Re: HP 21 replacement cells Message #2 Posted by Bernhard on 3 Oct 2005, 2:17 p.m., in response to message #1 by Bruce G.
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
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