Well, curiosity got the better of me again, so following on from my previous experiment with a cheap brand of NiMH cell I did the same experiment with an Eneloop. This is a cell with a previously tested actual capacity of about 1900 mAh.
The test protocol was the same as before: charge at 1600 mA and record voltage, charge, and temperature for later analysis.
So here is the result of experiment 1, using the automatic charging mode of the MH-C9000:
The first point of interest is that the cell didn't come close to the 0 ΔV point before the C9000 stopped charging. This was apparently due to the maximum voltage safety termination that the C9000 applies at 1.47 V. The second point of interest is that the cell remained quite cool at a maximum 34°C throughout the charge. This contrasts significantly with the previous cheap cell which got steadily hotter throughout the charge.
Charging stopped after 72 minutes with a supplied charge of 1754 mAh. A subsequent discharge at 500 mA give a measurement of 1763 mAh, so the charging efficiency was essentially 100%. Note how this compares with the 80% measured for the cheap cell.
Since the C9000 kindly stopped charging before the inflection point or the −ΔV point was reached, it became necessary to perform experiment 2. This was the same as experiment 1, charging at 1600 mA, but with manual charge termination instead of automatic:
(I have removed the experimental data points from the chart for clarity and have just shown the trend lines.)
This time the voltage reached a maximum of 1.54 V and the cell temperature started climbing rapidly. I pulled the plug when the temperature reached 40°C (poor Eneloop).
With this new test it is possible to look at the slope of the voltage curve and find the inflection point, which occurred at just a little over 1800 mAh:
Finally, we can examine the end of charge conditions in more detail:
The C9000 would have terminated at 1.47 V and 1720 mAh, working out at 1720/1900 = 90% of a full charge and an end point temperature of 34°C. The inflection point algorithm would have terminated just past 1800 mAh, this resulting in 95% of a full charge and much the same temperature of 34°C. Lastly the −ΔV signal would have been detected at about 1960 mAh and 38°C. Presumably this would have resulted in a 100% complete charge. I'm discharging the cell at the moment to see what charge it actually retained. (Edit: the discharge test at 500 mA showed 1834 mAh.)
I think the conclusion from testing the Eneloop is that it clearly demonstrates a much better level of performance than the cheap cell. It works at a higher voltage (the high charging voltage is reflected in higher discharge voltages), it accepts charge more efficiently, and it remains much cooler during charging.
Secondly, there is the interesting result that using the C9000 to charge Eneloops gives an early, low temperature charge termination that approximates what the inflection test would do. It means that to get a 100% charge you have to leave the cells on the charger for two hours of top-off charging (100 mA x 2 hours = 200 mAh, 1700 mAh + 200 mAh = 1900 mAh). On the other hand, the early termination will increase the cycle life of the cells.