well, I bought the Joytech PSP battery extender for my PSP and it has a Ni-MH battery inside, rated at 4.8V output with 2000mAh. The input is 5V 2A which is the same as the charger for my C3100. The plug is the same size too so I think its ok to use with my C3100.
Strangely, my C3000 charger is 5V 1A and my C3100 charger is 5V 2A (both came direct from japan in original packaging). Been using my C3000 charger with my C3100 because its smaller