Oh, I think I have a good one. I had an HP-25 calculator as old as myself, and couldn't use it. The original battery pack contained two sealed NiCd cells, which obviously failed many years ago. Most people replaced their NiCd cells with new ones, then with NiMh cells, or even alkaline AA batteries. This was always problematic: newer batteries were slightly larger and never fit well. Also, the power consumption of a calculator with an LED display was significant, so frequent battery replacements were needed. And the original HP charger was risky and could easily destroy the calculator.
So I designed and built a wirelessly (Qi) charged battery pack for it.
https://partsbox.com/blog/wireless-charging-for-a-hp-25-calc...
After a year of use, it's totally over-engineered and has so much energy and so little idle power consumption, that I have to remind myself to charge sometimes, the thing lasts for months.
I'm the only user. There are many people who wanted to buy one, but the step from a hobby design to small-scale production is a big one and it simply doesn't make business sense. Especially with Li-Po batteries being difficult to ship and potentially a hazard. I guess maybe if I found a manufacturer that would be willing to take the design and manufacture it on demand, taking over all of the shipping/support issues…
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