Espruino Wifi battery?

@allObjects I'm not sure you're considering the ESP8266 that's also on the Espruino WiFi? That accepts a maximum of 3.6v, and as you noted when fully charged the 3.6v NiMh battery could be a little over 3.6v.

@trusktr you've got 3 main options.

USB power bank

If you're at all worried about wiring up, I'd say just using a USB power bank is by far the best/easiest option.

Wiring direct

ONLY DO THIS IF YOU'RE SURE YOU ARE NOT GOING TO PLUG INTO USB WHEN THE BATTERY IS CONNECTED

Get a LiPo battery - any single cell battery (3.6/3.7v) will do fine, and using http://www.espruino.com/WiFi#pinout as a guide, wire:

• Battery black -> Espruino GND
  
• Battery red -> Espruino VUSB
  
  

Some stuff you could use:

Battery: https://www.adafruit.com/product/328
Battery charger: https://www.adafruit.com/product/1304
Socket for battery to plug into: https://www.adafruit.com/product/1862

Pretty much any LiPo battery would work (I tend to use old mobile phone batteries), but you'll find it way easier to charge if you have one with the 'JST PHR2' connector that everyone uses on it.

Nicest solution - separate power supply

Use a special 3.3v power supply like this one: https://www.adafruit.com/product/2745

The supply is designed to produce a steady 3.3v from a LiPo battery regardless of what the battery voltage is. It'll get the most life out of the battery and will allow you to plug USB in as well if you want to.

Using http://www.espruino.com/WiFi#pinout as a guide, you'd need to connect:

• Battery Black -> Espruino GND -> LM3671 GND
  
• Battery Red -> LM3671 Vin
  
• LM3671 3V -> Espruino 3.3v
  
• LM3671 En (leave unconnected)
  
  

Hope that helps - I think that covers the best options you have...