“Reuse old mobile phone batteries. I have been using used phone batteries in a bunch of projects recently after discovering an awesome little module on eBay. The module comes with a Li-ion charger and also a voltage regulator, allowing you to increase the voltage of the Li-ion battery from the normal 3.7vs to up to 30Vs!
The other great thing about using old mobile phone batteries is you can get them for free! There are plenty of places that have mobile phone recycling bins where you can score a few batteries, free of charge. I have one at my work, which I periodically raid for batteries.
The other good news is the modules are very cheap to buy at only around $2 each.
This ible shows you how to connect the module along with a micro USB adapter so you can use the battery as a phone charger. Its a simple project and will show you how to wire-up the module to use in any project of your choice.”