Thanks for reaching out to the community. While the battery is not a user replaceable part, we can definitely help you with that by contacting your local Bose Support. They will provide the best available options as far as getting the battery replaced.
If you want, you can also try rebooting the system processor to see if it helps. Here are the steps below:
Connect the speaker to power
Press and hold the Multi-function button for 10 seconds
While continuing to hold the Multi-function button, unplug the speaker from power
Release the Multi-function button
Reconnect it to power after, and if possible, let it charge to full so you can get a good idea how long the battery will last.