iFixit

The battery's a Li-ion 3.7V, 930 mAh, 3.5Wh unit.

Here's how the three major handheld gaming devices stack up in the battery department:

PSP Go: 930 mAh, unofficially user replaceable

DSi: 840 mAh, officially user replaceable

iPod touch 3rd Gen: 789 mAh, unofficially user replacable, requires difficult soldering

The PSP wins on capacity, and the DSi wins on serviceability. The touch, well, if you can't say something nice...