hp 41c battery pack rebuild


I've just acquired a 41c which needs the batery pack rebuilt, and was wondering if someone could share a little of their expertise / experience here in this forum. My questions are as follows:
1. What's the best way to get the battery pack apart...does it snap apart or is it glued?

2. What voltage does a working battery pack supply to the calculator? (looks like 5v to 6v from the configuration)

3. Since the wall adapter output is 8vac, am I correct in assuming that there is a rectifier built into the battery pack?

4. Whats with the center contact in the battery compartment...does it tap off an intermediate voltage or something?

Any and all information very much appreciated.
Thanks, Hal



1. It is seamed & does not snap apart: I use an Xacto Blade (or cutter for Europeans) to cut it apart cleanly, taking care not to break it.

2. Normal spec is 4x1.2V=4.8V which is enough. Up to 5 or 6V is not a problem as alcalines provide 6V anyway. I get 5V steady with my 4x 1/3 AA rebuit accupack.

3. Yep...with beautiful tiny fragile standard diodes.

4. Nein, it just links the 2 middle batteries. It is useless with a rebuilt rechargeable battery pack.

Small cells like 1/2N are hard to get. Therefore, you can use 1/3AAA for your first try but won't get a great capacity with these. When you get more practise, you can use 4x1/3AA which give you much more capacity but require careful grinding & sanding the accupack inside to give more room.

Friendly regards from France.


Fourqueux, Yvelines, France, Earth

Edited: 27 Sept 2005, 3:03 p.m. after one or more responses were posted


Thanks very much for the information. I think I can find the little N cells. If I have any problems, I'll post another message.
Best regards from Idaho (Northwestern US).


Yes Hal, I'm pretty sure you can find N cells quite easily.

However, those are too big to fit in the rechargeable pack. They can just serve as substitute for the dead-expensive N alcaline batteries in the non rechargeable battery container.

To recell the rechargeable pack, you'll want 4x 1/2 N cells which in my opinion are not found so easily at least here in Europe.

That's why I was advising 1/3 AAA or 1/3 AA+hard work.

Have a nice day!


Fourqueux, Yvelines, France, Earth

Edited: 27 Sept 2005, 3:03 p.m.


Hi Etienne:

I have rebuilt a lot of battery packs with NiMH 1/3AAA cells. They are 150mAh and last a lot. I have never seen a battery pack with 1/3AA cells. Do you have any photograph?. My email is jonjosebaibNOSPAM@euskalnet.net




> I have rebuilt a lot of battery packs with NiMH 1/3AAA cells.

Hi Jon, I'm doing the same thing with my HP41CX, but can you confirm me that the charge of these NiMH batteries is OK with the standard HP charger?

Are there any problems with this way?

Thanks for any info.

(Remove the double _NOSPAM_ from my e-mail address)


Hello Jon & all!

As requested, I propose a few pics of an Hp-41 battery pack recelled with 1/3 AA :

Hp-41C battery pack recelled with 1/3 AA

Just click "Start slideshow" to get bigger pictures.

Sorry for the lack of aesthetics but it's the 1st one I did...the others are gone to other Hp 41!

In a few days, I'll try to post a step by step description on the recelling when I can get new dead packs.

I intend also to refit an inexpensive alkaline J-type battery & will post the pics if I succeed.

The best solution till now for me is indeed the 1/3 AA cell for it's increased capacity.

Friendly regards from France!


Edited: 28 Sept 2005, 12:47 p.m.


Hi Claudio & Etienne

There isn´t any problem to charge the cells with the standar charger. It works fine. These cells have higher capacity (150mAh) than originals. So it is more difficult to overcharge them. If you take care to wait until the calculator shows BAT to recharge, you won´t have problems. I can send photographs to your email if needed.

The final aesthetic result is very good but capacity of the cells is, of course, lower than Etiennes pack. Etienne, just wondering if the contact is good when you insert the pack or you have to adjust it each time...

I have also succed using 2/3AAA NiMH cells (300mAh) with an standar alcaline holder. I found a plastic 12/10mm pipe that increases the diameter so the final dimensions are the same as N cells (28mm x 12mm ). But you have to charge them outside . There are chargers for that (MAHA,...)




Hi Jon,

Thank you for your tips!

On the recelled battery packs, I put the original metal contacts from the Hp pack.

Therefore the look & contact quality are the same, I do not have to reajust anything and the calc looks the same as normal.

The recelled pack is NOT thicker as the original because of the 2 hours spent grinding & sanding the inside.

You cannot see these original contacts on my pictures because....I've lost them when rebuilding my first pack!!!

Sorry, the next recelling "J-type" or 1/3AA will be more beautiful I hope!



Edited: 28 Sept 2005, 6:41 p.m.

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