The Museum of HP Calculators
HP Articles Forum
[Return to the Index ]
[ Previous | Next ]
Making replacements for 82001 Classic Battery
Posted by Don Shaffer on 20 Apr 2007, 6:01 p.m.
Building Replacement Classic Battery Packs
The basic HP Classic type battery 82001 is shown as a 3D model. Going from just having some of the old batteries to actually being able to make replacements was an interesting project for me. At first I studied the existing design spending much time making sketches how I could fabricate the covers. I had four old batteries available. One I had purchased as a spare for some tests where I needed to make sure I had a backup. Another was the original battery pack I had from my old HP-75. I also had two packs removed from an older HP voltmeter that used both of them.
I had used up the life on the hP-75 battery once and had to cut the battery cover open to replace the cells. This is a typical battery repair that has been done and steps for doing this given in the museum archives. The main issue with this was that you had to purchase a perfectly good AT&T cell phone type battery and cut it apart; then solder the old contact strips from the original battery carefully onto the new cell pack. Then the whole thing is hot glued back together again. You will find that cutting the covers apart into the two halves is not as easy as it seems, as you can ruin your old covers if you make the wrong movement, or alternatively cut yourself.
Also, after reading about the Nickel Cadmium cells you find that it is possible to damage them if they get too hot while soldering the contacts on. Again while doing this you should wear safety glasses in case you cause the cells to overpressurize. I wanted to find a better solution for this problem also. After searching the web I found that the normal way the cell straps and contact straps are connected to the cells is by using a battery tab welder. I understand that this service may be a lot more common than I had thought, so you may have a some local service shop that does this sort of thing right in your own city.
Going back to the other problem of molding the covers; at first, I tried some really basic methods taken from art and craft books. I tried making a reverse mold several times without any success. These attempts included plaster cast molding over the old cover half, using heat cured molding clay, poured rubber molding, and finally liquid metal mold material was tried. This last experiment turned out as a disaster, since the solvent in the liquid metal actually dissolved one of my original plastic battery halves.
What to do next? Well I though about asking for a mold making service, but when I looked into this, costs were pretty steep beyond what I could afford. So the next step was to try to find out how I could learn to make these myself using the plastic injection molding techniques.
I have had some CAD experience so modeling was not totally new to me. However getting the right software was the first item. One quickly finds that the costs of these 3D modeling packages can also be quite expensive. And after one prepares a mold design, then how do you go about getting it into the piece of metal? I first tried making the 3D models from basic sketches of the battery I wanted to make.
Making A Replacement Battery Mold
Getting to the point where I actually am cutting the mold halves took several years. This included learning more about how to do 3D solid design of molds, how to translate these solid models into CNC code, and then how to run the CNC milling machine to actually produce the mold halves. Along the way one has to obtain several important items, CNC tool path geometry software to translate the solid model to CNC 'G-code'. The actual CNC milling machine needs to be obtained or put together from parts and bits purchased from ebay the way I did it. Then tooling is also needed. You also need to have the CNC controller and oh yes, a computer to run the controller that takes the G-codes and sends them to the machine during the milling operations. As you can see in the photo I finally reached this point.
The next item on the list was getting a plastic press to inject the molds once they were finished. Again, after some research a book of plans was found and the press was built from scratch using angle iron, and other bar stock. Finally the launch day for pressing the first prototype part finally arrived.
Pressing First Battery Parts
Your basic black molded covers are shown here. I also had made a fixture to check the size of the battery cover half.
This allows me to check that the covers fit closely to the original shape of a Classic stype 82001 I wanted to replace. Of course the design is different in several ways. The original design has an intricate overlapping interlocking cover arrangement that is much too complicated in my opinion. Instead for the design I choose much simpler flat halves easily hot glued, or later cut open for cell replacement. Original packs also have fancy mortice and tenon style joints at each end apparently interlocking the halves tightly and making it difficult to cut apart. These must have been hot air welded together or other means as the end joints are barely discernable. I had also run the CNC machine mold cutting as fast as possible while still getting proper geometry. The tool had some slight overshot of the cutter making mold for the outer surface. At first I had planned to redo this with a new mold with this feature eliminated, but after pressing the first set of prototype parts I decided I liked the ribbed effect on the ends. This also differentiates my design from the original, so collectors can tell the new replacement parts from any of the original stock Classic packs marked with the part number and mold tool numbers.
Next was some experiementation using whatever items of plastic I could find for testing. Also getting some different colors as you see. So in addition to the standard black I could get red, green, blue, chocolate, or other colors. While I do not know how popular these would be, I tried what I could for pilot production runs.
Making the Contacts
Next and final part of the project for me, was figuring out how to make the battery terminal contacts. I had some of the originals to figure out the right size. After some investigation on the web you find out that the basic material used for this type part is solid Nickel sheet metal. The use of copper contacts is not the best choice since copper can corrode much eaiser, while the nickel is much more resistant to corrosion. The nickel comes as sheet, this of course must first be cut into strips, then shaped and formed to fit into the valley's formed between two AA cells on the battery pack. The longer end connection goes to one battery terminal, and a second shorter terminal strap goes connects on the other end. These of course must be put onto the correct side of the pack so the exposed terminals connect up with the proper connections in the device battery compartment. Again the details for these proper connections are given in the museum archives. Final assembly is too hot-glue the halves together once the nickel contacts and connector straps have been welded onto the cells
I want to thank the museum for a place where I could discuss this project. Batteries and cover sets are available. I am still working out the selecting capacities that people would like to obtain. So any suggestions will be studied carefully. Also any other ideas for another project such as this, let me know too. I already had a question about making replacement battery cover doors for some of the other HP's models. Please contact me at email email@example.com or if that does not work at firstname.lastname@example.org for more information or give comments.
Edited: 21 Apr 2007, 2:41 p.m.
[ Return to the Message Index ]
Go back to the main exhibit hall