Gday, try these few things...
=== Reset ===
# Disconnect the speaker from power.
# Hold the Power button for 10 seconds until the device powers off.
# Reconnect the speaker to the charger and check if it charges properly.
=== Update ===
Outdated firmware can cause charging issues:
# Download the ***Bose Updater Software*** from Bose's website.
# Connect your speaker to your computer via USB and follow the instructions to update the firmware.
# https://www.bose.co.nz/en_nz/support/articles/HC2474/productCodes/soundlink_mini_ii/article.html
Check the cable and such, otherwise could be some sort of damage to the battery.