Hi Matty, have read in heaps of places that power source is the ACK-E5
This guy gives a pretty good review/specifications of camera. This is p 3, but also good info on p 1 & 2.
http://www.steves-digicams.com/2008_...l_xsi_pg3.html
http://www.steves-digicams.com/2008_...si_battery.jpg
The Digital Rebel XSi is powered by a Canon Battery Pack LP-E5 rechargeable battery. At normal temperature (73°F and above) it's good for ~600 pictures without flash or ~500 pictures if the flash is used 50% of the time. At colder temperatures (32°F and below) that number drops to ~500 or ~400 if flash is used 50% of the time. The figures above are based on CIPA (Camera & Imaging Products Association) testing standards and are much improved over the battery supplied with the XTi.
The supplied LC-E5 compact charger required ~90 minutes to charge a fully depleted LP-E5 battery pack. It requires no separate power cord, it has fold-out prongs.
AC power can be supplied via AC Adapter Kit ACK-E5 (optional).
(Note that there is also a lithium "button" battery inside the battery compartment to retain camera settings during main battery changes.)