I am examining cheap, off-the-shelf hardware solution provides power for and ESP8266 with the least losses. The test setup uses an 18650 type Lithium Ion battery which, via the regulation circuitry being examined, powers a Wemos D1 Mini ESP8266 development board and a DHT11 digital temperature/humidity sensor.
I am testing two different peices of regulation circuitry at the moment. The first is an all-in-one board, designed as part of a cheap phone charger that you can buy on Ebay. The idea is that the user can swap out the battery inside for any spares they have lying around. It does both the protection and regulation, and provides 5V to the mini-USB port on the dev board.
The second setup uses a separate protection board with undervolt and short circuit protection, and a boost converter to bring the 2.4 to 4.2V of a lithium battery up to 5V, which is again, fed into a dev board.
I have two dev boards and sensors running simultaneously and I am periodically measuring the voltages of the batteries to determine which one degrades fastest. Both boards are running identical software. I plan to complete several tests in order to infer which board will let a battery last the longest.