Someone please tell me about these NMH Batterries... - RC Groups
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May 16, 2003, 05:42 PM
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Someone please tell me about these NMH Batterries...

Here is what I have, 16 New NMH 1800Mah AA Batterries and I will be useing them with my electric planes and I would like to know what rate should I be charging these at and how long? Also could someone tell me what would be the best motor size to run with them, like would a pack of 8 cells 9.6 volts be enough for say a speed 400 or two 280's?
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May 16, 2003, 07:08 PM
Too lazy to repair!
jperch's Avatar
To be completely safe, you should charge those cells at what is know as a C/10 rate. The "C" stands for the capacity of the cell. In your case C = 1800ma. So, you should charge them with a 180ma current.

That is the official recommendation. Now for the real use. If this is the first time these cells have been charged, then this is what I would do. First, build your pack. Then charge them as I described above at the C/10 rate for 14 hours. This will give all the cells a chance to equalize before you begin beating on them. When this charge is complete, discharge them at about a C rate (1800ma) until the pack reaches a voltage of about 0.9-1.0 volts per cell. Now you can start beating on them.

Since they are NiMH cells, I would not charge them at a rate higher then 2C (3600ma). The time it takes to do this should be about 30 minutes. However, I would highly recommend that you not rely on time. Invest in a good quality peak detecting charger. If you are going to charge faster than C/10, you really want your charger to be looking for the peak voltage and not rely on time. If you really want to be safe, some chargers have the ability to measure the temperature of the pack while it is charging. If the peak is not detected but the pack starts to get too warm, the charger will terminate the fast charge cycle. This is a very nice feature, but I have never found it necessary.

As for discharging. This depends on the internal impedance of the cells. Some here will refer to this as internal resistance or IR. To avoid confusion, I will also refer to it as IR for brevity. You can think of the IR as a resistor that is connected in series with the cell that you cannot disconnect. When current passes through a resistance, there is a voltage drop. We see this as the pack voltage dropping when we pull current from our battery packs. The higher the IR, the more the voltage will drop with a given amount of current. Since I am not familiar with your cells, I can't make any guesses about your cells' IR. Basically, you want the voltage to stay above 1.1 or at least 1.0 per cell when the pack is under load. Some may say 0.9V is also okay. It really depends on your application. Folks that really pull a lot of current, are typically willing to live with a greater drop because the have to.

What you might want to do is measure how much current your motor and prop pull with a pack size that you intend to use. Then try to put that much load on these cells and see how much the voltage drops. If it drops too much, you can't use them at that load. Try a smaller load.

I hope this helps,
May 16, 2003, 08:12 PM
Space Coast USA
hoppy's Avatar
Joe gave you the straight poop. I would recommend the manufacturers charge rate of 1C, or in your case, about 2A, to be safe. Some batteries do not take well to 2C charging.

The AA's will work with speed 280's and speed 300's forever and with speed 400's for awhile.