HP-29c Power (?) problems


Hello everybody,

At first I really do apologize for my english (well, better than my school's english, but not perfect).

I have a problem with my oooold HP-29c that I would like to expose here :

I stored this little machine years ago, WITH the battery pack inside (approximately during 20 years). When I realized it was a mistake, I put it away (about 6 months ago); true, this makes 19.5 years of ignorance.

Well, I put a charger with the battery pack inside the HP, and observed some kabbalistic signs on the display, those who seem to indicate some corrosion problems. But in fact, the battery compartment was very clean. Pressing the keys just made the display a little more silly (or mad), so I decided to open the beast.

It was pretty clean inside: so I just cleaned the keyboard with white vinager (50% v, 50% water), letting it into this solution during 15 minutes, pressing the keys meanwhile. I changed the liquid, using only water, and rinsed the keyboard. Then I tried to use a (cold) hairdryer to dry it, but it had no effect since the keyboard is constituted of two very very close plastic panels. So I just let it in a black box, in the sun light (here it is summer, and the temperature in the box should have reach the 50 ┬░celcius) during a whole week.

Finally, I reassembled the whole machine a week ago and it was OK! No more kabbalistic signs, keys were working for the best, and I even made a program to verify everything, and I was very, very happy (this machine belonged to my grandfather...).

Yesterday, I put away the battery pack to charge it in a HP-21. This is a pretty good battery pack for its age : while I am typing here, the machine is displaying Pi since two hours and a half without the charger.

But I just tried this morning to put it in the 29, just to use this machine a little more...

and then, after the usual 'error' message indicating the memory loss, the display and keyboard became one more time completely mad!

So what ?

- I can not believe this is a corrosion problem.

- I can not believe, (but I am not so sure) that the keyboard is so dirty that some dust has 'moved' in the machine last week.

But perhaps this is another thing that escaped to me. Perhaps battery requirements are different between an Hp-29c and an HP-21. Perhaps...

Well, if someone has an idea, or just want to suggest something... it would be a pleasure. Thank you for your attention.


If you didn't fry your 29C (charger with bad battery is a deadly combination), you should remove the NiCad cells and replace them with normal alkalines. Don't forget to put the charger in a safe place like the dust bin. ;)


One more time, thank's for your help, Marcus.

In fact, after having posted this, I let completely discharge the batteries. Then I put it back in the HP-29c, plug it to the adapter, and you know what? it works.

This means that the Hp-29c is NOT supporting the full charge of the pack (and this is the next question for me : why ?). I don't have any voltmeter yet, but I shall make some measurements next week-end.
I'll let you know, thank you.


Boy, are you lucky!! But don't you ever do this again!!! Life will not let you get away with this a second time.


Thank you for this, too.

The charger is a 82041A. The HP-29c is from Singapore, serial 1903S...; maybe the electronic of this latest generation is better?


Hi Xavier,

Rest assured, your english is excellent.

Yes, all the Woodstock series machines use the same battery pack.

I have a 29c (which I purchased new in 1978) that exhibits the same behavior as yours from time to time. It will work one day, then "go mad" the next. What I have found to be the cause of this problem is bad electrical contact between the battery pack and the machine, and even within the battery pack itself. If I remove the battery pack and scrub the contacts, and the ends of the batteries lightly with a pencil eraser (or sometimes an alcohol swab) it fixes the problem. If you are using a rebuilt battery pack, there may no longer be the spot welded metal strip to form the internal connection between the cells (at the closed end of the battery pack). If so, then the connection is made only by the internal spring (which also supplies the preload to hold the battery pack in place), and that interface must be kept clean and polished also. If the battery pack still has the center bridge, and shows no sign of being opened at the seam, it's probably factory original.

One more very important re-iteration I would make is that you never, ever connect the wall charger to your 29C. The battery pack also serves as the voltage regulator when the charger is connected, and if the battery pack has a bad contact, an over-voltage is applied to the machine, which can spell disaster. This would hold true even if you don't turn the 29C on while connected to the charger, as the CMOS circuitry (which implements the continuous memory) is exposed to battery voltage even when the machine is off. Using a 21 to charge the battery pack (as you mentioned) is OK as long as you don't turn the 21 on while connected to the charger.

I've cut the center bridge out of my battery pack, which lets me remove the cells and charge them externally (I use AA NiMH).

Hope this helps.

Best regards, Hal


Well, my battery pack is not so new, batteries have the writing 'Hong-Kong' on their rose-red plastic dresses... a pretty sexy battery pack, indeed.

Thank you for this advice, Hal, I think I'll do as you said, will use AA NiMH (and try to find them in Brazil). Do you know how much time may a program work until discharge ? Regards,


First of all, I must apologize for my whacky browser...I asked it to remember my password, and it also remembers the subject line at the time "Math Curiousity...an old fashioned approach" and inserts it every time. This must look absurd (thanks Google Chrome!)

If you are referring to how long will the calculator retain memory with the battery pack out, it's easily long enough for you to change the batteries (even if you have to remove them from the pack and re-insert charged cells). I recall I've had the battery out of my unit for an hour or so, with no loss of data. To be sure of data retention, you should have a spare pair of cells, ready to be exchanged with the discharged ones.



Thank you again, this advice will be very useful; I didn't know that, I used to know that the Hp-41 had this feature, but I'm very pleased to learn it for the 29.

Well, if I want to launch a BIG program (for ex., calculate Pi with 300 decimals), do you think one battery cells pair would be enough ? In other words, can this kind of power supply deliver a sufficient current during more than 24h, or better?

Best regards,


P.-S.: I realize that my question is a little silly: actually I only need to stop the program, change the pack, and run again, isn't it ? By the way, the autonomy of your cells is interesting, and if you know it...


Well, the original battery pack would run the machine for about 4 hours or so. So I'm thinking that even high capacity Nimh cells would give at best 6 hours (but that's only a guess). I'm sure they wouldn't last 24 hrs.



Thank you Hal. I had another questions about it (3v versus 2.5, or how to solder and charge the new pack), but I shall search it in the Forum, there are some threads about it. Have a nice day.



Just for closing this thread, finally I cut my battery pack and put new rechargeable NiMH cells in the HP.


Thank you for all advices. I'll do my best to help this way. Regards.

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