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Old May 27, 2007, 03:23 AM
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New NiMh batteries

I've been using LiPo's so long now, I've forgotten how to cycle a brand new NiMh pack!
Please remind me.

Thanks.
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Old May 27, 2007, 06:43 AM
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C/10 for 15 hours.
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Old May 27, 2007, 07:17 AM
The original Flying Pigs Sqd.
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And what would the result be if I charged at 1C?
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Old May 27, 2007, 07:52 AM
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Hello Up&Away,

Individual cells are easier to deal with than a pack. With a pack you have to watch for false peaking and get the cells in the pack in balance.

On top of this, you need to evenly distribute the electrolyte within the cell, and give the separator a chance to evenly soak up the electrolyte. This is best done at a rate that does not harm the cells when some of them go into an overcharge condition while the others are catching up. This charge is sometimes call the "forming" charge, but is more correctly known as a standard charge.

With individual cells, it is best to discharge them to 1.0 volt, then do a standard charge at 0.1C for 16 hours. After this, discharge at 0.5C and charge at 1C for a few cycles.

With a pack, it is better to start with the standard charge. This will totally eliminate the possibility of any cells going into a reverse charge and ruining them.

Start with the standard charge of 0.1C for 16 hours. Let the pack rest for 30 minutes. Discharge at 0.5 C to 1.0 volts per cell, followed by a 1C charge. Now you can do a few 1C discharge and 1C charge cycles, and you should be ready to go.

At the end of the charge, all the cells should be evenly warm. If you have a cold cell, your pack is not in balance and you need to spend a couple of hours topping off at 0.1C. At the end of your cycling, all the cells should end up evenly warm at the end of the charge.

To keep your cells vibrant during storage, discharge the pack down to 1.0 volts per cell and store them empty. If you anticipate long term storage, you may have to add a slight charge, something like 15 minutes at 1C. In storage, the pack should go through a charge/discharge cycle every 30 days. You can get away with longer periods of time between cycling them, but your pack will loose its "snap."

After long term storage (3 months or longer), you need to start the process again with the standard charge.

Tom
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Old May 27, 2007, 09:50 AM
The original Flying Pigs Sqd.
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So, after charging a brand new pack at 1C it should be discharged as slowly as possible, rested, and then charged at a low enough C for the charge to last approx. 16 hours?
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Old May 27, 2007, 11:13 AM
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Hello Up&Away,

That is not what I said... or at least not what I meant to say.

If you have already charged the pack at 1C, discharge it at 0.5C. Then do a standard charge of 0.1C for 16 hours. Then proceed with cycling.

0.1C is low enough that there will be minimal damage due to overcharging. This means that you could charge a pack that already had been charged without problems. However the electrolyte re-distribution and separator absorption of the electrolyte works better when starting with a discharged pack.

If this seems like a lot of extra effort, you can get almost the same results by simply cycling the cell about 10 times. 1.0C charging and 1.0C discharging. The problem is that some of the cells in the pack may suffer overcharge damage if the cells in the pack are not in balance.

I think there is also a slight risk of separator damage if you simply start out by cycling the pack. If your cells are high quality, you will most likely have no problems.

Tom

Edit to add:

The C rate referred to is related to the capacity of the cell or pack. A 1.0C charge rate on a 3300 mAh cell, or pack, would be 3.3 amps. A 1.0C discharge rate for the same would also be 3.3 amps. A 0.1C 16 hour charge would be a timed charge lasting 16 hours at a charge rate of 0.33 amps.

Also, a 0.5C discharge rate for the same 3300 cells would be discharging at 1.65 amps.
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Old May 27, 2007, 11:30 AM
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I cycled mine right into the trash
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Old May 27, 2007, 12:26 PM
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Hello Raydee,

As long as you got around 500 cycles from them, that is an interesting way to use them up...

Tom
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Old May 27, 2007, 01:06 PM
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I run mine in RC boats so I don't get anywhere near 500 cycles. Most of the boat racers are switching to Lipo since NiMh are only competitive for 30-50 cycles. At 90-100 amp draw Nimh cells just don't last long.
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Old May 27, 2007, 01:24 PM
The original Flying Pigs Sqd.
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Netanya, Israel
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Quote:
Originally Posted by SilverFoxCPF
Hello Up&Away,

That is not what I said... or at least not what I meant to say.

If you have already charged the pack at 1C, discharge it at 0.5C. Then do a standard charge of 0.1C for 16 hours. Then proceed with cycling.

0.1C is low enough that there will be minimal damage due to overcharging. This means that you could charge a pack that already had been charged without problems. However the electrolyte re-distribution and separator absorption of the electrolyte works better when starting with a discharged pack.

If this seems like a lot of extra effort, you can get almost the same results by simply cycling the cell about 10 times. 1.0C charging and 1.0C discharging. The problem is that some of the cells in the pack may suffer overcharge damage if the cells in the pack are not in balance.

I think there is also a slight risk of separator damage if you simply start out by cycling the pack. If your cells are high quality, you will most likely have no problems.

Tom

Edit to add:

The C rate referred to is related to the capacity of the cell or pack. A 1.0C charge rate on a 3300 mAh cell, or pack, would be 3.3 amps. A 1.0C discharge rate for the same would also be 3.3 amps. A 0.1C 16 hour charge would be a timed charge lasting 16 hours at a charge rate of 0.33 amps.

Also, a 0.5C discharge rate for the same 3300 cells would be discharging at 1.65 amps.
Yep, that's what I meant . Thanks.
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