NiMH AA discharge rate - RC Groups
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Apr 26, 2007, 07:56 PM
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Oreo's Avatar
Question

NiMH AA discharge rate


I'm trying to calculate the wattage potential of 8 NiMH AA's. I've got Duracell 2650 mAH AA's. On Duracell's website it says "discharges at 2 C efficiently." I calculate that at 5.3A x 1.2V x 8cells = 51watts. I need more watts. So it would be nice if the AA's will discharge faster then Duracell claims. Any thoughts?

For the reccord, any difference with AAA's?
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Apr 26, 2007, 08:30 PM
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Murocflyer's Avatar
Good question. I'd like to know also.

Frank
Apr 26, 2007, 09:14 PM
Thank you, early adopters.
Miami Mike's Avatar
According to the Maximum Power Transfer Theorem, you'll get the maximum possible power from a battery cell when you draw enough current to drop the voltage to half of its no-load voltage, and that happens when the resistance of the load is equal to the internal resistance of the cell.

Internal resistances of NiMh cells are given in manufacturer's data sheets, and if I recall correctly, 30 milliohms is a typical value for a AA cell. Using that value as an example and 1.25 volts as the no-load voltage, that means that the current draw delivering maximum power would be 1.25 / (.030 + .030) = 21 amps at 0.625 volts, which will deliver about 13 watts per cell to the load and also dissipate 13 watts within each cell. (For multiple-cell packs, the current will be the same but the voltage and power will be multiplied by the number of cells in the pack.)

The practical limitation of operating a battery this way is the amount of power dissipated within the cell, because heat destroys batteries. The amount of current a cell can actually tolerate will depend on time on vs. time off, cooling, and the battery lifetime that you'll accept.
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Apr 26, 2007, 10:18 PM
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Oreo's Avatar
So, from what you're saying Mike, it sounds like my 8S AA pack has the potential to deliver significantly more then 5A, if at the expense of the batteries themselves. If that's correct, then I have a power potential of well more then 50 watts, yes?
Apr 27, 2007, 07:01 AM
Thank you, early adopters.
Miami Mike's Avatar
I don't know the actual internal resistance figure for your battery cells, and I should also point out that it's likely to change as temperature and state of charge change, but if you can find the internal resistance in a data sheet then that's how you can make use of it.

Most of us use those cells to power radios, receivers, and servos, where current draw is relatively low and internal resistance isn't important. For higher power applications like powering a motor in a model, there are more suitable cells available with less Mah capacity but much lower internal resistances.
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Apr 27, 2007, 11:52 AM
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AA-cells are typical low discharge cells.
For high discharge you should look for other batteries. The shape of the cells will often tell you if they are low or high discharge capable. High discharge cells are usually short and thick. AA-cells are long and thin...
As a "rule of thumbs" high discharge NiMH could often deliver appr 10C...

KAN, Sanyo SCR and some GP-cells are some of your options.


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