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Old Jan 28, 2005, 12:41 AM   #1
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:roll:using AA rechargeables in my field work I found unexpected failures in brand new nimh AA cells. To test what capacity they had on a relative scale I used a 3 Volt Christmas tree lamp soldered across the battery holder terminals of a quartz analog clock I bought at the dollar store. Set it to 1200 O'clock, insert the fully charged battery and come back tomorrow to see what it did. So far nothing outperforms the Eveready 2500 mah NiMH, running over 15 hrs with estimated 100-140 ma lamp load. Now when I need something I can trust I use the Eveready 2500 mah or the Ray-O-Vac 15 min cells and charger. Both are really good except the smaller AAA Ray-O-Vac cells go "out-to-lunch" some times after a charge and recover sometime in the range of 5 minutes to 2 hours (internal pressure switch).
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Old Jan 28, 2005, 2:21 AM   #2
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Are all the batteries tested 2500mAh?
What are the "unexpected failures in brand new NiMH AA cells"?
And maybe you can post the performance of the other brands you tested.
How many tests where done?
And how many cells for each brands were used in the test?

Would really appreciate the infos you can share. Thanks.
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Old Jan 28, 2005, 9:56 AM   #3
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:idea:The testing was of necessity and not intended to be statistically significant or very scientific although times over 1000 minutes does amont to 4 significant figures.

The problem was that I bought a couple dozen NexCell AA NiMH cells rated 2100 MAH at a hamfest (fleamarket or radio amateurs). Whether I ruined them by my charging methods or they were rejects or floor sweepings I will never know. The problem is that some can only run the clock for 20 minutes but appear normal (no leaks or crystals at the end) and cannot deliver useful amounts of current. I discovered the problem when my GPS ran the batteries down and turned off while I was still loading equipment in the car for a job I was going out on.

Results on the NexCells were 772, 838, 415, 0, 31, 25,730, 43, 105, 662, & 240 minutes of operating the Christmas tree light until the clock stopped. I emailed NexCell asking if they were interested in doing an autopsy on them but got no reply.

Results on Energizer 2500mah so far were 914, 946, 946, 921, 1101, 833, 1034, 946 minutes of operation.

One Yuasadelta1450 mah cell tested 783 minutes.

Four "Power 2000" 2500 mah cells ran 808, 801, 747, & 716 minutes but this was using a set up of 4 new clocks that could very well have different cut off points than the original clock. The group of 4 clocks was just a quicker way to detect "duds" not intended to be identical to the original clock. I still have a couple dozen Power 2000 cells I have not tested.

The chargers I have used included Hitech IC Advanced Battery Charger Model IC-1012, the Ray-O-Vac PS-6, Panasonic BQ-88 (low tech constant current charger) and the Energizer CHDC (8 hour charger comes with 4x2500 AA cells).


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Old Jan 28, 2005, 6:48 PM   #4
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Did you use the same charger for all the batteries during each test? Do you have a detailed datasheet?
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Old Jan 29, 2005, 1:49 AM   #5
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:?My mission was to weed out the duds and I charged them with the Ray-O-Vac (non-15 min mode for ordinary batteries) and later did some with the Eveready charger.

I could see from repeat test on some cells the the type charger will be important to get consistency. Even so the bad cells stand out. I just tested 4 NexCell AA cells charged on the Eveready charger and got: 744, 554, 513, & 28 min respectively (the last one is obviously a dud.

When I get the duds weeded out I may go back and start retesting in a more scientific way. My first thought was to use a digital voltmeter into my laptop and write a program to do a cumulative mah total based on the voltage of the particular bulb andit's voltage-current characteristics. As an engineer I would really like to do it right but when I saw the clocks for a buck a piece and realized theycould time the life of a battery under load, cheap and dirty won out over engineering correctness.

Sources I have read tell me that battery capacity will actually increase during the first several cycles of use. We shall see. Don't have a data sheet, here is a photo.
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Old Jan 29, 2005, 11:19 AM   #6
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http://www.imaging-resource.com/ACCS/BATTS/BATTS.HTM
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Old Jan 29, 2005, 11:54 AM   #7
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:shock:Thanks for a most excellent resource. Takes the pressure off me of retesting everything in a scientific way. I still have to screen out my duds. Thanks much.
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Old Jan 30, 2005, 1:33 AM   #8
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:-)I think I probably damaged the cells myself. There are these holders that take 4 AA cells and parallel them as a pack the size of a D-cell. I think by taking one cell out and running it down, I ruined it by putting it back in parallel with the 3 full charged cells. I guess excessive current can ruin these NiMH cells. If that wasn't it then I probably cooked them by leaving them on a charger for weeks at a time.
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Old Jan 30, 2005, 1:33 AM   #9
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:-)I think I probably damaged the cells myself. There are these holders that take 4 AA cells and parallel them as a pack the size of a D-cell. I think by taking one cell out and running it down, I ruined it by putting it back in parallel with the 3 full charged cells. I guess excessive current can ruin these NiMH cells. If that wasn't it then I probably cooked them by leaving them on a charger for weeks at a time.
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Old Jan 30, 2005, 3:18 AM   #10
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palatineroger wrote:
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I ruined it by putting it back in parallel with the 3 full charged cells.
This is what many sources erroneously call 'reverse polarity'. It should be called something like 'over-discharge'. It means electrolysing the active material in the cell in the discharge sense, after it's all gone, driven by the cells that are still live. This will force the next possible electrochemical reaction to occur, for which the cell wasn't designed, or at best serious overheating, if another electrolysis reaction doesn't exist. Either way, it's likely to be irreversible.

Of course rechargeable cells are designed to operate with their polarity in either direction (charge or discharge). So 'reverse polarity' is a meaningless term that should be abandoned.

Because of this hazard, it is wise, as you know, to keep sets of cells in similar condition together, and always treat the whole set identically.
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