The rule of thumb is to discharge your batteries fully then recharge each time. Most users do not do this! Instead they use their PDA/Handheld PC for a period of time less than a complete discharge and then charge it. This uses up the number of cycles that the battery can provide. The problem is that most devices do not fully discharge the batteries. Also, if you leave your unit plugged in there is a trickle charge going to the battery. Over time this may reduce the battery life due to overcharging. These units were not designed to be continuously charging for extended periods of time.

The NiCad and NiMH batteries are subject to what is commonly referred to as the memory effect. Basically it is a change in the battery's ability to charge fully based on not being completely drained when it is charged. The best solution is to periodically (perhaps monthly) fully discharge your batteries by running your unit until the unit will no longer turn on. Then fully charge it. This cycle should be done at least twice in a row to keep the batteries in good condition.

The Lithium Ion and Lithium Polymer batteries are designed to be kept charged. If the batteries are fully discharged they can reduce the number of charging cycles. Further whey they fail, your PDA may not work at all. I have seen this in notebook PCs as well.