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Old Jun 16, 2006, 7:00 PM   #1
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For their initial charge, I charged my Powerex AA 2700s in my BC-900 charger @ 200 mA. I let them go for 18 hours, and all 4 batteries were only showing between 1200 and 1250 mA charge capacity. I took them out of the BC-900 this morning around 9, and put them in my MH-C204W charger about an hour ago, which charges @ 1.0 A, and they're stillcharging.

I'mwondering what may be causing this. Do I need to charge these @ a higher rate in my BC-900, or need I just charge them with the C204W from now on?

Thanks!


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Old Jun 17, 2006, 3:20 AM   #2
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Charging at such a low current (200 mA) makes it very difficult to sense end of charge. It is normal that your cells are getting undercharged. Personally, I wouldn't use 200 mA on the BC-900 for AA cells. Even for priming new cells 500 mA is more suitable.
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Old Jun 17, 2006, 8:26 AM   #3
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Thanks for your reply. They seem to have charged more thoroughly in the C204W, I charged them fully, then did a discharge/charge cycle, so they should be ready to go.

I found a possible explanation. The following was extracted from the article Charging Nickel -Based Batteries from batteryniversity.com:

"Nickel-metal-hydride should be rapid charged rather than slow charged. Because of poor overcharge absorption, the trickle charge must be lower than that of nickel-cadmium and is usually around 0.05C. This explains why the original nickel-cadmium charger cannot be used nickel-metal-hydride.

It is difficult, if not impossible, to slow-charge a nickel-metal-hydride. At a C?rate of 0.1-0.3C, the voltage and temperature profiles fail to exhibit defined characteristics to measure the full charge state accurately and the charger must rely on a timer. Harmful overcharge can occur if a partially or fully charged battery is charged with a fixed timer. The same occurs if the battery has aged and can only hold 50 instead of 100% charge. Overcharge could occur even though the battery feels cool to the touch."
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Old Jun 17, 2006, 5:54 PM   #4
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coppertrail wrote:
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It is difficult, if not impossible, to slow-charge a nickel-metal-hydride. At a C?rate of 0.1-0.3C, the voltage and temperature profiles fail to exhibit defined characteristics to measure the full charge state accurately and the charger must rely on a timer. Harmful overcharge can occur if a partially or fully charged battery is charged with a fixed timer. The same occurs if the battery has aged and can only hold 50 instead of 100% charge. Overcharge could occur even though the battery feels cool to the touch."
That's exactly what I mean. The magnitude of voltage drop, that most chargers use to terminate, depends on the charging current, i.e. charging at c/10 voltage drop may never be observed. At c/5 it is quite small initially and may not be detected by the sensing electronics until some overcharging occurrs.

style="BACKGROUND-COLOR: #000000"Back to more practical stuff, I have observed that the BC-900 is quite unreliable when charging high capacity NiMH cells at 200 mA. I don't know what methos the charger uses to terminate the charge at this low current but often AA NiMH cells that have been used together, and that are expected to be equally discharged (to +-10%) receive very uneven charging, say 300 % different.
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Old Jun 17, 2006, 9:30 PM   #5
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Here's what i've done:

* Charged them @ 200 mA for 18 hours on the BC-900

* Let them sit out of the charger for 9 hours.

* Charged them completely with the C204W and they trickle charged for about a few hours

*Performed a refresh cycle with the C204 and let them charge completely, thentrickle charge for a few hours.

* Put them in my camera

Would you say these batteries are "formed", or shouldI leave them charge a full 24 hours on the C204W to ensure that they're properly formed? What I'm getting at is this: Do you only have only one chance to "form" them, or can it be achieved after the initial charge?

Thanks again for your help . . .
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Old Jun 18, 2006, 10:50 AM   #6
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Hello Coppertrail,

It sounds like your BC-900 thought it saw a voltage drop and terminated the charge early. This is why you were able to continue the charge on your other charger.

A "proper" forming charge is done on a stupid charger that does not sense any end of charge signal. The end of charge signal is a timer that shuts things off. The charge rate is 1/10 of the capacity of the battery, and the charge time is 14-16 hours.

The BC-900 can be used for this, but you have to keep an eye on things. You can use the display button to determine the charge rate that the unit is charging at. If you see it drop below your initial charge rate, you need to unplug the charger and plug it back in to get it charging again. You may have to do this a few times with new cells.

With your 2700 mAh batteries, pick a rate that is close to 270 mA on the BC-900, set a timer for 14-16 hours, and check on the progress every so often. If the charge current drops on the BC-900, unplug it, plug it back in, set your charge current, and continue. When your timer goes off, stop the charge and pull the batteries off of the charger.

The purpose of the forming charge is to evenly distribute the electrolyte throughout the battery, and to balance the battery pack if several batteries are used together. Batteries become unbalanced through use and storage. I recommend doing a forming charge every 10-20 cycles depending on use, and before putting the battery into service after storage.

In light use, you can form a battery simply by using it. It may take 4-10 charge/discharge cycles to get to the full capacity and performance of the battery, but you will eventually get there. The problem with this method is that if your batteries are mis matched, your camera will get less shots than you think it should and you may have to recharge your batteries sooner.

Tom
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