HP41 Nicad pack and Batteries


I'm having a pack of 4 x 1/3 AA cells made up for my nicad pack a an approx cost of $US12. I'm going to have to modify the pack slightly to cater for the larger cells.

Has anyone done this before?

These cells are 110mAHr. Were the originals were 50 or 55mAHr (my box has gone missing)?

Unfortunatly the 1/2 N cells were not available, I had the choice of 1/3 AAA (55mAHr) or 1/3 AA. The 1/3 AAA are significantly thinner, and being a bit of a "Tim the Tool Man" I went for the big ones!

As for the adjustment to the pack, am I better off using a chainsaw or a pneumatic drill?


Steve, you have hit upon a problem all of us diehards have who refuse to give up their HP41's. My card reader died; but, it is about the only thing (other than the calculator itself) I don't dare try to take it apart and repair. Like Tim the Tool Man, my Achilles heel is my impulse to always assume everything is simple until proven otherwise, sometimes with dramatic results.

I now have the carcasses of about 3 battery packs on my drawing board at home. The one thing I can say I have done successfully is to separate the battery pack - very slowly using an exacto knife to separate only the glue and not cut the plastic too much. You have gone one further and found other batteries which will replace the old ones. The problem with replacing the old batteries with any new pack is the small power converter (AC to whatever volts, Can't remember now, DC) also contained in the battery pack module. This is the primary reason I quit attempting to rebuild the battery pack. I was also diverted by the fact that I found a surveying supplies store in Las Vegas about 5 years ago which still sold HP41 battery packs. Unfortunately I don't still live there and cannot remember the name. They were in southwest LV. If you want to pursue finding them, let me know and I'll try to dig through my files and find their name again. So far the last battery pack is still running; however, I have been very religious to run it all the way down and let it charge for at least a day afterward to ensure it is fully charged. Good luck, Roy


I spent several hours last night with a series of tools gently reshaping the nicad pack. It's amazing what a mm or two can so for clearances.

My pack now looks a lot different. Not only are the nicads exposed on the bottom of the pack, but also on the sides. As soon as I can lay my hands on the right tools, they'll be visible through the top too.

I have no doubt this pack will be fitted back together, but it won't look like any other pack you've ever seen :-)


You may want to try "Battery Company" out of Jacksonville, FL. They have tabbed 1/3N cells (N75P) which are a perfect fit. About $1.75 per cell.


The original batteries are 1/2 N (close to 1/3 AAA) are they not?

Although I beleive the designation N75P is correct.

Are they (Battery Company of Jacksonville) on the net at all?


Both Jerry Watterson from Sunn Battery Company of Jacksonville, and TNR Technical, refer to them as 1/3N cells. Regardless, I have rebuilt a dozen packs or so with these cells, and they work perfectly. Jerry Watterson, from Sunn Battery Company, can be reached at (904) 354-4508, or ptron1@hotmail.com


WRONG!!! They are not 1/3 AA or 1/2N cells. They're N-75 cells. They're rated at 75MaHr. Warning! I did business with Sunn Battery in Jacksonville ONCE and they charged my credit card over twice the price that they quoted me. Furthermore they messed up every battery pack that they put together. I had to tear apart every one of them and reweld them properly. I called and talked to Robert Calvert, the owner of Sunn Battery and he promised to correct the order and refund the over charge but he never did. Needless to say I don't recommend them. I have done business with TNR in Sanford Florida for many years and I have been very happy with their work. I think they have a website but I don't know the URL.

Joe Rigdon


As I said, TNR did mention that they are size 1/3N cells. As did Sunn Battery. Sorry to hear that your dealings with Sunn were not successful, but I have ordered from them about a dozen times, over 300 cells, and much prefer their quality, speed and prices to TNR. TNR Technical Inc.'s web site is www.batterystore.com, although I wouldn't recommend them over Sunn Battery.


Well good luck with them. Once bitten, twice shy if you know what I mean. I've been buying from TNR for well over fifteen years and I've probably bought over 5000 batteries from them with few complaints so I'll stick with them. Besides they're close by.



In reference to 1/2 N, that's just a reference to the size of the cell. The cell is physically 1/2 the length of an N cell.

The original cells were 55 mA Hr (at least in my old pack). Advances in technology have allowed the cells to have a higher capacity.

I remember that AA nicads were only 400 mA Hr at the time, and now they are up to (maybe beyond) 1000 mA Hr.

The cells are certainly not 1/3 AA (which are significantly larger) but I am fitting these to my pack because I want the extra capacity.

you have to be careful with designations of cells because there are the designations that refer to cell size (e.g. A, AA F, D, 5/4AA, 1/3AA, 1/3AAA, 1/2N) and other designations that refer to the capacity. My 1/3AA cells are designated P-11AA and in another place as 110N.

N75 probably just means that it is a Nicad with 75 mA Hr capacity. I'm sure it says nothing about the size of the cell (I could be wrong).

Be careful about the time you leave these cells on charge for. The Nicad pack charges them at around 18 mA (which is approx 4 Hr rate for 55 mA Hr cells, or 6 Hr rate for 75 mA cells, or 8.5 hour rate for 110 mA Hr cells. Notew that NONE of these is a 14 hour rate. Charging indefinitely at rates faster than the 14 hour rate can cause damage to nicads.


N-75 refers to it's capacity but it is also the battery model number. Here's the specifications of a few of the batteries.

model type capacity dia. length

N-50AAA (1/3AAA size) 50MaHr .41 .63

N-75 N-75 75 MaHr .51 .57

N-110AA 1/3AA 110 MaHr .57 .67

N-150A N cell 150 MaHr .47 1.16

N-250AAA AAA cell 250 MaHr .41 1.75

As you can see, you could probably make some other models fit but why bother? If you need higher capacity and rechargeability then use rechargeable N-cells. If you need HIGH capacity, use a pair of 3 volt Lithium cells. Even the AA cells have a capacity of over 1 Amp Hour. I'm running an opened up test 41 from one AA Lithium and it works fine except the low battery flag stays on.

BTW these specs are from Sunn Batteries catalog. These specs are for Sanyo cells. Other brands may be slightly different.

You should NEVER charge a NiCad at above 1/10 C rate IMO. You can fast charge them but you will significantly reduce their life. I have a set of 4 Sanyo AA cells that I bought in Thailand in 1974 that have never been fast charged and they still work fine!



Steve, I found that company I was telling you about that sold "new" HP rechargeable battery packs. I called today and got 1) the good news - they have two recahrgeable packs in stock for $20 each (and maybe some non rechargeable ones); 2) the bad news, the lady could not find the stocking number to know where they got them from. However, I remember when I got mine about 4 years ago, they only had one or two then. So their supply might be continuing if limited. The company is: Holmans Surveying Supplies 4445 S. Valley View, Ste.#2 Las Vegas, NV Ph: 702-222-1818 Homepage: http://www.holmans@nv.com

If you need a battery soon, you might want to check it out now since it is likely first come first serve and no telling how long until the next shipment. Good luck!



I'm the Service Manager at the above referenced company. We primarily repair surveying instruments and becuase the HP-41 was widely used in the surveying industry we get involved in a few repairs.

As mentioned, I had two of the N-75 NiCad sets in stock. I had ordered these a couple of years ago when my supplier told me they could no longer get them and these were all that they had left. I picked the $20 price out of thin air, if it is out of line let me know and we'll change accordingly. Since we can't find them in inventory, I just need to get close and cover the cost of paperwork and such.

Rhonda, the lady that Roy spoke to, has already recieved two calls on the batteries so I'm not sure that we still have them available. We are going to ask our battery suppliers if they can track down replacements, however, based on the comments in this thread I don't think we'll have much success either.

If you'd like to inquire, feel free to call us and talk to Rhonda or email me and we'll keep you updated as to what we can find.

FWIW, our page is at http://www.holmansnv.com/ However, I'm afraid there isn't much info there.

PS - since I'm posting here, can anyone reccomend someone to repair a keypad on a HP-41? I've got one with some 'soft' keys, the metal contact below the key has lost it's spring and isn't always successfully contacting. Someone has attempted to repair the thing before with limited success and I'd like to get it done right for my client. If nothing else, are the places suggested elsewhere at this site for repair worth trying?

Thanks for the interest

Chris Cothrun


I stayed up until 0230 to call you. My other half (who speaks American better than I) eventually spoke to Rhonda about life in the US, construction workers, the building market, engagememnt rings, schooling in Texas, heights, "other" details, zip codes, population, crime rate, vegimite and husbands.

Oh, and somewhere in there she also ordered a couple of HP41 nicad packs :-)

Even if we weren't first in line, Noelle enjoyed her half hour international phone call $-)

As for the keyboard, there is a thread above this one titled "HP41CV--Clean Keys" that may be helpful. I've not personally attempted to fix keys. I have a number of HP41s with various amounts of softness on keys.


Yes, Rhonda was quite thrilled to get a call from Australia and walked around the office all day doing her finest Australian accent. I do believe you have those battery packs coming to you.

She spoke to our battery supplier and they are going to make up cell sets so unless they don't come through we'll be able to sell them for $22.50 each plus shipping & handling. We don't have them yet, so I'll let anyone that is interested know when we do.

As for HP-41 repairs, I did see the above thread and I've even had someone email me so I should be OK.

Thanks again!

Chris Cothrun


Well, I'm confused ... I called Holmans the same day as the Australian's called. I said I was only wanted an Alakaline Battery Pack holder, that I did not want nickel cadmium Battery Pack. I was told I would be called if one was found. I have not been called, but today in the mail I received a box from Holmans. The invoice said it was a NiCd Batery Pack and total charges were $22.50. I opened it to find four batteries welded together, I guess ready to try and repair a genuine HP-41 NiCd battery pack. I hope a phone call Monday will clear my confussion ...

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