I also use the battery grip on my 300D. I ordered a "generic" extra battery from my local photography shop and this would simply not work (it discharged very rapidly, even after several hours in the charger). I took it back to the shop and asked for a replacement, which I got after a few days without any problems. This type is "VARTA P33 - 1500mAh - 7.2V Li-Ion" and performs better than my original Canon BP
-511 (which is normal because this only has a capacity of 1100 mAh).
: I do have another problem with the batteries in the battery grip (I wonder if someone else has had similar problems?). The original Canon battery fits easy in both the slots of the battery grip, the VARTA battery also fits into both slots, BUT is will only extract easily from the left slot (if I insert it the right slot, then it is very difficult to get it out...). Physical dimensions - with the naked eye - seem identical though...
Canon 347d mkII ;-) - EFS10-22mm f/3.5-4.5 USM - EFS18-55mm - EF28-105mm f3.5/4.5 - EF100-400mm f4.5-5.6l IS USM - ...