ESP8266 is so popular today, I think ESP8266 +18650 battery will be very easy to develop IOT projects.
I call this board Pocket 8266, I also put it on my tindie store: https://www.tindie.com/products/lspoplove/pocket-8...
This one has 10 pins and I think it is enough to connect most sensors.
It integrated 18650 battery charging system. It supports working and charging at the same time.
The 18650 inside could provide 1A current. Like NodeMCU this board could work in deep sleep mode.
Today I use this board made a simple project. Pocket 8266 read temperature and humidity of my house and uploaded the data to thingspeak at the same time.
Step 1: A Little More
This board has the same pin map as my another project: D-duino:https://www.tindie.com/stores/lspoplove/
That means Pocket8266 could use the same shields like D-duino.
I designed X1,X2,Grove shields for D-duino projects, Now Pocket 8266 could also use these shields.
Step 2: Upload to Thingspeak
Using Pocket 8266 to develop projects is very easy.
In this example I connected a DHT11 sensor to Pocket 8266.
I connected the data pin to D4.
The code is also easy: https://github.com/lspoplove/D-duino/blob/master/A... if you use adafruit library.
Pocket 8266 will upload data to thingspeak every 3 seconds.
Let's see how long will it work.