Awhile back our school purchased a 3D printer. Of course, the number one use I could think of was manufacturing robotics components. As I was interested in moving from Legos to Arduinos, I figured it would be a good idea to build a battery holder for Lego's lithium ion batteries so that I could use them as a power supply for my other projects. If you decide to build one of these, you should use a resettable fuse (or PTC) to prevent damage to your battery or circuit in the event of a short.