Designed for the Wemos Nano D1 in particular, this ESP8266 battery shield is designed to run an ESP8266 in deep sleep with periodic wakeup for many months from a small Li-Ion battery. Unlike the battery shield supplied from WeMos, this one runs the Wemos directly from the battery - no up or down conversion so the only power used is that from the ESP8266 itself. This shield does have a lower power buck converter to protect the ESP8266 from excessive voltages from the battery, limiting the max voltage to 3.3v, but allowing lower voltage than that. For more info on the idea, see this article (with comments) - https://arduinodiy.wordpress.com/2017/01/02/reviewing-the-wemos-battery-shield/