NiMH batteries only perform well for about a fraction of the cycles of NiCd's....300 cycles compared to 500 to 1000 cycles. NiMH are only suited for equipment and chargers that have been design for them. Voltage sensitive equipment (i.e. total stations) designed for NiCd's suffer soon after NiCd's have been swapped with NiMH. Non-voltage sensitive (i.e. cordless drills, motorized toys, lighting, etc.) don't care much about voltage stability. What causes this problem...internal resistance. NiMH cells have a higher internal resistance than NiCd's. This means when the equipment demands power from the cells, the cells fight back...at the expense of voltage. Those battery level numbers (or bars) on your total station screen are measuring voltage. The voltage they expect to see is characteristic of NiCd's and not NiMH's. If you load the battery with a high power demand (i.e. measuring distances, or tracking) the voltage drops. The higher the internal resistance, the more the voltage drops. Which cell (NiCd or NiMH) will most likely trip your "low battery warning" before full discharge...NiMH of course! Also, internal resistance gets worse from the day the NiMH starts working. NiMH start out working great in place of NiCd's. But just like the "rabbit and the hare" story, the NiMH run out of fuel while the old work-horse NiCd's pass them by to the prize at the finish line...lots of cycles!!! What causes this early burn out?...charging the NiMH's with a NiCd charger. The NiCd charger is designed for the characteristic "blip" in voltage that comes from the NiCd's when they are fully charged, telling the charger to shut off. NiMH don't "blip". They just keep accepting charger and dissipating it by heating up like crazy, killing their life span. Most total station battery packs are designed with temperature sensors that will eventually cut power from the charger to the battery when they heat up too much, but by then, damage has already set in. You can bet your packs are really warm after that! Now you know why. It is like using your stove to cook dinner and using the smoke detector as your "stop cooking" signal. Back to the internal resistance problem. After a few cycles and abuse by the NiCd charger, the NiMH cell is severely stressed. The internal resistance problem in the NiMH's has risen faster than normal and now causes the NiCd charger to run up it's voltage to try and overcome this problem. At this point, one of two things will happen: The charger runs out of voltage and never fully charges the battery again from that day forward. Or, the battery starts to heat up much faster and trips the temperature sensor early, shutting off the charge and never charging the battery fully from that day forward. Now you try and use the battery on the instrument and the high internal resistance drops the voltage too quick and cuts short the life of the battery on the instrument when in fact, much of the charge is still left in the battery. Back it goes on the charger and the torture repeats itself again. In about 1 to 1.5 years (compared to 3 to 5 years for NiCd's) of regular use you will be back to where you started when you thought, or were coerced into swapping your NiCd's for NiMH. Why do you think "swappers" refuse to give more than 90 days warranty, or at best, a "one year warranty" that is pro-rated and erodes down to almost nothing within one month of reaching a calender year? After reading all this you might ask, "how do you know all this will happen?" Mostly from experience and knowledge of the inner workings of your total station equipment and it's design. When NiMH's first came out many years ago, we experimented with using these cells exactly like we described above. After seeing most of these packs back at our shop in about 1 year, we learned our lesson. We hope our experience will help you make an educated decision on your battery needs.