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Old Jun 23, 2001, 06:19 PM
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How Do You Charge Your Nimh Packs?

I purchased a new 8cell 120mAh Nimh pack and placed it onto my FMA Supernova charger to charge the pack up. I cycled the pack three times and could only get about 80mAh out of it. I placed it back on the charger and tried a couple more times but could not get any more than the original 80mAh. I have the charger set for Nimh, 8 cells at 150mAh, a charge rate of 100mAh and a discharge rate of 500mAh. I can't figure out what I could be doing wrong? I understand that Nimh take a few cycles to get to full capacity but after about ten cycles I still can not get anywhere near 120mAh. When I plug the pack into the plane and run it up it seems to be lacking in power too. I run a 6 cell Nicad pack and I get more rpm’s from the motor. I contacted the dealer and he said to send the pack back but if I am doing something wrong I don't want to send it back and have them charge it up and find nothing wrong with the pack. I would really like to get this thing to work. Any help would be greatly appreciated...

Thank you,
Jeffrey Park
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Old Jun 23, 2001, 07:14 PM
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You should perform a slow charge for 12-14 hours at .01 amps to equalize the cells in the pack. You probably are having cells peak before others. When you discharge it (run motor) you notice the weak cells.

Good luck!

Steve
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Old Jun 23, 2001, 07:38 PM
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Quote:
Originally posted by stevem:
You should perform a slow charge for 12-14 hours at .01 amps to equalize the cells in the pack
No can do! I don't have a charger that goes down to .01 amp nor do I know of one that does. I am charging the pack with an FMA Supernova charger and do not want to have to buy another charger just to charge Nimh packs. If that is the case I will be sticking with Nicads.

Jeffrey Park

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Old Jun 23, 2001, 08:42 PM
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Quote:
Originally posted by OCModels:
I have the charger set for Nimh, 8 cells at 150mAh, a charge rate of 100mAh and a discharge rate of 500mAh.
Jeffery.

You are discharging at about 4C. I would think this would lower your discharge voltage and cause an early termination of your discharge cycle (cut off is usually about 0.9V/cell). The impedance of small NiMh cells would be pretty high.

I'm betting at a lower discharge rate you will achieve the 120 mAh capacity.

Rob
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Old Jun 23, 2001, 08:55 PM
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Quote:
Originally posted by rscamp:
Jeffery.

You are discharging at about 4C. I would think this would lower your discharge voltage and cause an early termination of your discharge cycle (cut off is usually about 0.9V/cell). The impedance of small NiMh cells would be pretty high.

I'm betting at a lower discharge rate you will achieve the 120 mAh capacity.

Rob
How can that be too high? The motor on my plane draws little over one amp. 500mAh would be about half what my motor is pulling.

Jeffrey Park


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Old Jun 23, 2001, 09:12 PM
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Smaller NIMH cells have a high internal resistance.
Are you sure that these cells will pull a 1 amp load? I doubt it. That is a 10C discharge, and even the best AA size (Sanyo 1600 Twicell) are at their limit at 16 amps (10C). At lower discharge rates you should get better than 80 mah. Can you discharge at less than 500 mah?

Just out of curiosity, why did you choose this particular cell/pack? Did anyone recommend it or have experience with it at these loads?

Shawn
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Old Jun 23, 2001, 09:53 PM
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Quote:
Originally posted by OCModels:
How can that be too high? The motor on my plane draws little over one amp. 500mAh would be about half what my motor is pulling.
Jeffery.

These are very small NiMh cells. Their internal resistance would be high enough to cause a relatively large voltage drop at .5A, let alone 1.0A.

You have already noticed that you are getting more power from your NiCds. In light of this, why would my previous message surprise you?

Cheers,

Rob
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Old Jun 23, 2001, 10:06 PM
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OK! I can understand that but why am I not getting a full pack when I charge it and why would it be so much Less than a 6 cell nicad pack??? I would think I would at least get the same if not better performance from an 8 cell Nimh pack. Am I wrong in assuming this? I went to 2 more cells to gain performance not lose it...

Jeffrey Park
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Old Jun 23, 2001, 11:07 PM
Rxb
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Are you using the auto charge setting on the FMA Supernova? The instructions tell you not to use it on NIMH packs just Nicad.

Rob
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Old Jun 23, 2001, 11:18 PM
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I rarely use the auto feature on this charger any more. I like being able to control the charge of each pack.

Jeffrey Park
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Old Jun 23, 2001, 11:37 PM
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To further clarify - The "capacity" of cells are given at a certain current. As that discharge current goes up, the usable capacity goes down. The 500 mah discharge may be very high for these cells and they can not perform at these levels.
Try a lower discharge and see if the numbers improve any. If so, then they are being pushed too hard.

Shawn
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