AFAIK there was never an official one and Chinese produced ones with additional battery are only good as grips. I've had one and that's how I learned to take the whole thing apart: when battery in the grip drops too low the whole thing stops working (which is kind of expected), but then PSP Go refuses to take charge! The cure is [relatively] simple: take PSP Go apart and disconnect the battery, connect it back and everything should work after that. This does not even affects the warranty: battery is covered with "warranty void if removed" sticker, but to reach connector you don't need to remove it. Still experience was too cumbersome for me thus I've stopped using grip (and no, I no longer have it).
If I understand correctly this is not even said grips fault, more the in-PSP battery-charging logic fault (it does not expect that "wall power" to go out when charge is low), but this does not make the experience less painful.