NiCad and NiMH batteries have similar discharge curves. They drop quickly, then stay almost constant. Finally they drop off very sharply when they are completely exhausted.
The batteries tested were on 700 mAh AA NiCad and 1200 mAh NiMH packs. The discharge curves below were generated by my battery charger and cycler (my own design that attaches to a PC for logging.) This design uses a unique method to plot discharge curves. The steps are as follows:
1. Apply load to batteries (approximately 500ma.)
2. Wait 4.8 seconds
3. Turn off current and wait 0.1 seconds.
4. Measure battery voltage.
5. Wait 0.1 seconds and repeat steps 1-5 until 1.0v/cell is reached
The above method will prevent any drops in the wires from effecting the results. Since the voltage is measured soon after the current is turned off, the battery does not have time to "self recover". This gives a value close to that of the actual load value.
In the plots below, the vertical scale is in volts. Time is measured across in minutes. No time scale is shown. I have placed two cursors in each plot. The red cursor is for the 50% point (in time, not voltage.) The green cursor is the full discharge point. The status bar on the bottom of the plot shows the voltage and time for each cursor in the appropriate color.
The small spikes in the plot are due to noise in my meter circuit (design is a prototype.) As you can see, the 50% point is at about 4.8 volts. The interesting thing is how the plot is nearly symmetrical about this point. At about 4.6 volts, the batteries really start to drop off. This is about as far you can "push" the batteries. I normally use 4.8 volts as the absolute limit. This gives me a good safety factor. The above plot showed that the batteries had a capacity of 720 mAh (86.7 minutes ate 500ma.) Slightly greater than their rated 700 mAh value.
As expected, the NiMH curve is almost the same except for the time factor. The 50% point again occurs at 4.8 volts. So, 4.7 volts would be my limit for NiMH batts. This graph calculated the capacity at 987 mAh (124.9 minutes at 500ma.) This battery was not peaked just before cycling so may not have had a full charge. Note how the last part of the curve the battery drops almost straight down, so you would have little warning if you reached this while flying.
I save the results for my packs. Later I can run the test again to see if the batteries are losing capacity.
These plots are for 4-cell packs. 8-cell (TX) packs would have exactly twice the voltage levels. When using a ESV meter with a load, the value measured may be slightly more or less due to differences in the way the voltages are measured and different loading.