Hi,

You may want to check the charger but I think that they are rated at 7v. Even the usb to charger adapters have a bump up chip so that they deliver 5.7 volts I think.

The minty box type solution would be good but you may want to look at a slightly higher output voltage from the chip and if so perhaps more than two AA otherwise they will drain pretty quickly. It won't be like strapping on an extra 2500mAh you will get loss in the conversion and then you will be charging the internal battery so you will also get loss there too.

Also I think that the charger is rated at 800mA so your chip should be able to drive that as well if you are trying to charge from flat or at least die gracefully.

Anyway all that info should be on the back of the charger I think.

Nokia do an external battery for some of their mobiles that use the same charger so it is possiable. You may want to look at some of the specs for these and compare with what you are putting together. ( DC-1 and DC-8 are the model numbers for these).

Anyway if you do get it working I would be interested in what you do get going.