HP 41CX Battery Modification

I have had this HP 41CX calculator since my university days. I have grown quite attached to it. I like it a lot. But it has one major flaw in my opinion: it runs on 4 N-size batteries. First, these batteries don't last very long in this calculator. Secondly, they have always been expensive to replace. With their decrease in popularity, this is even more so today and, worse yet, rechargeables are getting rare to come by. For this reason, this calculator had been left to collect dust ever since my last four N-size NiCads expired years ago.

DC-to-DC Boost Converter

The opportunity to revive this calculator presented itself when I received a few battery operated cell phone chargers which are based on the MC34063A DC-to-DC boost converters from On Semiconductor. They run on 2 AAA batteries and output approximately 5.6V regulated. In fact, by changing two of the surface mount resistors, the output voltage can be altered to say 5.0V suitable for powering many 5V digital circuits - Arduino for example. Below is a picture of one of the cell phone chargers with the top cover removed, revealing a tiny circuit board measuring approximately 2.1cm x 2.1cm.















With the bulky LED (on the reverse side of the board) removed and the input and output terminals detached, the mini circuit board can be placed quite comfortably inside two of the available expansion slots:















In the picture above, I have used a small piece of electrical tape to insulate the board against touching the copper contacts inside and used wire wrapping wires to make the input and output of the board available from the battery compartment.

Modifying The Battery Compartment

By comparison, hacking the battery compartment was a much bigger job given that I am not that mechanically inclined. Using a DREMEL, I carefully cut and sanded away the three plastic dividers in the original battery holder to make room for two AAAs to be placed horizontally as shown below:















The new holder was constructed from 4 pieces of small PCBs. The boards were held together by a small amount of crazy glue. The 4 original springs were moved from the bottom of the holder to the top and were held in place by soldering directly onto the top PCB. The boards were sanded down slightly to make sure that the new holder was able to slide smoothly in response to the compression of the springs.

There are four battery terminals. The middle two are tied together but are not connected to the rest of the circuit. Using the picture above, the leftmost terminal is negative while the rightmost
terminal is positive. In order to ensure a reliable contact with the battery terminals, an I-beam like coupler was constructed out of scrap pieces of PCBs with two big drops of solder in the center piece of the beam serving as points of contact.

Finally, the new battery holder was connected to the input of the boost converter board via the wire wrapping wires made available in the previous section; the output of the board was connected to the coupler constructed above. With some minor sanding and adjustments, the newly modified battery pack snapped into place just as it did originally. Upon pressing the "on" toggle button, I was greeted with a "Memory Lost" message indicating that the calculator had accepted the new batteries and was waiting for my input. After further testing, I confirmed that it was fully operational.















The Last Step

While this solution worked fine, there was one major drawback due to my negligence to consider the standby current incurred by the boost converter board. With a multimeter, I was shocked to discover that it drew a whopping 6.1mA all by itself. Given that a AAA battery is rated approximately at 1000mAh, a fresh set of batteries would run roughly for 7 days!!! One solution would be to replace the booster board with one that has an ultra low standby current. This Sparkfun module with a quiescent current in the order of uA would be an attractive option if not because it is too big to fit into the expansion slots. Finding the right replacement would likely be tricky. This prompted me to add a mini power switch to have the power completely cut off when the calculator is not in use. This would render the calculator's clock and calendar functions completely useless and I would be greeted with the "Memory Lost" message every time it is turned on. Given the alternative, I was perfectly happy to accept these tradeoffs. Finally, I found it convenient to add the power switch to the rear cover that covers the expansion slot directly below the booster converter board:















Final Thoughts

I am generally quite happy with the results of these modifications. Except for the clock and calendar functions, my 41CX is fully functional. Some day, when I come across a suitable replacement for the boost converter, I will make the switch and I will have the option to leave the power switch always on. Until then, I am once again a happy HP 41CX owner.


Useful References

Repairing an HP-41 CX by Stefan Vorkoetter
The Museum of HP Calculators