Modern LiPo Battery Pack for HP 'Classic' Calculators

This is my attempt at a LiPo-based battery pack for "Classic" HP Calculators, specifically HP-35, HP-45, and HP-55. The original NiCd packs are all dead by now, and rebuilding them is difficult - assuming you can even find a pack to rebuild. I thought it would be fun to use a rechargeable LiPo battery, and include the circuitry to re-charge via USB as well. 

LiPo chemistry has very specific care-and-feeding when it comes to recharging, You cannot use the HP AC charger to recharge this pack! Don't even try it. Not kidding.

The basic idea is to use a gold-flashed (ENIG) PCB as the contact substrate, and mount the LiPo, USB micro B receptacle, and charging circuitry on the opposite side. A pair of laser-cut 'endcaps' locate the assembly within the battery well. and allow the compartment cover to hold it in place.

More information and downloadable files are available on the Hackaday project page and at GitHub