Poor Man's Guides

Newsletter

July 2007

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Homemade Battery Load Tester

by Richard Lewis

In this newsletter I'll be showing you a very simple and inexpensive way to load test your batteries. This can work for car batteries for your homemade welder, or all the way up to forklift batteries like in my windmill book. If you have 6 volt batteries, you will have to hook two in series to make 12 volts, then test. If you have 24, 36, or 48 volt forklift batteries, then you will have to tap into just 12 volts at a time to test.

You want to have a known amperage for a length of time. An example would be 10 amps for 4 hours would be 40 amp hours. Or 5 amps for 10 hours would be 50 amp hours. You want a way to auto shutoff the tester so that it doesn't drain the battery too low and make it go bad. And you want the tester to show you exactly how long the test ran for. This should all be automated so you don't have to sit there and watch it. Believe me, it is long and boring.

The idea is really simple. You take a cheap inverter, usually about a 75 watt will do unless you need more than 5 amps. I've seen the cheap 75 watt inverters for $20 or so and sometimes on sale for half price. You can run 60 watts continuously off of it. The output of the inverter is connected to a cheap light socket with a 40 watt bulb ( for testing car batteries). You can also run multiple light sockets in parallel and use a bigger inverter for testing forklift batteries. Also, the output of the inverter should run a 120 volt ac analog clock. Some old stoves have them and you can find home clocks that run off ac in thrift stores.

You note the time on the clock, turn on the inverter and let it run. When the battery gets down to about 11 volts or so, the inverter auto shuts off. Then log the time on the clock as the stop time. Then take the hours it ran and multiply times the amps that the test ran at. An example would be 2 hours 43 minutes @ 3.3 amps. Take the minutes and divide by 60

( 43 / 60 = 0.72 ) and add that to the hours ( 0.72 + 2 = 2.72 hours ). Then multiply times the amps ( 2.72 * 3.3 = 8.97 amp hours ).

The reason I used a 40 watt bulb is to get 3.3 amps. The reason I wanted that rating is because a typical car battery is about 70 amp hours and rated at 20 hours to discharge. And 70 AH divided by 20 hours is 3.5 amps. If you were testing a forklift battery that was 24 volts at 700 amp hours, then you would test half at a time ( 12 volt sections ). And 700 ah / 20 hours would be 35 amps. That would be 420 watts ( 35 amps * 12 volts ) or 7 light bulbs, 60w each. You would need a 500 watt inverter or so to do that test.

In the above example, you can see the car battery tested at 9 amp hours instead of 70. That means that it is running at about 13% ( 9 / 70 ) of what it should be.

Load Chart

AC light bulb watts DC amps from battery

120 watts = 10 amps

60 watts = 5 amps

40 watts = 3.3 amps

20 watts = 1.7 amps

Richard

Newsletter

July 2007

--------------------------------------------------------------------------------

Homemade Battery Load Tester

by Richard Lewis

In this newsletter I'll be showing you a very simple and inexpensive way to load test your batteries. This can work for car batteries for your homemade welder, or all the way up to forklift batteries like in my windmill book. If you have 6 volt batteries, you will have to hook two in series to make 12 volts, then test. If you have 24, 36, or 48 volt forklift batteries, then you will have to tap into just 12 volts at a time to test.

You want to have a known amperage for a length of time. An example would be 10 amps for 4 hours would be 40 amp hours. Or 5 amps for 10 hours would be 50 amp hours. You want a way to auto shutoff the tester so that it doesn't drain the battery too low and make it go bad. And you want the tester to show you exactly how long the test ran for. This should all be automated so you don't have to sit there and watch it. Believe me, it is long and boring.

The idea is really simple. You take a cheap inverter, usually about a 75 watt will do unless you need more than 5 amps. I've seen the cheap 75 watt inverters for $20 or so and sometimes on sale for half price. You can run 60 watts continuously off of it. The output of the inverter is connected to a cheap light socket with a 40 watt bulb ( for testing car batteries). You can also run multiple light sockets in parallel and use a bigger inverter for testing forklift batteries. Also, the output of the inverter should run a 120 volt ac analog clock. Some old stoves have them and you can find home clocks that run off ac in thrift stores.

You note the time on the clock, turn on the inverter and let it run. When the battery gets down to about 11 volts or so, the inverter auto shuts off. Then log the time on the clock as the stop time. Then take the hours it ran and multiply times the amps that the test ran at. An example would be 2 hours 43 minutes @ 3.3 amps. Take the minutes and divide by 60

( 43 / 60 = 0.72 ) and add that to the hours ( 0.72 + 2 = 2.72 hours ). Then multiply times the amps ( 2.72 * 3.3 = 8.97 amp hours ).

The reason I used a 40 watt bulb is to get 3.3 amps. The reason I wanted that rating is because a typical car battery is about 70 amp hours and rated at 20 hours to discharge. And 70 AH divided by 20 hours is 3.5 amps. If you were testing a forklift battery that was 24 volts at 700 amp hours, then you would test half at a time ( 12 volt sections ). And 700 ah / 20 hours would be 35 amps. That would be 420 watts ( 35 amps * 12 volts ) or 7 light bulbs, 60w each. You would need a 500 watt inverter or so to do that test.

In the above example, you can see the car battery tested at 9 amp hours instead of 70. That means that it is running at about 13% ( 9 / 70 ) of what it should be.

Load Chart

AC light bulb watts DC amps from battery

120 watts = 10 amps

60 watts = 5 amps

40 watts = 3.3 amps

20 watts = 1.7 amps

Richard