I'm making a number of small devices such as temperature sensors, RF433 transmitter and receiver (for controlling and receiving devices => MQTT) mostly using EPS8266.
They work fine, and are USB powered because it's simple and works. They probably all use too much power to run off batteries but it would be nice if I could incorporate perhaps a small rechargable battery so that my temperature sensor for example could continue to work for a day or two if unplugged.
I could make something, but does anyone know of anything very cheap that is a plug in solution for this kind of thing. Small and cheap are essential.