HP-41CL & Battery


Given that the battery drain from the new 41CL by Monty is some ten times greater than a normal 41 while OFF, I am looking for a better battery solution than the normal N size batteries (or the even less capacity rechargeable counterparts).

How about building a new battery pack using cell phone batteries, like stacking two of these or some such. Any suggestion for a rechargeable replacement for the new line of 41s?


I might plug in the modified module with a USB connector to sustain power to the 41CL while at my desk. But that's no solution for beeing "on the road" with the 41CL. The stacking of cell phone batteries sound like a good idea!

The hint with the USB in a modified module came from Diego Diaz, see this thread!


We had a discussion on HP-41 battery options just a few weeks ago (cf this thread). While current LiIon and LiPo batteries for cell phones are not an option due to their voltage (3,6 V is too low and 7,2 V too high), the solution you suggest is a NiMH battery with 6 V nominal voltage, i.e. it probably is made of five cells instead of four in the original HP NiCd battery pack. Since freshly charged NiCd and NiMH cells may have voltages like 1,4 V and higher, the 5-cell pack may provide 7 Volts and more. That's why I would prefer a solution with the usual four cells.

Anyway, the essential point here is the higher power consumption of the 41CL. Let's take a look at what we can expect from different power sources:

  • 70 - 80 mAh: former 1/2-N-size NiCd cells in 82120A battery pack
  • 140 - 170 mAh: current 1/3-AAA-size NiMH cells for refurbished battery packs
  • 500 mAh: current N-size NiMH cells
  • 700 - 800 mAh: single-use N-size Alkaline cells
  • 2000 - 3000 mAh: current AA-size Alkaline cells (external battery packs)
  • 2000 - 3000 mAh: current AA-size NiMH cells (external battery packs)

So, what do we need for the 41CL? Without the card reader or wand, single-use N-Alkalines were a good option for the 41-series. In case of rechargeable batteries, even the original 82120A pack with less than 100 mAh capacity was fine. If you look at the figures again, there is an elegant solution with more than sufficient capacity: use simple rechargeable N-cells in the regular battery holder and charge them externally. Some more thoughts and ideas can be found in the thread I mentioned, including an external battery pack with four AA batteries (NiMH or alkaline).



Any easy way to ensure that the voltage doesn't exceed 6V?


Lithium cells have a higher voltage inherently, so 2 Li cells might suffice. I know they are available in AA size. Caution against overcharge, they may explode. Some are equipped with vharge limiters. Sam


Before we talk about "lithium batteries" we should define what this term refers to: Take a look at wikipedia to get an overview over the variety of various lithium-based battery designs.

In our case there are three major relevant groups:

  • Single-use Li-Mn batteries. Their cell voltage is 3 V or slightly higher. One of these may replace two regular 1.5 V alkaline batteries. There are 6-V-Versions like the 2CR5 used in many conventional photo cameras. As far as I know this technique is not used for standard-size AA or AAA batteries, simply because twice the voltage in the same package is not a good idea for usual consumer devices. ;-)
  • Single-use Li-FeS2 batteries with voltages roughly comparable to those of regular alkaline batteries (1,5 up to 1,8 V). In many cases these batteries can replace standard alkaline batteries while offering a significantly higger capacity.
  • Rechargeable Li-Ion or Li-Po batteries like they are used in most cell phones and digital cameras. Their nominal cell voltage is 3,6 or 3,7 V and can exceed 4 V when fully charged. Since the only options here are 3,6 or 7,2 V they are not particulary useful for applications that require 5 or 6 V nominal voltage.



Any easy way to ensure that the voltage doesn't exceed 6V?
It should be ok to exceed it a little. A brand-new alkaline is really more like 1.6V per cell, meaning 6.4V for four.

How about using two 4LR44 (6V) in parallel?


Using regular batteries in parallel without additional electronics generally is not a good idea. The 4LR44 in particular does not fit our needs very well either. As the name suggests, this battery is made of four LR44 button cells, the usual capacity is something like 100 mAh. There are more powerful versions like the 4SR44 (silver oxide) and 2CR1/3N (lithium), well known as power sources for photo cameras of the Seventies and Eighties, which can provide 150...200 mAh, but that's still rather low compared to other solutions (OK, twice that with two batteries in parallel, if you really want to do that) and last, not least: they are quite expensive.



Button-top Nicad N cells can be found for less than $3 each in 200 mah capacity. A set of 8 of these (to have one recharged set always ready) doesn't seem too expensive.

Flat top Nimh N cells with 40 mah are about $4-5 each, but then you have to deal with the flat top issue.

I will probably just use nicads...unless an easier, higher capacity solution can be found.


Well, NiCd batteries have been banned in many countries of the western world, so they are hard to get and most major manufacturers simply do not make them any longer. Over here in Europe, regular button-top N-size NiMH cells by GP (type GP50NH) are readily available for 3,49 Euros each from a source that is not exactly know for its lowest prices. Or for just 1,85 Euros from a well-reputed source on the internet, including VAT and other possible taxes. ;-) Their capacity is 500 mAh, more than twice that of the good old NiCd versions.

I always thought that "pay less, get more" is quite a good idea. ;-)



Links? No fair if no links! :-)


Here in Germany the two usual sources for electronic parts (and toys ;-)) are Conrad and Reichelt.

At Reichelt, the GP50NH can be found here.

The same battery (and a cheaper flat-top alternative at Conrad is listed here. There are additional discounts if you need more than one.

Yes, both webpages only speak German and I do not think that either of them will ship to the US, but I am sure you will find similar offers in your area as well.



For those in the US, I got 2 sets from here: GP50NH. price of the batteries is not bad just watch out for shipping options.


So, if I went ahead and made a new battery pack by stacking two of those I linked to - what could happen if they were freshly recharged and gave 7V output.

It would be interesting to order a couple, recharge them fully and see the output. If it is in the area of 5.5 - 6.5, it would be fine, right?

And then I would have my high capacity battery pack for the CL...


The board is designed to accept 6V. Applying a higher voltage will reduce the lifetime.


perhaps a silly question... why not have the final design of the 41CL board run off 2.4 volts derived from 2x AA cells, fitted horizontally in the existing 41 battery compartment? it would likely be easy enough to add the necessary step-up converter to the board design.


Compatibility. The 6V is on the Port connector for external peripherals to use. This would mean keeping a step-up converter running all of the time, meaning higher off-state current drain.

I believe that the board will run reliably at slightly less than 4V, although the battery warn annunciator will always be on in this case, which kind of defeats its purpose.



... would mean keeping a step-up converter running all of the time, meaning higher off-state current drain...

i'm not a 41 expert (never owned or even used one much really), but what is the point of powering external peripherals when the calculator is turned off?! any such peripheral, if it drew more than negligible power, would flatten the N-cell batteries quite rapidly anyway, denying itself ongoing power in short order.

and if this was an issue for a few odd devices under exceptional usage conditions, it would be easy enough to bring out the 6v feed line to a 2-pin header somewhere on the board that was accessible (such as in a spare corner of the battery compartment).

Edited: 10 June 2011, 1:27 p.m.


Robert, think of memory peripherals which rely on the 6V to buffer their contents or devices which may wake up the calculator.


Pressing the button on the Wand wakes up the calculator. I think that the printer can wake up the calculator that way also.


Would the non-stepped-up voltage be enough to preserve the memory and keep the clock going (these typically go on a lot less than normal operating voltage), and would a step-up converter come up fast enough for when an alarm or other stimulus tries to turn the 41 on? If so, you might still be able to go that route, use the step-up converter, and shut it down when the calc is asleep. I've designed step-up converters into products many times, but I don't know what the requirements for the 41 might be to retain memory and keep the clock going, and turn-on time.


there's a number of neat tricks that could be done - most obvious, have a capacitor that holds up the 6v rail when required and briefly start the converter every 10 seconds or so (when the calculator is off) to keep it topped up.

my reasoning is simply that it would be 'nicer' if the calculator could be run on more standard and obtainable AA batteries rather than N cells, while still fitting everything inside the existing body of the calculator (as opposed to people having to look at attaching an extended battery pack on the back).

and to answer the obvious question, Li-Ion cells with a service life of only a couple or three years aren't really a good match for a calculator who's lifespan thus far has been a quarter of a century :-)


Excellent points. So with the CL shipping, it should be a mini-market for replacement battery packs for someone with the skills to make this happen. I would be a buyer...

Possibly Related Threads...
Thread Author Replies Views Last Post
  HP-41CL setup troubleshooting Xavier A. (Brazil) 2 1,155 12-02-2013, 06:29 AM
Last Post: Xavier A. (Brazil)
  [41CL] New Extra Functions version Monte Dalrymple 0 676 11-08-2013, 04:32 PM
Last Post: Monte Dalrymple
  So, latest 41CL / Library 4 config is... Gene Wright 4 1,189 09-22-2013, 02:59 AM
Last Post: Ángel Martin
  HP-41CL anyone? Matt Agajanian 8 1,754 08-31-2013, 12:27 AM
Last Post: Sylvain Cote
  [41CL] A couple more rhetorical questions Monte Dalrymple 1 787 07-12-2013, 09:28 AM
Last Post: Ángel Martin
  41CL :TROUBLE IN FILE TRANsFER aurelio 22 3,896 06-18-2013, 03:44 PM
Last Post: aurelio
  [41CL] Another question for users Monte Dalrymple 28 4,721 06-03-2013, 10:04 AM
Last Post: Geir Isene
  [41CL] Updated Manual Monte Dalrymple 1 768 05-14-2013, 10:22 PM
Last Post: Matt Kernal
  HP-41CL & NoV(-64): Race condition? Geir Isene 11 2,076 05-03-2013, 01:59 PM
Last Post: Diego Diaz
  [41CL] Memory Reference posted Monte Dalrymple 12 2,346 05-01-2013, 03:39 PM
Last Post: Etienne Victoria

Forum Jump: