HP-25 Battery Pack


It is well known that a HP-25 should not be connected to the AC charger without proper accu pack. Now, I built a new pack (the original one of my old (1976) HP-25 got lost...)with NiCds in a plastic case, which fits well into the battery compartment.

Question: Is there any risk, if I connect to the AC charger (calculator switched OFF) and the batteries loose contact while being charged?




Question: Is there any risk, if I connect to the AC charger (calculator switched OFF) and the batteries loose contact while being charged?

Not as long as the switch is in the OFF position.

But it really is the best advice to place the charger in the most remote spot of your house and leave it there... Always charge the batteries outside the calculator. Since the original batteries only had 500mAh, your new NiMh last 5x as long with one charge and you will not have to charge them very often. If you use the original charger, it will take very very long to charge the larger batteries anyway.

Greetings, Max

Edited: 2 Oct 2009, 7:20 a.m.


Hi, Andreas;

I have the HP21 schematic diagram in hands, and it differs from the C (continuous memory) in many aspects. As you mentioned you have an HP25, not a 25C, I´d guess the power circuits may be closer to each other.

In the HP21, if the charger is connected and the ON/OFF switch is turned to OFF, there is only one diode and an 82 ohms resistor in series with the battery (half bridge rectifier, no capacitors); all of the remaining circuit is disconnected. Once you turn the calculator ON and the charger is connected, the 82 ohms resistor is shorted and the calculator is fed thru the battery in parallel with the charger plus the rectifier. The power line capacitor, a 60uF/6V, is in the calculator circuit.

If the HP25 has something closer to it, I see no problem if there is bad contact with the batteries while charging. The problem is that the batteries act as a voltage reference (regulator) while the calculator is ON. So, my suggestion is: allow it to charge for, say, about ten hours. Check if the batteries and the calculator case close to them are getting warm (82 ohms resistor). If you are going to use the calculator, unplug the charger and do so.

In any way, this does not apply the same for the 25C series.

Hope it helps.


Luiz (Brazil)

Edited: 2 Oct 2009, 7:29 a.m.


Hi Max & Luiz,

Thanks for your thoughts and input. To be on the safe side - I don't want to ruin my old HP-25 which guided me safely through my chemical engineering studies three decades ago - I build now an external charger, using the original AC adapter together with a 82 Ohm resistor (should be 0.5W since quite some energy will be killed) and a diode (1N4001). So I charge the accu pack outside with the original adapter. Not convenient, but safe.



PS: By chance, since yesterday, I am owner of a HP-42s


By chance, since yesterday, I am owner of a HP-42s
Congrats! I still have two working units, both upgraded to 32Kbytes RAM (I replaced them myself). One of them has one missing line and two missing rows in the LCD and many scratches in the keyboard faceplate, the other one is not mint, but looks great. Both work fine, though. I think the HP42S is the best choice for a stand alone calculator, mostly because I'm an electrical engineer and complex, matrices, integrals and SOLVE are our daily tools.


Luiz (Brazil)



Another practical and simple solution : throw this junk wall charger in the garbage!

Put 2 lithium cells into the machine.
It will last a long long time.
I reloaded this unit more than a year ago.


Edited: 7 Oct 2009, 5:49 a.m.


In the late 70's and 80's when I used an hp21, after buying a replacement pack or 2 I found I could pop the back off as you did and use nicads and charger from radio shack and saved a bunch of $$.



should just be carefull with the [+] pole while placing the battery in because of the contact shape...

Luiz (Brazil)


Meanwhile I decided not to use the original charger in combination with a self build battery pack. Instead, I put two Sanyo eneloop AA batteries into my self build pack and run the calculator. These eneloop AAs have 2000mA capacity, quite a lot compared with the original Nicads. I recharge them with an external NiMH charger.



Hi Luiz,

Cold you please confirm that the internal resistor is 82 Ohm, as you explained earlier? In previous postings 8.2 resp. 8.3 Ohm were mentioned. I m just building a simple external charger for Woodstock battery packs (original ones and self built ones) which uses the original AC charger as power source.



Hi, Andreas;

please, forgive me. You are correct: it is a 8.2-Ohm resistor instead of 82-ohm as I wrote.

I would edit the original post to avoid confusion and add a remark about your follow-up, but unfortunately it can no longer be edited.

Thanks a lot.

Luiz (Brazil)

Edited: 17 Oct 2009, 12:15 a.m.


Hi Luiz,

Thanks for the immediate information/correction, so I visit now an electronics shop to get the correct resistor and certainly little later my external charger will be functional....




Hi, Andreas;

given the size of the resistor, the time the calculator was produced and the amount of heat one can sense, I'd go for a 1Watt dissipation power. 1Watt resistors we find today are the same size as the 1/8 watt at the time the HP25 was produced...

Success! And please, let us know how it goes with the charger 8^)

Luiz (Brazil)

(P.S. - I'm sorry for taking too long to answer your first question, I just red it a few minutes before answering...)

Edited: 17 Oct 2009, 12:56 a.m.


Hi Luiz,

Among my electronic components I found a nice old 8.3 Ohm resistor with even 3W dissipation. Certainly oversized, it is like a small ceramic brick and in the cold season I may abuse it as a heater..

The basic construction of the charger is already finished, I need to wait until the paint got dried and then I wire the unit.

How to post here some photos? What is best method - I do not have an image host.




Halo, Andreas;

you may ask Dave Hicks to create a directory so you may upload your photos as you log in (the directory usually has your name on it) and then use the links to address them. This way, as Dave has already mentioned, the photos will be preserved and the links will never be broken (permanent address).

You can also create an account at www.photobucket.com (faster and easier than you can imagine...). I´ve been using it for a while (Geoff also uses it) and it works fine. See here. Any of these pictures may be added directly, like this:

My first MLDL 2000...

If you need further info, let us know.


Luiz (Brazil)

Edited: 17 Oct 2009, 9:49 a.m.


The reasons are known why Woodstocks better should not be run with the AC Adaptor. Therefore I built a simple and low cost external charger, which uses as power source the existing AC Adaptor.

On a base plate of plywood 7.5x10.0x0.6cm I glued a box of wooden stripes (1.5x0.5cm) in exactly the dimensions of the battery compartment of an HP-25 (or any other Woodstock...)

In one end of this box I cut out an area of 1.5x0.8cm, which gives place to the retainer of the original battery pack and furthermore facilitates to remove the battery pack from the box. In the other end I drilled two 2.00mm holes in a distance of 14mm for the charging contacts. These contacts are M2x16mm screws.With nuts and washers the correct distance of 53.5mm can be easily adjusted.

As connector to the AC Adaptor's plug two screws (M2x12mm) are used (head removed with cutting pliers). Drill two holes, diameter 2.0mm in a small wood stripe (glued on the base plate) and fix the screws with a 2k adhesive in the bores.

Now, the electrical part comes. Connect left connector in the box with the left connector to the charger with a 8.2 Ohm resistor (white body in the photo), soldering tags may be helpful and connect correspondently the two right connectors with a diode (I used a standard 1N4001). The "positive" end of the diode is marked with a silver or white ring, this needs to direct to the connector in the box.

The AC voltage is 9.0V, at the charging contacts (idle) it 4.2V DC.

Finished - or continue with painting.

Materials: Plywood, wood stripes, 8.2 Ohm resistor (1W dissipation), diode 1N4001, soldering tags, 4 M2 screws with washers and nuts, wood glue, 2k adhesive, paint.

Tools: Jigsaw, sand paper, soldering iron, solder

Cost: approx. 1$

Time demand: 3hrs.



Edited: 19 Oct 2009, 6:20 a.m.


Halo, Andreas;

first of all, thanks for your feedback. I see you found a 'guests place' at the museum so you could store your pictures. About the charger, I always believed (and still do) that the best solutions are the simpler ones, and most of all, one may need heavy brainy activity to come up with a working, simple solution. The pictures tell by themselves, it is beautifully done. And it works, what else do we need?

Thanks again, I´ll soon try my own based on your directions.


Luiz (Brazil)


Hallo Andreas,

Gute Arbeit ! This is now a new function for an old & unused HP accessory. Thank you for having realized it.

Edited: 20 Oct 2009, 7:35 a.m.

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