Your Body has an approximate battery life of 18 months using 4 LR3 batteries.
You can check the battery level by opening the Withings App and going to Devices. The battery level is located next to the Body.
https://support.withings.com/hc/zh-cn/sections/6212630854033-Body
https://support.withings.com/hc/zh-cn/sections/115001378708-Maintenance