Find a battery that will fit into this player

[autofils]

Xbrav: Good for you; that's great if you are using 1,5v cells like Duracell alkaline, but since I used re-chargable Ni-Mh AA cells (2600mAh capacity) the voltage regulator is not needed. Ni-Mh cells are nominally 1.2v; so 4 cells gives you 4.8v and totally safe via the USB port.

You could also use NiCad re-chargable cells which are also 1.2v per cell, but typically NiCads only have 600mAh capacity, so go for the higher capacity Ni-Mh cells at a bit more cost. They are worth the price. You can use an old NiCad battery charger to recharge the AA cells.

[Solttu]

Gameboy micro battery has right volt value and the mAh value is twice as big as the original one has.
The size of the GB Micro battery is almost just a same as Chipod nanos have.

Or you can use "two" batteries, but the another one has changeable 9v-battery and you get 3h more playing time. You have to do it yourself:
http://www.instructables.com/id/EO7VZ73NWXEP286ZV3/