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Old 11-26-2015, 08:41 PM   #1
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Question Battery AH testing

Hello everyone, Thanks for allowing me into your group, I need some help please...I have a opportunity to purchase some used batts from a tornado alarm (siren) systems, I know nothing more about them until I go to take a look, my main question is does anyone know how to test the AH that are left in these batts, or know where I can purchase a cheap tester to tell me? Thanks in advance for ant comments/directions or suggestions.

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Old 11-26-2015, 08:55 PM   #2
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I've never heard of a meter to test AH. I believe AH rating is based on the design of the battery and the number and size of the plates in it. There is a meter to load test the batts. A load test is a test to assure that the batt. will still produce when it is in use. I would think Harbor Freight would have a load tester.

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Old 11-26-2015, 08:56 PM   #3
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Welcome to iRV2.

Personally, I believe used batteries are a real risk, they are being replaced for a reason.
A load tester would be your best bet, if you're set on purchasing them.
For more info this web site could provide some insight.

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Old 11-26-2015, 09:18 PM   #4
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There are a lot of issues but the biggest one is what is the intended use of the battery? Starting batteries are short term high current so the testers at HF or the gas station are designed that way. Deep cycle storage batteries for the house are low drain long time discharge. If you have the manufacturer's data sheet and can build something like a 20 Amp constant current load that you plot the voltage over time you can evaluate the battery that way. Run it for an hour or several depending and see how far the charge drops then compare to the spec. Accuracy will depend on your setup and how careful you are. Do a search and there is a lot of information out there.
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Old 11-26-2015, 09:24 PM   #5
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May be a good buy depending on age. In my working days at a Water District we had several diesel emergency standby generators that used 8D batteries some had single, some had duel a couple had four batteries. Because their importance we replaced the batteries every two years. Ours were starting batteries don't know what they would use for a tornado siren, maybe deep cycle.
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Old 11-26-2015, 09:47 PM   #6
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1. Fully charge the batteries.

2. Disconnect the battery in your car/light truck

3. Attach a battery to be tested (2 in series if they are 6 volt) to the car/truck with jumper cables.

4. Turn on the vehicle's headlights and monitor the voltage over time.

5. When you have dropped to 11.7 volts, you are at about 50% capacity and can compute the amp. hours based on your headlights and tail lights drawing 12 amps total.

Example: Battery reaches 11.7 volts at 10 hours. That's 12x10 = 120 amp hours drawn to reach 50%. Total capacity would be approx 240 AH.

Note that in actual use, you do not want to discharge batteries below 50%, so this test will give you the actual usable capacity. Also note that most capacity charts mention 12.0 volts being the 50% point, but that is a reading taken after removing the load and letting the batteries rest at least 12 hours before checking the voltage. In my experience, the 11.7 volt reading under light load equates to the 12.0 volt resting reading. Certainly close enough for real-world use, IMO.
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Old 11-26-2015, 10:34 PM   #7
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Batty AH testing

Load test box is something I keep around. Available for $50-75.00. Handy to have.

Need to ensure the battery has a good charge before testing.
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Old 11-27-2015, 09:15 AM   #8
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Thanks everyone for the input, I propose to set up a small PV system to power my bedroom via 2-3 of these batts being that is where I spend 99.9% of my life, I am pretty much disabled and can't/don't get up and around except to eat dinner (supper here in the south USA) and to go to the Living room to watch my youngest Grandson play (3 years, what a trip) I want to power my 24 X 7 products, TV, radio/clock, laptop, 9 watt LED light or 12 watt CFL. I have PV panels from 20-40 years ago that are unused and of different watts ect. I hope to set up a ATS for this circuit and use a relay to drop out DC power to inverter @ predetermined voltage and allow AC back over until batt voltage is built up to predetermined voltage, repeat, repeat. I had purchased 6 new 6v 180 ah batts before my injury(10.5 years ago) and they have set in the shed new in their boxes and ruined, what a shame, they were LA and sort of pricy at the time. Now that I barely survive on a small monthly income I hope to save a few bucks on power bill by using what I have, except for batts, ATS, dc voltage switch (pretty much a rely) not knowing the capacity of the batts prior to purchase increases the chance of reduced usage, or possible failure. There are AH testers (trimetric, pentimetric) out there, but about $350.00 or so, and I can't afford that no way, no how. I don't have the time to do a charge/discharge test, these are located at a mans business, although he told me I can pick and choose, so I will take my Fluke DMM, digital thermometer, SG tester, and torpedo level, these test I know how to do, just hoping someone knows of a quick AH test that is cheap, and does not require history to calculate remaining capacity, There are some "meters" on ebay from China that states it will read AH, but asking a non proper English speaking person questions is quiet a task....
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Old 11-27-2015, 10:37 AM   #9
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Ask why changed.

If calender then proceed.

On past life managed about 500 cell sites which are operated with backup batteries.

They budget for battery replacement by in use time or equipment upgrade.

Alarm companies usually require good maintenance due to nature of the operation so failed batteries usually are replaced as found.

So what will they let you do for testing?

Assuming you will not be able to charge and have limited time do the following.

Confirm via a Google search the manufacturer of the battery to get the data sheet as well as where the date code is located and how to decode it.

Go to harbor freight and get their 500 amp load brick and a volt meter if you do not already have one.

Have a sharpie to mark your score on the batteries.

Selecting batteries.

First Look for the newest ones.

If they replace as needed there may be a few months old in the group of years old.

Mark the date in easy to see place.

Next measure and write voltage in easy to see place.

Assume taken out of live site they should have been floating and full charge but over time self discharge.

Highest static voltage in better batteries...usually.

Next use the load brick and apply the same load...Use max and try to do a 5 count so test can be repeated and note the reading on the meter.

Last re measure the static voltage on the batteries and note it.

Now select from those the batteries that have the highest voltages and readings.

Last test.

Pick up a non selected battery and hold it 1 foot above ground and drop it flat...miss your toes.


If it fails then be weary...

We had a manufacturer defect that caused a 960 battery replacement of 160 amp hour batteries and the above was a test to determine if the group was involved.

Do NOT do above for your selected battery as it could damage it but a well built battery should not be damaged by one "hard road bump" , the test is to see how the line of batteries you are looking at are built.

If it fails then test a few more same way and choose units that pass.

All of this is assuming you are able to select from the scrap pile and buy for scrap price.

To test for actual capacity requires a full charge and timed discharge.

A headlight works wonders.

If you can take a collection of batteries to further test then that will allow more testing.

If not grab twice what you need.
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Old 11-27-2015, 10:46 AM   #10
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Just saw last post.

There is no instrument on the market that can indicate capacity in minutes.

Just follow steps above to get the best of what is there then charge for a day or three to insure battery is floating.

Clip on a headlight and see how long it takes to get the voltage to milestones.

Say 12.5 to 10.5 in 0.5 volt steps on one battery.

10.5 is fully discharged.

That will tell you capacity of any battery fully discharged if you know the current in amps.

Reason for mile stone measurement is other batteries can be checked for time at those voltages and compared.

Charted in a spreadsheet you can select best batteries and easily retest later.

If yiu can get for scrap price buy twice what you think you need plus a couple more...

If VRLA they have a long design life.

Search the forum for much more reading.
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Old 11-27-2015, 10:49 AM   #11
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Originally Posted by Skip426 View Post
Welcome to iRV2.
Personally, I believe used batteries are a real risk, they are being replaced for a reason.
A load tester would be your best bet, if you're set on purchasing them.
For more info this web site could provide some insight.
Many corporations, private companies and municipalities with "unlimited budgets" routinely replace perfectly good batteries.
(Some lucky RVers are able to take advantage of that policy).
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