The EOS-300D takes the Canon BP-511 or BP-512. It has the capacity to hold one battery. If the 300D is being used with the BG-E1 grip, it can hold two of these batteries for extended shooting.
Canon has stated that the BP-511 will push 600 pictures without flash and about 400 pictures using the internal flash 50% of the time. It should also be noted that using an IS (Image Stabilization) lens will cost about 30% of your battery life.

Batteries currently used in the Battery Grip BG-E1:
2x GP VCL002 Accu BP-511 Li-ion 7.4V/1300mAh 74gram 55x38x21
This makes together a 2600mAh power source, good for 1420 pictures without flash and 945 pictures using the internal flash 50% of the time!
So it must be possible to make 3400 pictures without flash and 2300 pictures using the internal flash 50% of the time, using all batteries in my bag: 4xGP and 1xCanon battery.
If I can't charge any battery over-night, I'm still not stuck, I guess.

The are sold by Alternate for € 49.-

The Battery Grip BG-E1:

This grip provides you not only with more power, but also a better ballance when you use a more heavy lens such as the EF 24-70mm f/2.8L USM