Making New Battery Packs for the HP-35 Calculator

Douglas W. Jones.
University of Iowa.

Today, an old HP-35 Calculator is likely to be as useful as it was when it was new, but finding new battery packs for one is difficult. Thus, most people who use one today must put up with the tether to its plug-in power supply.

Fortunately, HP-35 battery packs are fairly simple to make. Three 1.25 volt NiCd cells, wired in series, produce exactly the 3.75 volts the calculator is rated at, and three AA sized penlight cells fit comfortably inside the calculator's battery compartment. Furthermore, the battery charger side of the HP-35 power supply is well matched to the rated 45 milliamp trickle charge current of the AA NiCd cells I bought from Radio Shack.

Before going into detail on the battery pack, it is worth documenting the details of the HP-35 "wall wart" power supply and battery charger. This unit contains a transformer, a voltage regulator, and a current limiter, with the connection to the HP-35 through a 3-pin plug, as follows:

Figure 1 
     |      |     gnd
     |   ---|---
     |  |  bo   |
     |  |ao  co |
     |   -|---|-   + 
     | -  |o o--o o---
      -o  | S1   S2
      Bat | 

The plug is shown as viewed from the bottom of the calculator. The switch S2 is the on-off switch on the front of the calculator. Switch S1 is a spring shim that shorts pins a and c when the calculator is unplugged from the power supply, allowing the battery to power the calculator.

When the supply is plugged in, battery charging is provided by a current limited 50 milliamp 30 volt supply between pins a and b, while operating power is provided by a regulated supply between b and c; a 3.75 volts, 0.15 amp supply should suffice, but the HP supply I have puts out 5 volts.

I gleaned the above by disassembling both my power supply and calculator; I don't recommend disassembling HP-35 calculators if they work, but mine was broken when I got it. After disassembly, I found that the only problem was dirt under the slider of the on-off switch; and after cleaning and reassembly, it works quite well.

To to make a new battery pack, start with three AA NiCd cells arranged side by side with the positive ends pointing in alternate directions, and bundle them together with vinyl electrical tape, as shown:

Figure 2 
           _-_  ___  _-_
          | + ||   || + |
          |             |
          |             |
          |             |
          |             |

The next step is to jumper the cells together and add contact pads at each end. Note that you can sometimes find bundles of 3 AA NiCd cells, pre-jumpered and bundled in shrink-wrap. Buying these can save a bit of work. I gently clamped my stack of cells in a vice to solder the necessary connections to each end. To prepare the batteries for jumpering, I melted a button of solder on each end of each cell. Be careful soldering to batteries, the heat boils a bit of the electrolyte inside the battery, and you'll do the least damage by using a very hot iron very briefly; I used a soldering gun. Also be careful not to fill any vent holes in the battery with solder.

I used brass shim stock for the jumpers between cells and for the contact pads, and I connected the pads to the cells with stranded hook-up wire. The contact pads should be 3/8 inch wide and between 5/8 and 7/8 inch long. The connections to each contact pad should be soldered on the back of the pad, and the wires and jumpers should be soldered as shown:

Figure 3 
           _-_  _/_  \-_
          | \ ||   || + |
          |   \         |
          |   _\   __   |
          |  | +| | -|  |
          |  |  | |  |  |
            \___/-    `

It is critical to get the polarity right! Make your battery pack exactly as shown above, with the positive pad at the bottom left side of the pack. If you accidentally build a mirror image of this, you will either end up reverse charging your NiCd cells (which can damage them) or you will end up putting a reversed voltage into the calculator (which might damage it).

Finally, wrap tape over each end of the battery pack, as shown:

Figure 4 
           _-____/   \-_
          |             |
          |             |
          |             |
          |             |
          |  |  | |  |  |
          |_      ____ _|
            \___/-    -

The contact pads should be securely held by both their top and bottom edges so that they are between 1/8 and 1/4 inch apart. The tape covering the top half of the battery pack should cover the pads to a point about 1/4 inch beyond the center line of the battery pack, and the tape over the bottom half of the batter pack should extend about 1/4 inch from the bottom shoulders of the battery shells. I used a narrow strip of tape around the pack for the bottom end, while a full width piece of tape worked fine around the top. At both the top and bottom, I taped over the solder and wires at the ends of the battery pack before securing everything with tape around the pack.

The battery pack I made this way came out a bit on the small side; better small than too large, but as a result, it occasionally slips out of registration with the spring finger contacts inside the HP-35 battery compartment. The problem is not severe enough to make me consider rebuilding my new battery pack; the 3/8 inch squares of exposed contact pad are big enough that it can slop around quite a bit inside the calculator and still make good contact, and the geometry is such that if the battery pack is ever accidentally inserted backwards, it won't hurt anything because it only makes electrical contact when it's inserted the right way.

I don't guarantee that you won't damage your valuable antique calculator by following my instructions, and I certainly don't want to be held responsible for any errors you might make in trying to duplicate my work. Nonetheless, if you feel confident in your ability to handle a soldering gun and electrical tape, and if you know enough electronics to verify my reverse engineering of the HP specifications for the HP battery charger, you should find these notes useful.