Cypher Labs AlgoRhythm Solo disassembly (multiple pictures)

Nice catch.

It does look like battery replacement will be fairly easy.

Would a bigger battery be possible (presuming there are bigger)?

i wouldnt mess with it. you could theoretically change for a higher capacity battery with the same chemistry and same exact voltage, but it would be a risky thing to do, as lipoly batteries have been known to explode if under or overcharged. the charger chip will have been programmed with low and high voltage cutoff thresholds for this battery specifically and will not work with anything else. a higher mah/watt hours rating with everything else the same will work, but i wouldnt recommend it. 

presumably you will be able to upgrade the firmware when available too given the 2nd interface connected to the mcu and dsp. looks like a bit of fun could be had with upgrading the clocks and perhaps the buffer stage. its a pretty neat layout actually i must say.

hmmm, actually i use a good deal of lithium batteries that are top shelf and dont have it, they rely on the charger to do that and the operator knowing what they are doing, the chances of your mp3 player battery having one are a bit slim, higher capacity but same voltage and same battery chemistry means nothing and is perfectly safe, the charger will simply go about its business as it works by cutting off at a specific voltage (for lipo this veans alternating between constant current and constant voltage modes, not capacity) but changing the battery chemistry with a lipoly or higher voltage is a recipe for disaster, as the thresholds will be all wrong.