Sony PSP1000 Battery 3.7 V 1500 mah Li-Ion,

The PSP-1000 is an external battery that is moulded to fit perfect along the bottom of the Sony PSP so it virtually becomes part of the machine. With the PSP-1000 the total battery capacity available to the PSP is effectively doubled* as it combines with the internal battery giving the PSP double* the run time without changing or charging batteries as would be the case with the internal battery only.

(assuming original battery is 1500mAh)
PAB-PSP1000