Why its never get to battery full?

@markjohnkie5249 this could simply be caused by an old battery that can no longer take a full charge. It is also possible that this is related to a bad power management IC etc. For now and as a first known-well starting point, replace the battery.

Since iFixit does not yet have a specific guide for this task, use something like these instructions JBL PartyBbox 100

to disassemble your PB100. Here is where the battery is located

Replacement batteries are available at many online stores.

While you work on your PartBox, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!