I would like to build one.
Can you help me with this request.
have a good new year Thank you, Greg.

 

[kchriste]

Well, all you need is a 0.3Ω 500W resistor and a voltmeter.

 

[Leftyretro]

The following shows a typical "electronic load" circuit. This would not handle 40amps and would have to be modified.

Variable High Power “Resistor” For Power Supply Testing | Free Circuit Diagram


I would be tempted to use a large N-channel MOSFET that could be driven by the pot adjustable pin. Keep in mind that electronic loads will only handle the current for a short while unless a lot of thought goes into heat management in the way of large heat sinks and fans. A continuous 40 amp 12V load is almost 500 watts of heat being disappated, so plan and deal with the heat.

Lefty

 

[Hayato]

Maybe some NiCr wire?

 

[rjvh]

I am looking for a circuit ,12V, 40Amp load tester for a car battery I would like to build one.
Can you help me with this request.
have a good new year Thank you, Greg.

All you need is a 0.3Ω resistor 480 Watt and a voltmeter and 1 safety fence for the resistor (heater)

Robert-Jan

 

[Boncuk]

Or you need my 1KW variable current sink design. The size is 600X600X200mm, occupied by some electronic parts and heat sinks + fans.

Boncuk

 

[flat5]

Some wire in a big jar of water. Might work for a few minutes.

 

[bassnut]

Maybe some NiCr wire?

Thats the simplest, let it glow

The hard part is analysing the result in terms of usefull capacity.

 

[duffy]

Nichrome changes resistance when it heats up, don't use that. I kind of like flat5's idea... about two-hundred feet of twelve-gauge solid enameled wire in a bucket of water would be about .3 ohms and ought to handle 500 watts.

 

[Matsukaze]

You can buy a small load tester for about $15. I doubt that you can build one that cheap.

You can see the insides of a larger 500A tester at this site. It uses a pile of carbon disks as its resistance element. Changing the pressure on the disks changes the resistance of the pile.

 

[Hayato]

Nichrome changes resistance when it heats up, don't use that. I kind of like flat5's idea... about two-hundred feet of twelve-gauge solid enameled wire in a bucket of water would be about .3 ohms and ought to handle 500 watts.

Cooper changes more its resistance properties with the temperature than nichrome.

Actually, temperature coefficient of cooper is 10x higher than nichrome. That's why nichrome is used as heating element.


Take a look at the graphic:

 

[duffy]

Thanks!

So you could use nichrome wire - two feet of 1/4" x .018" nichrome ribbon would be .24 ohms cold, .29 ohms at 1000°F, and would only cost about $6.

Or you could buy a big-ass power resistor for $50 - $100.

 

[Gregory]

Could I use copper bars or brass bars for a big resistor to do the same thing as I have them on hand 100 x 4mm x 25mm I have 4 of them.

 

[Leftyretro]

You can buy a small load tester for about $15. I doubt that you can build one that cheap.

You can see the insides of a larger 500A tester at this site. It uses a pile of carbon disks as its resistance element. Changing the pressure on the disks changes the resistance of the pile.

Boy, that bad boy must spark a bunch when you place the clamps on a good battery.

Lefty

 

[Hayato]

Could I use copper bars or brass bars for a big resistor to do the same thing as I have them on hand 100 x 4mm x 25mm I have 4 of them.

When you use that, the resistance will raise significantly, as well.

Why do you need a 40A battery tester? That's a lot of current.

 

[duffy]

Shoot, just buy that $15 Matsukaze gave a link to.

 

[Gregory]

I require a load tester to test different 12V Car battery's . I have chosen 40 Amp as a all round current Do you think this to high if so what current would you select.


By making this load tester helps me to understand more and to learn

 

[Hayato]

I think a 10 amp will do fine.

40 Ah is not equal to 40 A. I think you confused that.

Most of cars have batteries around 40 Ah, not 40A. 40 Ah means that the battery can source 40 A for 1 hour, it's a "capacity" measurement.

A 40 A tester will be bulky, and will not test well. Because you can have a 40 Ah battery with 800A cold start current, so 40A will not be sufficient to test that.

There are several ways to test the battery charge.
The voltage, the internal resistence (this one you do with load), charge retainment...

 

[duffy]

Well then your next step would be the meter. Normally I would recommend digital, but for this I would say use the kind with a moving needle. Reason being that this is probably be the only occasion you will ever get to make a scale that reads Amps, Volts, and Watts all at the same time!

 

[Gregory]

Could I use stainless steel Bars as a resistor. Stainless has chrome and nickel