After announcing the brand new slimline PSP at E3 one of the big bullet points that Sony hit on while trying to sell the redesign to the audience, both in attendance and worldwide, was that the new PSP's battery would last longer for gaming on the move.
However, it has now come to light that this information was more of a small white lie by the company, rather than actual fact.
If you have the battery from your original PSP put in the slimline, you'll be able to get 8-10 hours play, which is the number Sony touted at E3. However, the new PSP will come packed with a smaller battery, one that will only run for about 5 hours while gaming, which is a fair few hours less. Also, if you try and put the original battery in you PSP it won't fully fit in the slot and bulge out of the system, kinda like the same problem you will get if you put a GBA cart in the DS Lite.