LG ELECTRONICS NR18650 Lithium-Ion Accumulator 18650 Battery 3.6V 3350mAh Rechargeable - reviews - There is 1 review.

Great capacity.
The capacity is very nice, but this is based on older technology and you can not draw lots of current continuously.

On my builds, when the battery gets to about 25% I need to turn the volume down. Continuing to play at or near max level will cause the BCPB3 to enter protection mode. It stops any current and I need to connect it to the charger for 1 second or so to bring it back On.

This is most likely due to the battery dropping voltage too much under higher load, causing the undervoltage protection to kick in, even though, you are actually well above 3v for each cell.