The Noise Masking Sleepbuds feature a battery protection mode which preserves battery power when the system is unplugged and unused for a certain amount of time. You can find out more about this on page 24 of the user guide.
The batteries should last for many years of use, so it is unlikely a replacement will ever be needed. If you do have battery issues, however, please reach out to support in your country for service.
I hope this helps you and anyone else on the community who has the same query.