One great thing about Canon Powershot SX10 and Sx20 cameras is both use AA batteries not those Canon Lithium camera batteries. Canon Batteries are pretty expensive so keeping backup batteries would be costly. Fortunately Canon SX10 IS users can use rechargeable AA batteries. The Camera uses 4 Rechargeable AA Batteries So if you plan to have a full day field trip then you need to keep at least 1 extra battery set. at least 2 extra battery sets if you plan to use the SX10 IS internal flash.
I received free a rechargeable battery set for free from the shop which is not included in original package. A brand called Camelion. I have to say these rechargeable batteries are great. Important thing is they keep the power for more days after recharge.
Be careful if you are going to buy Sony AA Rechargeable Batteries. There are China made ones which don't recharged at all. Even Ebay Auctions have these fake china batteries So buy from a reputable place like Amazon.com. Panasonic Eneloop Rechargeable AA Batteries are good I know mnny who use these brand and happy with them. Energizer is another popular rechargeable battery brand you can find everywhere. I have mixed feelings about this brand these are good though not the best.