Here's a unique sound for charging, it reminds me of some 70's horror/mystery movie..
Just SSH it to your UISounds folder (Don’t forget to keep/make a copy of your stock one)
Edit: Here's another one too I had for a while....
These below are MP3's so if you D/L them there you need to covert them to a CAF..
Listen to charging too
here
Listen to Beep-Beep
here
The Zip attachments are already correctly named .caf files and are all ready to SSH 
YOU MUST DOWNLOAD THE ONE FROM THE ATTACHED ZIP FILES BELOW!!! NOT THE FILE AT WHERE YOU LISTENED TO IT!!! THEY ARE MP3's AND WILL NOT WORK...