NiHM batteries in HP-25


Dear all,

I'm in the process of rebuilding a battery pack for an HP-25 recently acquired. NiCd batteries are somehow difficult to find in all-purpose shops, and then I've got 1.2V 2300 mA/h NiHM batteries. My questions are:

1. Can they damage the calc, even if the voltage is the same as NiCds? 2. Will the HP charger be able to charge them? If so, how long could it take?

Thanks for your feedback!

(Sorry for the similar message with wrong Subject)


I use NiMH batteries in my HP-25 (1.2 V, 1900 mAh), and have had no problems so far. If the voltage is right (and 1.2 V is the same as what the original NiCd cells were rated for), I don't see how they could damage the calculator... In fact, I have often heard from people who even powered their Woodstocks using 1.5 V alkalines with no problems.

I don't have a charger for my HP-25, so I have to charge the batteries outside the calculator, but I'm sure others on this forum will be able to give advice on that issue.

- Thomas


I don't think there is any problem with NiMH. Charging would take longer though as a 2500 mAH battery has almost 5 times the capacity of the original NiCad in the 25.


Hi Rodolfo, folks;

I read many posts about this and I also added my own worries about this fact. As mentioned, the 2300mAh NiMH batteries have almost four times current capacity of the original HP25/25C pack (all HP2x, Woodstock series); if I am not wrong, they are rated 650mAh. Because of this, when fully charged the NiMH pack allows about 4 times operating time, and also needs four times charging time. If the HP25 internal circuits were designed to charge a 2300mAh battery pack, it would feed the pack with about 1/10 of the nominal pack current, that is equivalent to 230mAmps. Based on its original pack, this current is limited to approx. 65mAmps. With these figures, the charging time rises to about four times the recommended 8 to 12 hours, i.e., 1½ to 2 days chargind time.

I also use many high capacity NiMH in my Spices, and although I have the original chargers, I do the same as Thomas Okken: I charge them with external chargers, outside the calculator. Mainly because I don't like the idea of having a charger circuit design to operate for 8 to 12 hours being in operation for 36 to 48 hours. Yes, it's a fact that the batteries will not overcharge if the calculator is left plugged in the Ac outlet, but in this case, the NiMH will drain charging current for as long as their internal resistence is higher than normal.

The positive fact is that the NiMH will not be warm as it happens with the original pack when charging, because the applied charging current is about ¼ the recommended charging current.

I think that other considerations may come from other contributors, so please wait for other posts if you want additional info.


Luiz (Brazil)

Edited: 1 June 2005, 5:44 p.m.


I agree with Luiz, I converted the batteries in a number of my old HPs (35, 25C, 67, 34C) to 1500mAH NiMH cells. So far, everything is working fine.

The 3x charge time is a pain, but since I don't use them daily, I don't mind leaving them on the charger longer. It is also why I chose the 1500mAH cells instead of the higher capacity ones.

In choosing cells, I pick cells made by well known manufacturers, rather than judging it by capacity alone. I prefer the ones made in Japan to the ones from China.

With regard to the charging circuit, when everything reaches a steady state, I don't think running it longer would have a big adverse effect on service life.

Woodstocks are known to be fragile machines when it comes to charging. So charging it off the calculator (such as with the reserve battery pack charger) is a good idea. If you must charge it within the calculator, you may want to install the zener diode for added protection.


Thanks all for your responses,

I've rebuilt the pack with no apparent problems. Unfortunately, calc seems to malfunction: if the statistical register are cleared (after making a mean calculation, for instance) it works no more (it doesn't take any more data). A pity, since otherwise the calc feeling and size is truly sweet.

To be honest, the calcs I'm getting seem to be plagued by small flaws (unsteady functioning in a HP-80, bad ON-OFF switch in an apparently mint HP-33E, the HP-25 cited above...) I suppose it is the danger of buying things you can't see before from people who doesn't seem to know much about them...

Anyway, I like them so much.... :-)


Hi, Rodolfo;

first of all, congratulations! That's good news, indeed!

Now about this:

Unfortunately, calc seems to malfunction: if the statistical register are cleared (after making a mean calculation, for instance) it works no more (it doesn't take any more data). A pity, since otherwise the calc feeling and size is truly sweet.
I'd like to help you and understand the problem. What I don't get is the fact that computing mean would cause statistical data to be 'erased'? Or you are telling that after computing mean and there is no other computation to be done you clear statistics data with
And when you say that it works no more you mean the calculator freezes? I once repaired two Woodstocks (HP22 and HP27) with the same problem, althought they had different behavior. One was completely dead and the other had unreadable display. I noticed some oxidation between the ACT (main processor) and the mainboard. I decided to remove all IC's and clean the residual oxidation, that was actually shortenning the cooper trails. Both calcualtors returned to normal operation (I have some pictures stored somewhere in here...). I'd just like to know what is hapenning to your HP25. BTW, is it a regular HP25 or a continuous memory HP25C?

I hope we can help you setting your HP25 back to normal condition.


Luiz (Brazil)


Thanks a lot, Luiz

I knew you are one of the most helpful posters of this forum, but didn't expect my HP-25 problem would attract your attention. Thanks again.

What happens is:

1. If I enter data for calculating mean and sdev, the screen shows "1.00", "2.00", ..., and so on for each introduced value, and so the statistical data can be obtained
2. If I clear the stat registers to calculate a new mean, the memory doesn't seem to accept new data, since the screen shows "1.00" for each introduced value, and the statistical calculations render "Error"

The same procedure renders perfect calculation all the time in my 33E, and so I don't believe I'm doing something wrong...

Probably an internal chip has gone nuts... doesn't it? Anyway I use a 48G for my daily work, but it's a pity for me (who loved HP calcs since youth, and because of that I'm getting a small and cheap collection) seeing old HPs slowly dying. I guess many people here feels this way...

By the way, it's a HP-25, not a HP-25C


Hi, Rodolfo;

thank you for taking your time to describe the problem 8^)

Indeed, this is completely new for me. Your description refers to what seems to be a problem related to SW instead of hardware. Just a question: if you store zero in all registers separately with:

and so, will this happen the same? Or have you tried to start a new statistics calculation without computing MEAN or any other? (senseless, I agree; what would entering data and not computing statistics functions be good for, han?) Now it is just a curosity of mine, because if with the first time it works fine, how come the others it doesn't? By taking software behavior it would be easier to explain, but even so it is still complicated...

And about the problem attracting my attention: posts in here tease me! I read them all, but unfortunately I have no way to answer some of them. If I have the answer or if I can try to help? Well, I'm already reading, why not to be usefull and make some new friends? That's what makes life worth living... Oops! Maybe not the subject itself... ;^)

If you have the time to test and let us know abotu, I'm sure others will also post and try to go ahead with the discussion. But please do not bother yourself in testing just for this purpose, just wait for having a chance to do so. These Woodstocks, as you mention correctly, are rare 'specimen', and for as long as they can be kept alive, better for us. I have my own HP25C with bad ROM chip: although I can key in numbers, perform some stack operations and store keystroke codes in program memory, the simple [ENTER] pressed in run mode returns a display full of zeroes. I removed one of the ROM chips (don't remember which) and it kept the same, meaning that chip is dead, and I tested another one from another calculator and it worked fine. Well, I cannot find a replacement unless it comes from a donator, but this HP25C is in fact resting for a while, as are two other of my own calcs and at least some other six or seven calculators from friends and contributors in here... Effective spare time for repairing is rare for these days, but I'm getting 'back to the korkbench' in these vacations... At least I hope so!

Best regards and success! (and thanks for the kind words ;^)

Luiz (Brazil)

Edited: 6 June 2005, 8:30 a.m. after one or more responses were posted


Dear Luiz,

Thanks for your kind message. Once at home, I've been checking the calc and it seems to me that it's a memory problem:

1. STO x doesn't store anything. RCL x always gives 0
2. Programs are not recorded
3. Sometimes stat calculation does not work even after the calc has just been switch on

I've read sometimes about old HP25s having dead memory chips. Maybe mine is one of them!

As I said, it's a pity, since that little thing is lovely. Regards from Spain!


Hi Rodolfo,

Spain must be beautiful this time of the year.

I encountered the exact same problem you referred to in my 25C. Assuming your memory chips are not fried by a shorted battery (remember I told you about the fragile charging circuit in Woodstock), then you may want to take a look at this link by Katie:

I performed the prescribed changes, and my 25C has been working beautifully ever since. While I had the calculator apart, I also added the Zener diode to minimize charging mishap. If you are good with electronics repair and soldering, you can get the repair done in an hour or so.

In my case, the memory chips came back to life when the circuit is warm. In my case, the warming is done by charging the calculator (with a good battery) for a few hours. Once I unplug it, the circuit cools, and the problem returns. That's how I determined that the memory chips in my 25C was still good.

Good luck.


Dear Gordon,

Spain is indeed beautiful now, but it's getting hotter day bay day...

Thanks for your feedback. I had read the linked article long before I got my HP25, but had totally forgotten it. I'll leave the calc on (don't have a charger yet) just to see if warming up makes any difference. Nevertheless, a dolphin is more likely to peel an orange than I to perform succesfully an electronic repair!

Thanks again and best regards

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