HP35 Scientific Calculator Battery adapter and charger

thingiverse

I designed this little battery carrier to replace the old nicad battery pack and wall adapter for an HP35 Scientific calculator. The idea is to get rid of the nicads and the big wall adapter. I use a 3.7V Lipo battery 503450. This is a 5mm x 34mm x 50mm lipo cell that is charged using a widely available TP4056 USB to Lipo charge adapter board. I have since adjusted the location of the TP4056 a bit to the right. The battery door clip was crashing into the USB connector. Place yours accordingly. You can charge using a USB charger. And the capacity of the Lipo is 1000mAH instead of 300mAH like the nicads. This is important because the calculator draws 70mA quiescent. The terminals are thin brass plate from the hobby shop. I bent it into angle bars and placed them in the channels on the 3d printed carrier. This is an important design feature because the recessed bars prevent short circuits when the terminals come into contact with a metal object like a desktop. Read: Fire danger! Make sure the brass terminals are sufficiently recessed into the channels so they aren't exposed to the surface of the carrier. Enjoy!