The built-in battery life is short
Due to the characteristics of the built-in battery, repeatedly charging it may shorten the battery life. If you feel that the battery life has become extremely short after fully charging it, try the following procedure.
In order to completely exhaust the built-in battery, continue to use the speaker until it automatically turns off even if the charge indicator is flashing. After completely running out of the built-in battery, immediately charge it to its maximum capacity (built-in battery refresh). The built-in battery will be refreshed and the battery life may improve.
If the battery life still does not improve, it could be the built-in battery has expired. Consult your nearest Sony dealer.