The main feature of this project is the nickel strip looped through the 3d print. This allows the nickel to contact the terminals of the stereo just as a set of 6 "D" cell batteries would. 

I have used a BMS inside the pack to make sure the cell do not over charge or over discharge

A barrel jack on the back side allows for charger. 

A suitable charger would be a 9v 2amp power brick.