Tale about my dad's HP-35

[robjordan]

(12-13-2022 12:49 AM)lrdheat Wrote:  Great story. I have my dad’s HP 35 in pristine condition. He wisely packed it away with the battery pack removed.
How might I get a working battery pack that would fit the HP 35 case?

I'm glad your dad took better care of his. Mine also left me two pristine HP-28S, and a 49G... yes, recently voted worst ever HP calculator

My battery solution, maybe not ideal for regular use but a very easy improvisation for testing. I used a slim LiPo battery of 350mAh, which should be good for 2 or 3 hours use. Mounted it on a stack of business cards, trimmed to fit the battery compartment, and glued together for stiffness. I added a TP4056 charging / battery protection module, which are 10 a penny from Aliexpress. Wrapped two copper strips around from the top (battery) side of the card, to the bottom (spring contact side). Two strips of stiff but compressible packing material to keep the copper tape in contact with the spring clips, but avoid squashing them. Another thin layer of foam above the card to apply a little pressure from the battery cover, down through the stack. Attached photos should make this a bit clearer.

I'm slightly wary of fully-charging the LiPo cell to 4.2V, since the original battery voltage was 3.75. On the other hand I think I've read that the original charger supplied more than 3.75V to the calculator when running on mains electricity. Can anyone confirm?

[Edit: I found this thread which confirms 4.2V is likely safe, but also has Tony's good idea to add a schottky diode to drop the voltage a little and protect against reverse insertion]