IPaq rx3115 working like PC not Pocket PC, need to plugin always

You need to purchase a new removable battery for your RX
The removable batts usually only last about a year (like cell phone batteries).
Although it is showing 100% charge, the battery is not storing enough power to run the ipaq

The rx series uses 2 batteries, one is the removable one u can see/access when u remove the back cover. The other batt is a small round batt which stores the basic info for your system (so you don't lose your data when u swap out the removable battery).

This small (backup) batt is similar to the cmos battery on a laptop or pc.
You cannot easily replace this, as the unit needs to be disassembled to access this batt.
If you are tech-y (and brave), the replacement button batteries are sold on ebay for $9-12

Normally, if your removable batt is bad, you could still use your unit plugged in, and could
remove from AC power without losing data. Not the case if your replaceable batt is exhausted.
When both batteries are bad, you lose everything when u remove unit from AC power.

My workaround without having to disassemble unit:
Purchase and fully charge new removable batt (They have huge capacity ones available for like $15 - better than the original !).
Load up all your programs & settings the way you like.
Perform full backup to SD card (& also on PC via active sync - to be safe)
Now the only time you should lose your data (like hard reset) is if you allow the battery
to completely drain, in which case you can restore your backup.
If you purchase 2 removable batts, you can easily swapout without losing data if you
use the usb charge cable & leave ipaq plugged in while you change battery

Hope this helps