@PotatoBattery You have probably already found the solution for your issue, but for the sake of variety, here is my take on the matter. The battery is a generic lithium phone battery, 2300mAh in this case. The solar cell is a 5,5V, 30mAh one. It is enclosed in resin. The battery charger is a 1A lithium charger with battery protection. I've replaced the programming resistor with a 10k, to better suit the solar cell output current capabilities. The battery gets 15mA, on a sunny day. All the components are from eBay. And you can buy the Arudino board here, on mysensors.org. Everything fit perfectly.