Don't be tempted by third-party batteries: they may promise longer battery life, but Ruriweb has proved that they're unreliable, and dangerous too. For comparison purposes, note that PSP is packaged with a 1800mAh battery. Sony offers a longer-life 2200mAh version as well.
Ruriweb tested three third-party batteries and they all had incredibly disappointing results. The 2200mAh battery produced only 2000mAh of power. Two other 2600mAh batteries produced only 1500mAh of power: that's less than the battery that comes with your system! To make things even worse, one of the batteries actually caused massive damage to the system (pictured right). The official Sony battery may be pricey, but (ironically) it's the best battery solution by far.
I cant say i agree, i found the 2600mah battery to be very decent, agree or disagree via comments