Discharge Cut-Off Comparison

This test will demonstrate how different discharge cut-off values determine the capacity and cycle count of NiMH batteries in high drain applications. The cut-off values in question are

• 0.5V (to simulate overdischarge)
• 0.9V
• 1.0V
• 1.1V

All tests are carried out on VARTA 2100 cells for the same reasons I used them in my High Current vs Low Current test.

The test conditions are the same for all cells - they are cycled with 1.5A current and charged using 0dV algorithm.

Results

Conclusion

This test simulated 4 high-drain devices, each with a different cut-off. The devices with 0.5V and 0.9V extracted roughly the same amount of capacity from the cell in the same number of cycles. This confirms there isn’t much capacity left below 0.9V and it does not make sense to discharge the cell deeper.

The device with 1.0V extracted less capacity from the cell as it hit the cut-off sooner. This was expected. But here comes the surprise. One would expect higher cycle count from partially discharged cell, right? Well, not so much for high-drain applications where internal impedance plays an important role. The bigger the cell’s impedance the bigger the voltage drop on it (100mV for every 66 mOhm at 1.5A current) leaving less and less voltage available for the device.

The same effect is more prevalent for the 1.1V device. First it’s unable to extract the cell’s whole capacity due to high cut-off, then it starts hitting the cut-off sooner as the internal impedance rises. 1.1V cut-off is way too high for high-drain applications.

Note: the 1.1V cell was still in good shape when the test finished (I was able to get some more cycles out of it when discharged to 0.9V). The 1.0V and especially the 0.9V and 0.5V ones were totally worn down.