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Author Topic: Want to build a Gel Cell battery tester  (Read 7979 times)
KC4JCX
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« on: July 06, 2012, 06:09:32 AM »

I have been given a bunch of 7Ah batteries that I would like to use as backup power.
Would like to know the best way to test them.
I found articles on building chargers but none on testers.

Any help would be greatly appreciated

Thanks,

Randy
KC4JCX
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G3RZP
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Posts: 4954




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« Reply #1 on: July 06, 2012, 07:02:42 AM »

There used to be - at least over here - testers for car batteries. These were a physically big resistor  made of nichrome tape and a voltmeter: the resistor was something like 0.2 ohms, there were two substantial prods for connecting to the battery and a voltmeter.

Probably a good sized resistor that would pull maybe 5 amps and voltmeter: you don't have the resistor connected for longer than it takes to read the volts, though.
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AA4PB
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« Reply #2 on: July 06, 2012, 08:15:26 AM »

A resistor load, a voltmeter, and a clock  should do it. Normally, you want to test at a 10-hour discharge rate. For a 7AH battery that is 0.7A which at 12V takes a 17 Ohm, 8.5W resistor.

For a reasonably accurate test, make sure the battery is fully charged. Then connect the load and a voltmeter and see how long it takes to discharge the battery down to 10 volts. If it takes 10 hours then you've got a good battery. If it takes only 5 hours then the battery is half capacity (3.5 AH). The capacity will vary some with temperature so don't do your test at extreme temperatures.

Of course you could get real fancy and get a PC based voltmeter that would automatically record the voltage over time and give you a graph of the discharge rate. You could also get more accuracy by building a current limiting circuit into the load so that it adjusts the load resistance such that it always draws 0.7A regardless of the voltage. A 17 Ohm resistor will draw 0.7A at 12V but only 0.58A at 10V - which will make your battery look a little bit better than it really is.
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W5FYI
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« Reply #3 on: July 06, 2012, 08:15:40 AM »

Here is an idea for a fairly realistic battery test: First, determine the average current you expect to draw from the rig. This will be the transmit current x transmit time + receive current x receive time. Next, find some filament lamps that in series/parallel combination will equal, or come close, to that figure. Charge up the battery, connect a voltmeter across its terminals, and connect the lamps. Note the difference between the start time and the time the battery reaches 11 volts, and that gives you a fairly good estimate of operating time with your particular radio.

Let's say you have a radio that consumes 1 amp in the receive mode, and draws 4 amps when transmitting. And let's say you spend about 50 minutes listening and 10 minutes transmitting out of every hour. That means, every hour the rig draws 5/6 x 1 + 1/6 x 4, or 5/6 + 4/6 = 9/6 = 1.5 amps.

So you need a lamp bank that draws 1.5 amps. An 1141 or 1142 bulb comes awfully close (1.44 amps at 12.8V), so wire one across a fully charged 12V battery and start your timer. When a voltmeter attached to the battery's terminals reads 11V, stop the timer. The time from start-of-test to end-of-test will give you a reasonable expectation of the amount of time you can get from using that particular gel cell. GL
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K5LXP
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« Reply #4 on: July 06, 2012, 03:18:35 PM »

W5FYI pretty much nails it.  For years I used an 1156 tail light bulb as my 7Ah gel test constant.  I take them all the way down to 10.5V but as long as you're consistent from battery to battery and test to test, you'll get a good relative merit.

The missing factor in the above advice is the charge cycle that precedes the test.  If the battery isn't 100% charged, then any test you do will yield an incorrect value.  If you have a gel cell charger you trust, use that or go with a bench supply that's both current and voltage limited.  Be sure to give the battery an adequate soak time after the bulk phase.  With my automatic charger I would leave it connected for at least 24 hours before doing a capacity test.  If you get an abnormal result, repeat the charge cycle and try again, as batteries that have sat a long time will take a few cycles to recover.  I write the date and Ah right on the battery with a sharpie so there's no confusing the merit of each unit.


Mark K5LXP
Albuquerque, NM
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G3RZP
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« Reply #5 on: July 06, 2012, 03:30:06 PM »

How do long term, low current tests check for high internal resistance?
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AA4PB
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« Reply #6 on: July 06, 2012, 06:21:48 PM »

Most 7AH gel cell batteries are specified by the mfg to supply 7AH capacity with a 10-hour load down to 10V. If you test it at a heavier load then it won't supply 7AH. Only by testing at the mfg specs will you get a capacity number that you can compare to the mfgs specs.

If the battery has a high internal resistance then you won't get the 7AH capacity either. You will see the battery voltage immediatly drop when you apply the 10-hour load whereas a fully charged, good battery will maintain 12V or more for several hours at a 10-hour load. Remember, we are talking about a fairly small 7AH gel cell here. If you were testing an automotive starting battery (designed to supply large currents for a short time) then you would use a different load test (a lot more current) to discover high internal resistance.

The test with a load matched to the radio average current will give you a pretty good idea of how long your radio will operate on the battery but it probably won't provide the 7AH specified capacity. It can also be used to compare several batteries *used with your radio* but it won't give you a number that you can compare to the original battery mfg specs (i.e. its rated AH capacity).

Gel Cell batteries in parallel (assuming all 12V batteries) don't need to be matched very closely because the voltage will equalize. If you have a 7AH battery in parallel with a 3AH battery (either by design or because one battery is weak) you will get a total fairly close to 10AH.

Batteries in series is a totally different matter. The smaller capacity battery will discharge first and then be reverse charged by the other batteries in the series string. This usually overheats and completely destroys the weaker battery in the string. Batteries in a series string need to be pretty closely matched in their actual AH capacity.
« Last Edit: July 06, 2012, 06:26:34 PM by AA4PB » Logged
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