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Discharge data on 4/3 Fat A NiMH Cells

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Capacity versus discharge rate

This graph shows the effective capacity of two NiMH 4/3 Fat A cells. The PowerStream has a rating of 3850 mAH, and the Panasonic has a rating of 4500 mAH. As can be seen, the effective capacity of the Panasonic drops quickly as the discharge current increases. The PowerStream cells do not have such a severe drop off. The point is, that at low currents the difference between the cells is 650 mAH, at 3.5 amps the difference is only 200 mAH.

The reason for this is simple. In order to squeeze more capcity into the cells, Panasonic used thinner foil electrodes, which increased the impedance (resistance). At higher currents the voltage drop through the electrodes is severe. This is a good tradeoff if you are drawing low currents, but if you are drawing high currents, you might be better off buying the less expensive PowerStream cell.

Voltage versus time for different discharge currents

This curve shows the PowerStream 4/3 Fat A NiMH cell in a 12 volt, 10 cell pack. It shows the useful discharge time you can expect at different discharge currents. The end-of-discharge condition is usually considered to be 1 volt per cell, or 10 volts for this pack. You can see that there isn't much capacity left between 1 volt and zero volts per cell.

PowerStream 4/3 Fat A:
Current Nominal Rate Measured run time Expected run time Ratio Equivalent AH
0.525 A C/7 440 440 1 3.85
1.02 A C/4 226 226 1 3.85
1.5 A C/2.5 150 153 0.98 3.78
2.0 A C/2 114 115 0.99 3.80
2.5 A C/1.5 91 92 0.98 3.79
2.98 A C/1.3 75 77.5 0.97 3.72
3.5 A C/1.1 64 66 0.97 3.73
4.0 A C/1 56 57.8 0.97 3.73
4.5 A C/0.86 50 51.8 0.97 3.72
5.0 A C/0.77 44 min 46.2 0.95 3.67

In this chart the "expected run time" is what you would expect if you took the capacity measured at a C/20 discharge rate and divided it by the the discharge current. You can see that even at 0.77C, the PowerStream cells are still up to 95% of their 20 hour capacity. This indicates a high quality, well constructed cell that was designed for high current capability.

A similar chart shows the PanaSonic 4500 mAH 4/3 Fat A

Panasonic 4/3 Fat A:
Current Nominal
Rate
Measured Run Time Expected Run Time Ratio Equivalent AH
0.9 C/5 300 300 1 4.50
1.49 C/3 178 181 0.98 4.42
1.98 C/2 128 136 0.94 4.22
3.51 C/1.3 70 77 0.91 4.10

As you can see, at C/1.3 the capacity is already down to 91% of the rated capacity, whereas the PowerStream has 97% at C/1.3, and still has 95% at C/0.77.

For a chart of the sizes and weights of cylindrical batteries click here:Battery size chart
For more battery engineering resources click here:Battery Engineering Resources

Note: We don't inflate our specifications as many companies do. The capacity in amp-hours is guaranteed, and will be exceeded most of the time. "Usable cycles" is a guideline, but not guaranteed because we have no control over abuse of the cells.


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