While I do not have a Trackpad, I do have a Magic Mouse and wireless keyboard and I use rechargeable NiMh batteries in both and I have made the following observations.
Although alkaline cells are rated at 1.5 volts and NiMH cells at 1.2 volts, during discharge the alkaline voltage eventually drops below that of NiMH. NiMH batteries offer a flatter discharge curve, particularly at higher current draw.
Because of the output voltage differential and because the charge level measurements in the Mac are calibrated for alkaline batteries, even properly charged NiMh batteries fresh from the charger will not show 100% charge, in fact the maximum charge level they will indicate is not far from your 81% charge level. In addition, the discharge curve of the NiMh batteries together with the lower initial voltage means the reported % of power remaining that is reported in OS X is a relatively inaccurate representation of the charge level or remaining battery life of NiMh batteries.
The key factor is when does the battery voltage fall below the point necessary to properly power the bluetooth transmitter in the remote device. The best indication of this is when your Trackpad, Magic Mouse, or wireless keyboard looses contact with your Mac or when the low battery warning appears on the screen. Then it is definitely time to recharge the NiMh batteries.
