I used a MAX40200 ideal diode on the output to protect the circuit on my Woodstock battery pack. See my project here https://hackaday.io/project/189308
For your application, given the voltages in play, I guess a beefy schottky diode would work just as well.
for charging the Woodstock pack I used a USB connector, but you need to remove the pack to charge it. I am tinkering with the idea of adding a Qi receiver for wireless charging. There is much less available space than in the 82033 though…

CYUL

Hey @CYULMaker very cool idea. I'm not a fan of the NiCads but you inspired me to build a drop in battery box based on two 18650 LiPo cells in series followed by a switching Buck regulator to create a stable 4.5Vdc supply. I think I have the schematic worked for my use, which is without an external transformer input. However, before I share that I want to make it tolerant at least of someone accidentally hooking hook up the OEM +12vac transformer input. I don't need it to charge given the 18650's will last for months but I just don't want that 12vac to damage the calc. 

YodaFred24018

Soulsister

0.2mm layer, 6 walls, 25% infill

D&D&D Printer

cover

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<boostme><boosttitle>Boost Me</boosttitle></boostme>Fits HP 91, 92, 97, 97S, HP Printer 82143A, or HP Cassette Drive 82161A.&nbsp;See instructions at <a target="_blank" rel="noopener noreferrer ugc" href="https://www.schwalvenberg.com/en/hp-41cx/115-hp-82033a-battery-pack-refurbishment">https://www.schwalvenberg.com/en/hp-41cx/115-hp-82033a-battery-pack-refurbishment</a>&nbsp;1 - No need to destroy an original to retrieve the shell2 - I provided the circular separator to isolate the cells at the end3 - The two Y shaped bits are meant for you to form Nickel strips into proper shaped contacts. I also had some good results using simple kitchen aluminum foil (but keeping the plastic bits inside for strength).&nbsp;&nbsp;

HP 82033A - HP 97 Battery Pack Shell 

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