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A number of folks have rebuilt the battery packs for Woodstock calculators (HP21/22/25(c)/27/29C) using commercially available AA-size rechargeable batteries, and I'm embarking on that project for a couple of my calculators.

A question: what batteries seem to be good candidates?

My findings so far:
- not all AA batteries have the same form factor. In fact Wikipedia says: "An AA cell measures 49.2–50.5 mm (1.94–1.99 in) in length, including the button terminal—and 13.5–14.5 mm (0.53–0.57 in) in diameter. The positive terminal button should be a minimum 1 mm high and a maximum 5.5 mm in diameter, the flat negative terminal should be a minimum diameter of 7 mm." In my experience there is a lot of variation in the height of the positive terminal button, which matters as it controls how much of the button protrudes from the plastic battery carrier, which determines how well the battery interfaces with the contact on the calculator's printed circuit board.
- These batteries have positive terminal buttons that are too short for effective use in Woodstocks: La Crosse Tech NiMH 2600 mAh; Sanyo HR-3U 1.2v NiMH
- Sanyo eneloop (HR-3UTG) NiMH are just too long overall; most commercially available springs + this battery = too long to fit in the plastic battery carrier.
- I have had best luck with Amazon Basics AA/HR6 NiMH batteries (vintage 2011, since that's what I had around the house). Nice positive button length, overall length is good.

If you have had good luck with a specific brand/model of AA rechargeable battery, please share your experience.
I am using Tecxus NiMh Mignon rechargable batteries (1,2V, 2100mAh) for all my classics and woodstock calculators.

NiMH LSD technology, so extremely small selfdischarge rates and with flat tops. They fit perfectly in calculators' battery cases. Cells have industrial grade and are well balanced to each other.

For standard woodstock calculators I use single cells with soldering tags and solder them together/remove unnecessary tags to get 2 cell pack. For classics and e.g. HP-29C there are preconfigured 3/4-cell packs available. Original chargers can be used but of course it takes 24-48 hours to recharge a fully discharged pack. On the other side small charging rates (80-160 mA) compared to battery capacity have the advantage that you will not overcharge cells if you forget them for a few days ;-).
as a side issue, I am finding the battery terminals off the motherboard for woodstocks show some variation and sometime do not always extend far enough to always allow contact with a flat end of a battery. This is especially true when corrosion has been cleaned off the terminals. This is something you may want to check while you have the battery case disassembled and can visually inspect battery case and terminal positions. Where this has been an issue for me I have been soldering a small brass shim around the end of the terminal to allow connections
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