There are three possible ways to address this problem-

 

a) Usually device will get Power OFF, when the Battery voltage is lower than 3.3V

 

b) Consumption of Battery depends on the Sound Volume and quality of Music

c) Power OFF the device and again connect with the Mobile Phone just to avoid any synchronization problem (causing fluctuation in Battery percentage shown in the mobile phone) between Mobile Phone and BT Device

d) RESET the device