Your Wireless Scale has an approximate autonomy of 8 months using four standard AAA batteries.
You can check the battery level by opening the Health Mate app and going to Devices. The battery level is located next to the Wireless Scale.
https://support.withings.com/hc/ja/sections/6212668113809-Wireless-Scale
https://support.withings.com/hc/ja/sections/200608188-FAQ
