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. The circuit on this PCB is designed to safely work with LiPo battery packs in the 1000-1400mA range, and an LED illuminates while the battery is charging. An Ideal Diode prevents any sort of damage to the LiPo pack if the HP AC Charger is connected to the calculator while the battery pack is inserted. The HP AC charger will not charge the battery, however - this is only accomplished via the USB micro B connector.

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. Complete bill of materials is available on GitHub, see link below.

More information and downloadable files are available on the Hackaday Project Page and at GitHub