Batteries are rated at a number of charges, by some off the wall agency. What a battery is saying, when it says "200 charges", is that the battery should accept 200 full charges from the specified low-voltage minimum. Falling below that voltage will reduce the number of full charges. Charging above the battery charge-level will reduce the number of charges.
However, you do not only have 200 charges. You can charge it at any time between FULL and LOW. If it is mostly full every time you charge it, you may get over 600 charges. That is 600 not-full charges, or 600 1/3 charges. (400 would be 2/3 charges, and 3/3 would be the 200 full charges.)
As for the battery and the device... using it while the voltage is higher, is always better than using it when the battery is near low. The lower the voltage gets, the more power it takes to produce the same work. This overheats and stresses the battery. The same thing happens if you have a bad charger. A charger that over-charges all the time, will overcharge more frequently if you charge the battery when it is mostly full. (You may still get 200 charges, but you only get 200 1/3 charges, as opposed to the 600 you would have gotten if the charger was not over-charging.)
The only way to know for sure, is to test the battery voltage before and after a charge. Allow the battery to sit for an hour, and test it again. If the battery voltage has dropped more than 0.5v from full-charge you measured an hour ago... It has been over-charged, and has normalized back to what it should be charged to. If the voltage after a charge is close to the voltage before the charge, on a battery that alerted you it was low, than the battery is near death. It no longer has the ability to hold a charge at the charge level. (Thus, in essence it is over-charging every time, and this is what decays batteries. Each time you use them, they loose more and more power, permanently in the chemical process.)
Just make sure your charger is plugged into a surge/noise suppressor, and remove it as soon as it is finished filling. (The charge light does not indicate "Full", it indicates "Charged". Charged is how you describe a battery ready for use, not a battery that has a full-charge. Mine takes about 20 minutes after the charged indicator shows it is ready.)
Just for the record... My palm computer battery has a 300 charge cycle life. I have had it for over five years, running on the same battery, and I charge it every day. (But I use it every day.)
On the opposite side... My laptop has a battery that is the same age, and same charge cycle life. However, the laptop rarely ever came off the charger, and the battery died four years ago. At the moment, it still sits inside the laptop, and provides about five seconds of life after the power dies. (That is what happens when you charge it while it is full, or mostly full, and the charger continues to feed power into the battery that is slowly dieing. EG, it feeds 12.5v into the battery that has died down to a 11.8v ability. So it is always 0.7v overcharged, and continues to decay as it charges.)
Just a rule of thumb... You may use 2-3 batteries in a day. Those will last about 4-8 months in these high-drain devices (182 days avg). (Or they may only last a month if you always run them down to low, and over-charge them.)
This is why the screwdriver and USB and booster pack are loved so much.
