Your Body+ has an approximate battery life of 18 months using four standard AAA 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/en-us/sections/4411046153745-Body
https://support.withings.com/hc/en-us/sections/204132387-Maintenance