there's some space if you leave out the metal cradle also the plactics can be adapted a little. this gives enough space to fit a mibilephone battery, stripped out of plastics etc, the thing is to find one that fits the space best while also having a big enough capacity.
what's worth considering is also if it makes sense, as the batterys discharge themseff too. i mean, even with a theoretical capacity of lets say 2000 mAh instead of 750mAh, will ir really hold 2,4x as long? or will it dicharge it self anyway, so you'd need to charge every 2 months anyway. for now i have a 1000 mAh and 2 cables sticing out through the audio jack to measure the voltage from time to time and see the progress
