ESP8266 battery experiments continue. Here’s an ESP-07 module with some bodge work to read the actual battery voltage. Running off two AA batteries, the little green board is a step-up SMPS, capable of running down to 0.8v, in other words it should suck those batteries dry.
I grabbed some weak alkaline batteries to start with, combined voltage is only 2.2v (dead pretty much), but the smps turns those lemons into tasty 3.3v lemonade.