I made an ESP-01 temperature sensor that sends data to Home Assistant via MQTT. Based on a prototype using a smaller battery I'm hoping it will last 4+ months. : r/esp8266
NodeMCU ESP8266 http relay control with 12v battery monitor and wifi configuration – Part 1 | Be cheap and DIY:
Battery Powered ESP8266 WiFi Temperature and Humidity Logger
Battery Powered ESP8266 WiFi Temperature and Humidity Logger
Battery powered weather station with ESP8266 and BME280 — glsk
WIFI based battery voltage monitor : r/esp8266
IoT based Lithium Battery Monitoring System using ESP8266
Power ESP32/ESP8266 with Solar Panels and Battery | Random Nerd Tutorials