Optimal charge for each individual Ni-Mh battery!
Ni-Mh batteries exhibit an interesting phenomenon. As they are charging the voltage of the individual battery increases until the battery reaches full capacity. At this point the voltage takes a slight but measurable drop.
The best Ni-Mh battery chargers take advantage of this quirk by incorporating a computer chip that takes note when the battery reaches this peak (delta) voltage and terminates the charge process.
Less advanced chargers usually use a timer to terminate the charge process which may result in an under or an overcharged battery. They may also require the charging of two or four batteries at a time. A more advanced charger will allow the charging of any number of batteries at a time through the use of individual circuits for each battery space.
More advanced Ni-Mh battery chargers use Negative Delta V charge control, individual circuits for each battery and incorporate timers and thermal cutoff for overcharge protection. Some also have extra features such as discharge, refresh and will record the milliAmphour (mAh) capacity of individual batteries for peak performance. Due to Nimh batteries tendency to self discharge, most chargers will continue to trickle charge batteries after charging so you can leave them on the charger and they will stay at peak voltage until you are ready to use them.