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12V battery load testing method

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Junior Member level 2
Sep 18, 2015
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iam designing a 12v battery load testing unit . i already designed switching resistance .75ohms 2W(parallel load) load using mosfet. now i looking for advance method for testing battery capacity. Help me with your new idea.
and thanks in advance..

I guess your battery chemistry is lead-acid? Then its capacity is rated at a 20 hour current. If its capacity is rated at 20Ah then you load it with 1A and if it lasts for 20 hours or more it is good.
Your 0.75 ohms load will draw 16A if the battery is huge and its voltage does not drop too much. I think 16A of current is too high for your battery for longer than a few minutes.
As your DC current would be 12/.75 = 16A a (192W), and you are using a 2 W resistor, you can only draw current for 2/192 (~1%) of the time. So if you keep pulsing away and measure the drop in voltage between current and no current, you can measure the internal resistance of the battery ( R = dV/16). The problem is unless you know how the internal resistance relates to the internal resistance, it will not get you very far.
The normal way would be to put the battery on a constant load of C/10 and measure the time it takes to get to the "end voltage". Pulsing it at 1% of the current will take 10 times longer.
Is this a 12 Volt Car Battery?
Or just a Small Sealed Lead Acid Battery?

Your .75 ohms might be Ok for a car battery, but it needs to be a Much Higher WATTAGE RESISTANCE.
iam going to test the batteries voltage health so that i can distribute to client without any flaws in it . this is why i need some testing circuit. by the way iam going to test the batteries only for 3sec or 5sec..

sorry the wattage of load is 20W. can u tell me the different possible way to test the 12v batteries.current rating may be varied from 40 to 50Ah according to the batteries

You can buy cheap ready-made car battery testers everywhere. I don't see the purpose of a MOSFET switch for this instrument.

Strictly speaking, the tester doesn't check the battery capacity (which is impossible without discharging the battery completely) but the charge state and internal resistance, the latter is an important criterion to determine battery health.
A 3 or 5 Second Test Won't tell you anything and a 20 Watt load is not good for very much current.

Using a Solenoid from a Car Starter motor would be better than a Mosfet for a good High Current Test.

Assuming the battery is Fully Charged, A load test of 50 Amps for a full minute would be more appropriate.
And if the battery is good, the battery voltage should not go below 12 Volts during the test.
That would Indicate it has a Low Internal Resistance.

This test would at least give a Good Comparison for comparing Multiple batteries.
Car battery testers are made by and used by companies who want to sell a fool a new battery. When getting an oil, lube and filter they want to "check" my brakes and rotate my tires so they can sell me new ones. They also "test" my battery. Of course the tests are rigged to show "failures".

I have driven for many years and sometimes for the life of a car on those "failures".
An electronics magazine had an article describing how to make a car battery tester, from a length of steel band commonly used to wrap bundles of large items for transport.

The resistance is very low, so you'll need several feet of the stuff. Hammer several nails into a board. Loop the steel band around the nails, zig-zag fashion. Air cooling is automatic.

You must make a very-low-ohm electrical transition from steel to the battery terminal. This is a challenge, and it is up to you to decide whether to weld copper cables to the steel, or use a nut-and-bolt method which requires clean contacts. Etc.
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