HP 21 replacement cells

[Bruce G.]

I see that a user has recommended Digi-Key part no P092 as a replacement cell for the hp-21. These are 1000 mah AA size cells rather than the 700mah AA size cells HP used. Will that cause any problems?

I can handle the mechanical part of actually replacing the cells but I do not know much about batteries or their terminology.

Thanks for any help.

Bruce G.

[Bernhard]

Hi Bruce,

from my experience with HP2X it is possible to use any NiCd or NiMH cells regardless of capacity - those with more mA-hours will run longer, but won't damage the charging circuits nor the calculator.
Be careful, though: the original HP cells are 49.0 to 49.5mm long (including the center contact nose). Some cells on the market are significantly longer (such as 53.0 mm) and this might bend and break the contacts in the HP. Do not use those. I have successfully used cells with spot-welded contact strips for solder connection with a total cell length of 48.0 to 49.0mm. The contact strips on the HP's contact side are removed or cut back, and those on the spiral spring side are soldered together. Beware the spiral spring and watch for proper insulation of the cells. If the spiral spring shortens a cell, the heat may melt the battery holder tightly into the HP's case, and it may catch fire. I always put some thin insulation plastic below the contact strips after soldering the cells together to make absolutely sure they nor the spring can't ever touch the outer metal can of the cell.

Regards,
Bernhard

[Bruce G.]

Bernhard,

Thanks for the tip, I would have assumed all AA cells were a standard size. I will look for some cells locally, it sounds as though any cell from 700 mah to 1100 mah will be fine. I agree with your notes regarding shorting.

Thanks again.

Bruce

[Bernhard]

Hi Bruce,

any NiCd or NiMH cell with any capacity will do as long as the diameter and length fits the parameters. The spot-welded contact strip variety has the advantage that it indeed was designed to be soldered. If you try to solder cells without such strips, the cells may be damaged and this may cause leaks. One final hint: try your local model aircraft / model car outfit. They should have such cells for solder connection.

regards,
Bernhard

[David Smith]

Any good battery store is able to properly spot weld the two cells together. You want the strap to be such that the cells are loose and can move back and forth a little bit. You do not want them glued together.