There's a thread that's probably a couple of years old that tells exactly how to do it but I believe it goes something like this...
- Rename the extension of your config file. (i.e. Change My Config.pcf to My Config.zip).
- Open the zip file (Windows built-in compressed folders doesn't work so you'll need WinZip or one like it).
- Find the battery icons (there are five).
- Replace them with your own graphics. Make sure you use the same file names.
- Change the extension back to .pcf.
