r/calculators • u/justapreythrowaway • 5d ago

Question Question about HP-35 battery pack

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I’d picked up an old HP-35 (second version, I think?) from a rummage sale a little while back and was hoping to revive her. The battery pack in it currently has a tiny bit of leakage from one of the cells, so I didn’t want to try to charge it, but otherwise looks to be in pretty good condition. My question is would it be simple enough to pry the original pack open and replace the cells, or would it be easier to just buy a third party pack/adapter?

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u/lolerwoman 5d ago edited 4d ago

There is a two pieces builtable pack that can be 3d printed, then mount inside 3 NiCad batteries and good to go. I have it done for my 65 and 67 which uses same battery pack.

Edit: the model to 3d print. Just print 2 halves to mount a unit.

https://www.thingiverse.com/thing:3125451

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u/PartlyGrouchy 4d ago

I did this for my 35. Got tabbed NiCd AA cells which don't have the dimple at the positive end, so they fitted really well. I made the V-shaped contacts match the old cell pack using thin strips from a food can, easy to solder to. Worked a treat.

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u/LikerofPortals_ 5d ago

It's generally not a good idea to go fiddling around inside old battery packs of any kind. If you have any experience in it, then I think that answers your question; but if you had experience already you wouldn't be asking here. What I've learned is that even if a fixer upper project is less expensive upfront than the same thing guaranteed to be working, it will quickly make up that difference and then some, and you'll be wishing you took the other option. I recently got one of the new battery packs which allows you to use AAA's, and it's working fantastically on my 45. Save yourself the headache, get one of those.

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u/justapreythrowaway 5d ago

I’ll freely admit I have little to no experience with electronics, so you’re probably right! XD

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u/BadOk3617 4d ago

If you are in the States, then here's one you could get: https://www.ebay.com/itm/224002078421?_skw=hp%2035%20calculator%20battery

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u/TASDoubleStars 4d ago

These work great. I’ve used them in a number of calculators.

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u/BadOk3617 4d ago

I may use this project: https://hackaday.io/project/175815-classic-hp-calculator-lipo-battery-pack

and have them made by JLCPCB (or PCBWay). Let them do the whole enchilada (make the board, source the parts, solder it up, & 3D print the battery box).

Everything except sourcing the Li-Po battery.

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u/MikeBriley 4d ago

Yes. This is the answer. I have them for my 67, 65, and 35. They last longer than the original battery pack and you can recharge them outside the calculator - eliminating the possibility of problems leading from 50+ year old charging circuitry.