Rebuilding a HP41 battery pack

I have several old HP41 battery packs that require rebuilding so I have been perusing some old articles on the topic. Unfortunately I have never figured out how to search this forum efficiently and am quite sure that I have missed relevant data. At any rate, since I absolutely hate soldering anything I decided today to take a trip down to the local Batteries Plus and see what they had available. The only thing they had was a 50MAh NiCd cell that specifies a charge rate of 5ma for 14-16 hours. Since these are the cells that were already in the pack (from a trip to B+ some 15 years ago) I went ahead and had them rebuild 2 packs for me.

On returning home I got online and found a NiMh 1/3 AAA that is rated at 170 MAh and that got me thinking...

Looking at the 82059D charger sitting in front of me I see that it has an output of 8VAC with a maximum power output of 3VA. If I remember my high school and college physics that translates into a max current of 375MA. So, given that the output is AC and that 375MA would absolutely toast a 1/3 AAA cell rated at 50MAh (that would be a whopping 7.5C rate for a very small cell) I am assuming that there is some circuitry that regulates the charging current. If this is so, and if the charge rate is limited to 5MA or thereabouts, it would seem to me that replacing a 50MAh cell with a 170MAh cell would push the charge time to upwards of 34 hours. So my question is: Has anyone tried a higher capacity cell in a 41 pack and what sort of capacity, charge times, and run times are you seeing?



Edited: 16 May 2011, 5:15 p.m.


Hi Marwan,

I use GP cell GP17AAAH1A1H cells for my rebuilds, and change nothing else. I charge overnight, so 12 hours (ish), but I couldn't tell you how long that charge lasts. But, you could change R1 (see in the battery pack circuit to increase the charge rate.



Hi John,

Thanks for the info. This is the cell I located online:
[link:|101|1|%2cN~2147384805-4294765403-4290305461&Title=1.2V+170MAH+NIMH+1%2f3+AAA]NiMh Cell[/link]

Sorry, I can't seem to get the darn link to display correctly.

It appears very similar to the cell you are using. According to the specs for your battery they recommend 16 hours at 17 MA. My guess is that you are not getting a full charge. However, I have to wonder, if the charge rate of the unmodified pack is right around 5MA, whether you could keep the pack plugged in indefinitely without fear of overcharging and only unplug it when away from your desk. I mention this possibility because a C/34 rate is extremely low and would generally be considered a safe trickle charge rate (at least for a NiCd and I do realize that we are talking NiMh here).

I think that I will grab 4 of these and get out the old soldering iron and give it a try. Admittedly, as stated in my initial posting, I am terrible with a soldering iron but I can always hijack the services of a friend.



Edited: 16 May 2011, 10:45 p.m.


Hi Marwan,

Yes I am probably undercharging the GP cells, I may change the value of R1 so I see ~17mA going into the cells, and that should suffice.

Before you buy the cells, just make sure they have solder tabs. I clicked through that link, and I couldn't really tell from the pic on that page. The GP17AAAH1A1H definitely do have solder tabs.

If you need help sourcing suitable cells let me know, I don't think I have any spares, but I can possibly get some for you, or help you to find a supplier in the US which would make more sense as I'm in Australia.

Also, I have a documented procedure for doing the cell replacement, so I can mail that to you if you like.



Hi John,

I would love to get hold of that procedure. I'll contact you offline and send you my email. I think that I found a US source for the cells you are using but I'll have to take another look in the morning.

Thanks for your help!



Here is some information you might find useful:

  • The 82059 (or 82066) charger is rated at 8 V AC (!) and 3 VA, but this is not relevant at all here. The AC-adapter does not push current into the batteries, it's the charging circuit that sets the charging current. The PCB inside the 82120A battery pack sets this current to something like 15 mA (DC of course). This means, that a completely empty (BAT announciator on) common 150 or 170 mAh 1/3-AAA-size NiMH-battery will be charged within 12-14 hours.

  • 1/3 AAA is the right size here, and the net knows various instructions on how to refurbish an old 82120A-pack with current NiMH batteries.

  • Soldering tabs are nice, but there is a better solution. For instance, my local battery distributor does not only stock this type of "industrial use" batteries, they also offer custom-built packs with nicely welded (!) tabs, even shrink-wrapped, if you wish. Maybe you will find a similar service in your area, or at least on the net.

  • There is another option: Use N-size NiMH cells. Their capacity is 500 mA, i.e. three times the 1/3 AAA type. Use these cells in the regular 41-series battery holder and charge them externally. You can even use an old AA-charger rated at 45-60 or 100...120 mA, since back in the Eighties 500 mA was the usual capacity for AA-size NiCds. Charging time = 1,2 * capacity / current. There are mechanical adapters to fit N-batteries in AA-chargers. But beware - do not use chargers with higher currents.


  • #7

    Hi Dieter,

    You confirmed some of the information that I suspected. What I was not aware of was the 15MA charge rate. This seems somewhat high for a 1/3 AAA cell of that period (early to mid 80's) but I will accept this info as fact. If so, obviously replacing with a higher capacity NiMh cell without any changes to the internal circuitry becomes trivial. It would also seem that charge times for an original capacity NiCd rated at 50MAh should be kept around 4 hours.

    At one point I tried the rechargeable 'N' cell approach. For various reasons (primarily the quality of the cells available at the time) this did not work very well for me. I am going to look into this again as it would be a great option.

    Now for a completely different approach... I fly electric powered radio control aircraft and there are a plethora of packs available for these planes. I have sitting in front of me a 3.7V (single cell) 400mAh LiPo pack that is about 30mm*25mm*8mm in size. This pack would easily fit in the space available in a 41 battery pack. A singe cell LiPo pack has a nominal voltage of 3.7 V and a full charge voltage of 4.2 V. Will a 41 run off this voltage and if not would the next size up (7.4/8.4) destroy it? Of course this approach would require a very specialized charger (which I have) and probably require that it be charged outside of the calculator.

    Thanks again for the information that you provided. Any thoughts on the above idea would be appreciated.



    This topic - rebuilding a 82120A pack and related questions - has been discussed here not too long ago. You will find further information there.

    The 82120A battery pack did not use 1/3 AAA size cells, but 1/2 N size batteries. This size is no longer available, but 1/3 AAA is a nice (slightly smaller) replacement. The original batteries had a capacity of about 70-80 mAh, so an empty pack was fully charged after 7 hours. Now think of thousands and millions of users who left their pack charging for 10 or 12 hours, or even longer...

    The charging current indeed is about 15 mA. A friend of mine measured 15,6 mA in his pack, and this is confirmed by a quick look at the schematics. Please take a look at this site. The AC-adapter 82066 (European version) provides the usual nominal 8 V AC, but since there is virtually no load the effective voltage is more like 11 V AC. Behind the rectifier and the smoothing capacitor the voltage should be something like 15 V DC, just right for the following 12 V DC-regulator. The charging circuit finally is essentially a 365 Ohm resistor across which the voltage is 12 V minus the 6,2 V Zener diode minus roughly 0,6 V drop caused by D6. Thus the nominal charging current is (12 - 6,2 - 0,6) V / 365 Ohm = 14 mA. Voila.

    There are slight differences between earlier and later 82120A packs, there were even packs with three pins. However, I assume the basic schematics are similar.

    Using Li-Ion or Li-Po batteries is an option that comes to mind soon once you think about alternative solutions. But the voltage will be a problem. The BAT announciator comes on at 4 Volts, and two cells (around 8 Volts) are too much in my humble opinion.



    Hi Dieter,

    Since I had some difficulty searching the forum for this topic the information you provided is useful and appreciated. Your opinion that 7.4 - 8.4 volts is too high for the 41 will keep me from trying the 2 cell LiPo option at this time. The information on the battery annunciator voltage is also useful. The LiPo cell I was considering is designed for extremely high discharge rates (this particular cell is rated at 20C) so it might be that a single cell solution would actually work since the extremely low current requirement imposed by the 41 would cause little or no voltage depression for a large part of the cells life. When I have some time I might try to test this theory and see if a single cell LiPo works.

    I have also done an online search for rechargeable NiMh N cells and find that they are actually quite easy to locate. The problem is that I can't find adapters to charge them in a standard AA/AAA charger. I am still looking into this as it appears to be an attractive option.

    Thanks again for all your help!



    The mentioned AA-adapter is readily available in many stores that sell electronic components. I do not know where you live, but here's an example from a major German supplier (Conrad Electronics). There even is a PDF data sheet. As you can see the adapter is made of aluminium. Real cowboys of course will make their own adapters. ;-) I have a set of four, handmade by a friend with far better skills in this area than mine.

    Charging N-cells can be done in many ways. A long time ago I built a simple constant-current charger that can be set to 75 and 150 mA (basically the same design as the one used in the 82120A pack). Simply put four N-cells in a usual battery holder (like this one) and connect the charger of your choice.



    Hi Dieter,

    I thought about charging serially but if at all possible I would prefer to charge cells individually. I will continue to search for a commercial solution and if I can't find one I'll try the build-my-own approach or go for charging them in series.

    When I get the time I am going to try a single cell LiPo battery and I'll post my findings.

    Again, thank you for all the information you have provided.


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