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Old Jun 24, 2007, 3:25 PM   #1
mrb
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Hi -

I finally ordered and received my new Maha C9000, alongwith some batteries. The charger is fairly simple to use, but I have a few questions regarding the information on the display:

1. I believe that the display gives the instantaneous charge or discharge rate (mA), instantaneous voltage, and elapsed time for that particular cycle for each cell. My question is regarding the capacity reading (mAh). Does this value mean anything during the charge/discharge cycles? I saw values up around 3700mAh while it was running break-in on my new Powerex 2700's.

2. I have seen some info on the Powerex cells saying that their actual capacity is often higher than the rated capacity. After a break-in on my 8 brand new Powerex 2700 cells, I received the following "available capacity" at the end of the cycle:

1st four: 2661, 2693, 2724, 2732

2nd four: 2653, 2659, 2669, 2688

Are these capacities where they should be for new cells following a break-in?

Isn't it best to use cells together with similar capacities?
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Old Jun 25, 2007, 10:49 AM   #2
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Hello Mrb,

The amount of charge put back in is interesting with NiMh charging, but the cell is rated for its discharge capacity. The discharge number is the more important one to look at.

On a normal 1 - 2 hour charge, you should end up putting 5 - 10% more than you took out. At lower charge rates the % you put in increases because the charge efficiency drops at lower charge rates.

The break in mode uses a very low charge rate and put 160% back in over 16 hours. This is the same method that is supposed to be used by the battery manufacturers to get the labeled capacity of the batteries. Once again, the capacity that we are interested in comes from the results of the 5 hour discharge.

Your break in numbers look reasonable.

The idea of matching on capacity is to group cells together that only differ by a maximum of around 2%. With your cells, you have 6 that fall into this range by setting aside the highest capacity cells for another 2 cell application. You can play with the grouping to get the difference between the highest and lowest capacity within 2% of each other.

If you need two sets of 4 cells, I would group them like this:

2732, 2724, 2693, 2688 and

2669, 2661, 2659, 2653.

As you can see, the second group is actually closer matched.

Tom
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Old Jun 26, 2007, 4:57 PM   #3
mrb
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Thank you Tom!

I also got a set of 4 IPower 2100 mAh AA'sfree with my order from Thomas Distributing. The available capacities after break-in were:

1164, 1282, 1468, 1637

This seems like quite a range and they seem low. Any opinions?
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Old Jun 28, 2007, 12:43 AM   #4
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Hello Mrb,

My humble opinion is that cells that come up with less than 80% of their labeled capacity should be considered "crap" cells. It is a good thing they were free. It won't hurt as much when you recycle them...

Tom
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Old Jun 28, 2007, 10:27 PM   #5
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Hi Tom -

I appreciate your "humble opinion". That's what I was afraid of. Like you said, at least I did not pay for them!
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