[Answered] HP-82120A battery pack

[grsbanks]

Does anyone have a reference for the Ni-Cad cells in the HP-82120A battery back for HP-41C?

I've just acquired a 41CV that has one of these and, unsurprisingly, the cells in the battery pack are dead. They look to be the same diameter as a standard AA cell but maybe 1/3 the length.

[Dieter]

(11-12-2018 10:02 AM)grsbanks Wrote:  Does anyone have a reference for the Ni-Cad cells in the HP-82120A battery back for HP-41C?

Originally the pack had four 1/2 N sized NiCds. These are no longer available. Refurbishing a 82120A today usually is done with 1/3 AAA cells, and NiMH instead of NiCd. Here the typical capacity is 170 mAh which is about twice the original 1/2 N capacity. The circuit board inside the 82120A sets the charging current to about 15 mA which is just fine for a 12-15 hour charge if the battery is completely empty (!).

This topic has been discussed several times both here and elsewhere, so you should find instructions and tips on how to do the conversion.

Dieter

[grsbanks]

I did a bit of research after posting that query (I know, should have done it before...) and have come to the same conclusion.

I'm going on holiday in a week but when I get back I'll get myself some 1/3 AAA cells.

[Dieter]

(11-12-2018 02:49 PM)grsbanks Wrote:  I'm going on holiday in a week but when I get back I'll get myself some 1/3 AAA cells.

Fine. You may get four flat-top cells with Z-shaped soldering tabs, or maybe you can also do it this way:

Some time ago a friend of mine planned to rebuild a 82120A pack, and so I called my local battery supplier who also offers custom-made replacement packs for power tools, laptops and other devices. So they assembled a complete NiMH pack, consisting of four 1/3 AAA NiMHs, professionally welded (!) and shrink-wrapped. As desired, they even left a small gap between the cells so that the contacts at the ends nicely aligned with the case and their counterparts in the calculator. :-)

Dieter