Lithium Ion batteries do not have a memory and so nothing can be gained from allowing the batteries to become fully depleted.  In fact, full depletion of Lithium Ion batteries can shorten their life and can even damage the battery.

The iPAQ has a battery sensor in it that may need to be recalibrated every now and then.  For example, when I first started using an extended battery in my iPAQ hx2415, the iPAQ did not recognize the extended life of the battery and kept responding to the battery as if it were the old battery.  After I ran the battery sensor calibration procedure on the device, the iPAQ properly recognized the extended battery.

This procedure was originally posted on HP’s website, but the page was removed some time ago and so the link I have for it no longer works.

For a Windows Mobile 2003 iPAQ

1. Charge the unit for 30 minutes (no more, no Less) only powered by AC Adapter and 22-pin connector and not connected to the PC/Cradle (make sure the unit is switched/powered off).
2. Remove the power and perform full 'Hardware Reset' and then 'Soft Reset'.
3. Charge the unit again for 4 hours connected to ac power only.

For a WM5.0 (or later OS) iPAQ

This recalibration procedure was posted long before WM5.0 devices were around, so I modified it to make it applicable to WM5.0 devices.  Success with this procedure with newer Windows Mobile units is mixed:

1. Charge the unit for 30 minutes (no more, no Less) only powered by AC Adapter and 22-pin connector and not connected to the PC/Cradle (make sure the unit is switched/powered off).
2. Remove the power and perform full 'Hardware Reset' (or clean reset) and then 'Soft Reset'.
3. Charge the unit again for 4 hours connected to ac power only.

Final Notes

All data, user settings, and user-installed software will be deleted from the unit when this procedure is performed, so be sure to sync your data to a computer beforehand.  Be prepared to spend some time afterwards reinstalling applications and setting up email accounts, wifi connections, etc.  Restoring from a backup file is not advisable since it could reset the unit’s battery sensor back to the way it was before the recalibration procedure was performed.

Make sure the unit remains switched/powered off during this charge period.  Following the procedure exactly as outlined in the steps above enables full calibration of the battery with the unit.

After a software reset the power Leds (Light Emitting Diodes) should now be on and the battery should be showing 100%. If the unit still fails on power do the following checks:

• Test the AC Adapter at another location/power source.
• Swap test The AC Adapter if possible.

If the recalibration procedure does not resolve your battery issue, you may need to purchase another battery