Solar cell charger for the battery

Hi,

The 3V pin on Puck.js is connected straight to the battery, and Puck.js draws so little power that you can use a small solar cell which can be left connected to the battery (batteries are usually fine with a small amount of 'trickle' charging even if they are already full).

There are a few really easy things you could do. The only thing is to watch out for is to make sure you don't put too much voltage (over 3.6v) into Puck.js:

• Use a supercapacitor and solar cell (use a zener diode across it to stop the voltage going too high)
  
• Use a LiFe (Lithium Iron) battery and solar cell (these have the perfect voltage when fully charged)
  
• Use 2x AA or AAA NiMh batteries and a solar cell
  
• Use a Lithium Ion battery but drop the voltage by 0.7v with a diode before connecting it to Puck.js
  
  

Sadly what you can't do (but which would have been perfect) is to use a rechargeable Lithium Ion CR2032 battery (they do exist) and insert that in to Espruino, as the voltage of the battery is too high. While LiPo/LiIon batteries say '3.7v' they can be as high as 4.2 when they come off the charger.

You could do the same trick as with the supercapacitor though - putting a zener diode between 0 and 3v to ensure the voltage never gets too high. You could then put a half-charged CR2032 rechargeable battery in it and use the solar panel to charge the battery (but to only around 75% charge). It wouldn't be amazing for the battery, but probably wouldn't be too bad.

Hope that helps!