The upcoming PSP Lite will include a smaller battery for the smaller system. The battery that's included with the system will run for about 5 hours while gaming. However, if you have the battery from your original PSP, you'll be able to get 8-10 hours. The problem with this solution? The battery will bulge out of the system. Sony will offer a new backplate for those interested in adding the old battery to the new system.
Does that make things clearer?