HP 97 Battery Question


I can power my HP 97 using a standard 6-volt Lithium battery. Is there any problem powering the calculator using a standard 6-volt dc power supply connected to its battery terminals?



theoreticaly, no. There is no problem at all, BUT (and read it as a loud and clear BUT) you MUST use a completely protected, regulated, 100% noiseless 4.8 to 5.2 Vcc power supply, not a 6 Vcc, because original cells are NiCad and their voltage is about 1.2 to 1.3 Vcc. 6 Vcc is not achieved even when charging them, so it's good preactice keeping it under 5.2 Vcc.

The "completely protected, regulated, 100% noiseless" is intended to copy batteries behavior, and that's what internal circuits expect to be fed with: clean, pure constant-current voltage supply. If you grab the first cheap 6Vcc power converter and measure it wihtout anything connected to it, you'll find about 7.5 Vcc, with a considerable 60hz/120Hz ripple (rectified and bad filtered), what will probably destroy your beloved HP97 after a few sections. IOt will probably resist bravely after a few hours/days/weeks, but everytime you switch it ON, all of the circuits will receive a brief 8.5 to 8 Vcc peak voltage (capacitor charging) before diodes .7 Vcc drop each (1.4 total)allow it to go close to 6 Vcc.

Spending some time developing a regulator with forward protection or making sure you find a regulated/proteced 5.2 Vcc power supply that will not float while the HP97 is reading/writing a card or printing while runnning a program (peak power demand) is a better choice.

Or rebuilding the battery with existing Motorolla cellphones battery packs... I rebuilt two of mine with these and they work way fine.

My US$ 0.02.

luiz C. Vieira - Brazil



Can you say me which Motorolla cellphones battery packs works fine?.




Hi, Jon;

I opened (WITH MUCH CARE NOT TO SHORTEN THEIR TERMINALS) at least ten battery packs from TELE T.A.C. series, being both SNN4258D and SNN4019E the ones I used most. I prefer the SNN4258D because it has NiMH cells (usually isolated with green plastic). There are other types I remember being disassembled, but I do not have their codes.

One thing that called my attention is that there are different batteries inside packs with same ID #. In fact, what matters is that they must keep the same Ah relation (Ampère/hour), and the cells inside are found in some different shape. I'd like to find N-size NiMH (HP41) but I think I'll never find them...

What I noticed is that in most cases, dammaged packs have one (worst case was two) bad cell, and unfortunately they are usually in the middle (Murphy's law); at least in one case the dammaged cell was in the edge so the other four ones were kept with their original metal contacts (no need to further soldering except for the terminals).

In general the cells are sub-C equivalent with a lower diameter, so you'll need something to keep them in place inside the HP97 battery holder (and all topcats plus HP82143A, HP82161/82162 and a few others use the same package). The existence of the holder eases the job, you do not need to keep batteries firmly inside the equipment, just inside the holder (the small cage). I used a small piece of thick rubber in the middle of the cage so both batteries and contacts were kept in place. As I have only one 82143A (at least so far) that uses this pack, I rebuilt only one for testing, but I have the cage for a spare that I'll rebuilt and keep for emergency.

I hope it helps a bit. Anyway, I was informed a bit ago that my provider found another host for its customer pages and the one I had to place images is already available. I'll check for this and, if applicable, I'll scan the battery pack and place images there. Keep tunned!

Best regards.

Luiz C. Vieira - Brazil


Hi Jon, folks;

I thought I could get my pictures back in great style. Look below:

1 - HP97 (and others) battery holder without cells

2 - Original Hewlett-Packard NiCad battery cells

3 - Rebuilt battery pack with Motorolla®-pack cells

4 - The remaining cell from the pack of five used in (3)

5 - Another set of cells (one missing) from another Motorolla® pack

6 - Original Motorolla® SNN4258D

7 - Opened SNN4258D with different (smaller) cells

Items (4) and (5) (cells) were taken form two different SNN4258D, as are the cells in (7), being this the newer SNN4258D I have. All of them are not good (low performance) and I identified at least one troubled cell on each of them. The one shown in (4) has a higher internal resistence: charges faster than the others, but goes drops to low voltage either faster.

Hope the image helps having an idea. All items were scanned in the same "shot", keeping the same proportion. Images were retouched and background was chosen as for bweing neutral to all of them.

I immensely accept comments, suggestions, corrections, etc.


Luiz C. Vieira - Brazil



You have done a fantastic work. Thanks for the idea.
I don´t have these bateriaes available at this moment but I will look for something similar.




I have found an old battery pack I used for the camera with 5x1.2 v. of similar length. The cells could be ok (less diameter but...). I know at least one of them is failing. How can I check each cell?




Hi, Jon;

When I have this sort of situation, I must open the pack and access each cell's connection. Then I use an external power source that may deliver from Vbatt + 15% (min) till Vbatt + 40% (max), being Vbatt the nominal pack valtage. If you have five cells you should use an external supply from 7Vcc to 9.5Vcc with a series resistor enough to limit current intensity to a maximum of 20 mA. Then I'd check for voltage over each cell at the very moment the power is connected and after about one hour, measuring form time to time. The bad cell will be the slowest one to reach nominal voltage if it is shorted OR the fastest one if it has higher internal resistance. If you allow higher current, say, 30 to 35 mA, the cell that is NOT warm at the finger touch is the one with problems. I prefer measuring voltages.

I hope it helps. If you need further advice, let us know. There are others here, like Norm Hill, Glynn, Randy amongst other brainy guys, that deal either with analog devices or with batteries and they will surely add valuable info.

Best regards and success.

Luiz C. Vieira - Brazil

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