The Sense-U Baby uses a CR2032 coin battery and the battery life depending on usage and other settings. Pls make sure to turn off the device and close the APP from your smartphone background when you don't use it. In the meantime, please remove the battery. Please update the firmware to the latest version (Sense-U app->Setup->Help->Firmware Update) which reduces the power consumption dramatically. These measures can extend the life of the battery.