12v Battery Charger Circuit Diagram using LM317 (12V Power Supply)
Amazon.com: Power Queen 14.6V 20A LiFePO4 Battery Charger, 2-Stage Automatic Smart Battery Charger and Maintenance, LiFePO4 Lithium Batteries Charger, Suitable for 12V (12.8V) Lipo Lithium Iron Phosphate Battery : Automotive
arduino - Is it okay to connect 2 lithium ion battery in parallel to a charger that can maintenance 4.2V? (more detail in post) - Electrical Engineering Stack Exchange
Arduino Powered Solar Battery Charger : 7 Steps - Instructables