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HP Calculator Battery Pack Rebuild

By Chris Codella, W2PA

August 2009

Hewlett-Packard produced the first pocket-sized calculators capable of doing trigonometric and other scientific and mathematical functions.  The first generation of them, including the HP-35, 45, 55, and 65 were made beginning in the early 1970s, along with the HP-80 for solving problems in business and finance.  While expensive, they were ground breaking technology for the time and revolutionized how engineers, scientists and students worked, retiring the slide rule forever.  Any real engineer uses a calculator with RPN.  ;-)

But if you're reading this article you already know all that and are interested in getting your classic HP calculator running on batteries again, so let's proceed.

For a lot more information about HP calculators see the following sites:
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HP was still selling replacement battery packs for the classic calculators in the early 1990s, and I was able to purchase a box of them at discount through a friend (W2SQ) who worked there.  But before I could use up even one of them, in less than 10 years they all became incapable of retaining a charge.  By this time HP had stopped selling them.  I continued to use my HP-35 on AC power only, but usually defaulted to my newer HP-32S and HP-11C more frequently, for no other reason than being able to pull them out and use them on batteries. 

I recently decided to try replacing the cells in the HP packs with newer, maybe higher capacity cells.  Rather than use NiMH or other more modern types I chose to replace them with new NiCd batteries just in case it made any difference in the charging process while inside the calculator.

The first challenge was to get the old cells out of the battery pack.  Having five packs to work with afforded me the opportunity to try different methods and not worry about wrecking my only one.  So after prying two of them apart in different ways, I discovered enough about how they were stuck together to get it right.  Note that this procedure will only work on the newer replacement packs.  Since I didn't have an original one from the 1970s to play with, I can't say whether or not it would work on them. 

The two halves of the pack are held together by tabs inside the flat ends of the frame.  These are the only places where the plastic pack frame is glued together (or, as I've read elsewhere, ultrasonically bonded).  Break these bonds and it opens right up with no cutting necessary.  The trick is to apply pressure to the right spot.  I found that if I used a small screwdriver to pry the inside corner of the plastic frame apart, leveraging the screwdriver shaft against the end of one of the batteries (as shown below), the tab bond closest to the corner would break first and then I could break the rest by simply prying it further apart with my fingers.  Doing it this way allowed me to open up the pack without breaking any part of the plastic frame.

Once open, the batteries came out easily.  They are linked together with metal straps that look like they are spot welded to the terminals. 

On either end of the three-battery chain, similar metal straps are formed into V-shaped contacts that nestle between the cells and form the contacting surfaces for the spring contacts inside the calculator's battery compartment.  I clipped these special contact pads off the old batteries so I could reuse them later. 

I had read some on-line articles about how replacement packs could be made by soldering wires to new AA batteries, but it seemed to me to be a dicey and potentially dangerous process that could lead to damaging the new cells by excessive heat from the soldering iron.  A short Internet search turned up several places that sold AA-sized NiCd cells that already had straps or tabs welded to both ends, all ready to solder in.  I bought six, intending to make two new packs.  Here's a picture of three of them, showing the solder tabs - the positive tabs have covers, and the positive end of each battery has an indented ring around it.  According to what I've read, the original cells had a 700mA-hr capacity.  These new ones are rated for 1000mA-hrs so they should last longer.  I anticipate they'll also take longer to charge.

Now the procedure was simple - solder together three of the tabbed batteries, then solder on the V-shaped contact pads I had clipped from the old dead packs.

Once reinserted into the plastic frame, I put a dab of Elmer's Glue on each of the six plastic tabs, which had had their previously bonded surfaces broken free earlier, closed it up, put a couple of books on top to hold it together while the glue dried and...  Here it is!  My newly assembled classic HP calculator battery pack, ready for use.

And finally, here's my newly re-energized HP-35 running on batteries for the first time in about a decade, along side its original leather carrying case.  Of the four versions that HP produced for sale, this one is a version-two HP-35, made during the 49th week of 1972 (mid-November), according to the serial number.

Copyright © 2009 Christopher F. Codella, W2PA.  All rights reserved.