Batteries and analog stuff (or: Hi, Norm! I'm back)


Hello, Analog-related guys.

Norm, have a look at this post (I found it!) written in January 23.

You se, I have mentioned about that concern of your when using new, "high-powered" NiCads in vintage calculators. The NiMH is also a concern, of course.

That's why I thought about using AAA-Nicads instead of AA-type. Also, I intend building a battery pack tfor the HP82143A with new, 850mAH AA-NiCads because my pack is showing definitive signs of retirement request. I have a spare case so I'll build a new pack with AA-Nicads. I'll try to find 1,000 mAh AA-type NiCads. What's the size of the batteries used in the HP82143A pack? It's the same pack used it Topcats (HP91/92/95C/97).

There are other experiences described by other contributors in that thread.


Best regards.

Luiz C. Vieira - Brazil


Hi Luiz,
the batteries used in the HP82143A pack are named 'Sub-C'. It is the same type used to 99% in electro motorized model aircrafts. The highest available capacity, at the moment, is 3300mAh NiMH(GP-Batteries or Sanyo). These types have nearly 3 times the capacity of the original NiCds. The 'chance' to charge them to death is very less compared to the originals, IMHO.


Hallo, Andreas;

I will try to track them around here. Have you used any of these in Vintage calculators? Say, have you at least seen a set of these (not a 3.3 Ah, instead a lower-capacity) as part of HP97 or HP82143A battery packs?

Viele danke, Andreas.

Luiz C. Vieira - Brazil


I use 2200maH nicads in my HP9x and 82143A, etc packs. I would like to use the NiMH cells, but for some reason they are ungodly expensive compared with the NiCad cells (about three times as much). The AA NiMH cells are only around 20 percent more expensive than NiCads.


Hi everybody,
We had such a long chain going earlier, looks like we
are going to continue it.


Well, one thought was AAA vs AA. When i said 'concern'
I didn't mean it negatively. I meant that I would use
the highest capacity batteries I could possibly get,
thermonuclear if possible, so that these "red LED" calculators would run much longer than originally before
requiring a charge. Why cut the capacity back down
by dropping to AAA? Don't follow that thought.
I was surprised nobody has tried rechargeable Lithium?
Maybe I'll be the first one. Sure, if there's a
technical problem it'll set your calculator on fire,
but why fear the worst.


We were homing in on the current output of these chargers.
I've got an HP-25C charger that says "1.8VA" not "0.8VA".
Any further opinions on that or shall we let "1.8VA"
be the official number (10VAC, 1.8VA = 180mA max output).


The acceptable leakage thru a clamp part was said
that should be perhaps only 10 - 50 uA max (at 3V).

That was based on the battery being 70mAH.
Is everybody sure about that spec of 70mAH for
2 series connected AA batteries? Sure it is
not 700 mAH ????????


I really hope we can nail this down. (a recommended
mod for the HP-25C, to add a specific zener item).

I was recommending a 4.7V 1W zener, the other
guy was recommending a 6.2V 1W zener.

My concern was overheat if the charger is on with
no battery pack present.

the other guys concern is leakage in the part while
the battery sits idle.

Suggest that I am coming around to agreeing with the
other guy about not wanting leakage. I just also don't
want it to overheat. The 6.2V definitely has no
leakage at 3V, its I/V curve is very sharp for a 6.2V

If somebody is willing to step forward and do mod's,
I'm willing to mail out a few sample Motorola zeners
from my basement stash.

Or we can chat this out a little more,
from this fresh new perch atop the forum board.

- Norm


The 82120A pack uses 1/2N sized Nicads. 70 maH... yes seventy... These cells are no longer made by anybody. The closest thing now that will fit is either 50maH 1/3AAA or a 150maH NiMH cell that is impossible to find (unless you want to buy a 1500 piece case).


Hi David,

Well, that does explain some discrepancies.. We were
originally just trying to figure out best-mod for an HP-25C
which uses "AA" cells. Apparently the original
capacity must be somewhere around 700 mAH for the
batteries in HP-25C, however, I have no spec.

When you mentioned 70 mAH capacity, I thought it referred
to the AA batteries in HP-25C. Sure, if you have
micro-batteries, then you might have other choices of
zener. It would be more important to have a sharp transition in the conductivity of the zener.

A 6.2V zener has one of the sharpest transitions due to
the choices of doping in the silicon junction. As the
spec voltage falls (5.6V, 5.1V, 4.7V, 4.3V, 3.9V etc)
the transition gets softer & softer and is so gradual
as to be useless for our purposes.

In your application, sure, a 6.2V makes more sense
because of the limited available power, and the lower
operating currents in all various operating situations.

It might even turn out that your recommendation of
6.2V is a fair one for protection of the HP-25C. I will find out before we give up on this.

Anybody got a mAH spec for the factory original AA
cells? And what can I increase it to with NiMH or


The original woodstock cells were 500maH. Later increased to 600maH then 700 maH. I think the 6V zener will be just fine. The is no way the battery alone will cause it to conduct. What you are protecting against is the half wave peaks of the AC adpater. This voltage does go through a low ohm (2W) resistor and a diode before being applied to the battery.



I disappeared for a day. Had to work on
the furnace. Thanks for the 500 mAH specification,
later upgraded to 700mAH, for the AA cells used
in HP-25C or HP-34C. And did I hear right
that the NiMH in AA size would give me 3300 mAH ??

That would solve the problem of HP-34C draining
down the battery too fast. And it wouldn't even
require any Lithium technologies, which admittedly
cost far more than NiMH.

But I would really be grateful if you could tell
me your preferred source for buying those batteries.
Do you get them from an internet dealer ?

I am definitely coming around to the notion of
the 6V zener. Could you tell me the value of the
resistor that is in series? Where is it? Inside
of the wall-charger ? And is it just half-wave
not full? That schematic is gone from before.

So we might consider part # 1N4735A, however, I
would still like to conduct an over-heating test
in actual use in an HP-25C .


The 3300maH cells are the sub-C size used in the HP97 and the HP41 printers. The largest AA cells you wil probably find are 1800maH. There are some new 2100-2200 maH AA cells out there, but are pretty difficult to find. A while back, Fry's had 1800maH cells for 59 cents each in four packs. I like to get my cells welded together, so I pay $2.50 each at the local battery store and they weld them for free.


you're getting closer, closer to a store to which
I could go.

Thank you for the information about 1800 mAH.

I know where Fry's is. I can go to the one in
Wilsonville, OR. But you're still a little
vague about which brand of battery I should
be looking for. Is it with flashlight batteries
like "Energizer" or off someplace else ?

You mentioned "the local battery store". I've never
heard of a store that would weld the cells together.
Any suggestion how I find it? Is it there for
purposes of construction guys with cordless tools,
did you just go to the yellow pages and look
under 'batteries' ?

Yeah, welded sounds desirable. Thanks for the tip.

I haven't lost interest in this, will check back
in a day.



Yeah - head for the Frys - and go to the regular battery area. You will find a wide selection of regular and rechargable batteries. In my experience (at Frys and elsewhere), pick the cheapest rechargables that you can find. (If they happen to be a brand name, so much the better.) If there's a Radio Shack closer to home, you might just go there and pay a small premium (but save some gas - which here in Las Vegas could be substantial!).

The battery fix-it stores have names like Batteries Plus. They are basically a "batteries are us" type of place, and cater to a very wide market. The yellow pages would probably be a good place to start. Again, in my experience, they will almost certainly have what you need, at perhaps a slight premium, but (as noted by others) they will be happy to spot weld your new battery collection together (included in the price) with or without tabs and perhaps even put in the plastic case which you bring in. I had the local one rebuild an HP97 pack, and the results were quite nice.


The two main national chains are Batteries Plus and Batteries Etc. They can open up just about any battery pack that yoy bring them and replace the cells for a very reasonable fee. Most major cities also have independent places that specialize in supplying industrial batteries.

The NiMH cells that Fry's had advertised were from some unknown Chinese source. The name probably changes weekly. The most common cell suppliers with a stable name are GP (Gold Peak) or Sanyo.


Hi Luiz,
yes, I am using them - the ones earlier available with 2700mAh - in my HP 82161 drive and HP 82162 printer since half a year and the 3000mAh types in my HP 2225B printers. The 3300mAh are brand new and became available just when I was rebuilding my HP 2225Bs battery packs.
There is no concern about overheating the charger or charging circuit. It is built for charging the original battery pack (4.8V/1200mAh) in about 5h. That is the reason why many battery packs are charged to death! Many people have had the charger connected permanently to the drive or the printer. This is poison for the Standard NiCds. There are special types available that can withstand overcharge for a longer time, but they are hard to get, very expensive and have relatively low capacity. So, IMHO, the risk of overcharging the 3300mAh NiMHs drops.


Hi, Andreas;

thank you for the informations you posted. One more question: as seen in many previous posts, my concerns are mostly about battery life and, in other aspaect, battery leakage. What was the specified charging time for the former original 4.8V/1,200mAh batteries? If it is designed to charged in about 5 hours, then we have about 20%-figure charging current, say, considering fewer loss, more than 240mAmps, right?

If you are using the same circuit and if this circuit delivers the same 240mAmps fo the bateries, as you are using a 3,300 mAh batteries, charging time should increase to about 12 hours if you want to increase operation time. If you keep the same 5 hours charging time, you'll keep the same operation time (a bit more, as new batteries are more efficient and charging curent is lower than expected). Two questions:

1 - Did you change the charging circuit OR use an external charger?

2 - isn't it better allowing longer charging periods in higher capacity batteries so you'll have bigger operating time?

I based the second question in the fact that NiMH batteries operating below their nominal capacity do not suffer the same memory effect observed in NiCads when used this same way. Is it correct?

I know I've been a lot incisive and annoying in these concerns, and I did not find (yet) time and adequate equipment (load patterns, recorders, precision measurement instruments OR RS232/USB connectible data collectors and a good SW) to proceed with experiments. Norm, if you're also reading, that's why I did not answer to your specific requests for lab testings: personal inadequate "financial" (temporary, I hope) condition. I'm running for better days so I can seat in front of my analog and digital set (many of them I built, a few I bought) and proceed with the so demmanded experiments.

Thank you all for contributing with a subject I believe is very important and, in some aspects, crytical enough to cause "injuries" in our dearest handheld calculators. Leakage, dammaged batteries will surelly be avoided if used correctly.

I think as soon as we have some more experimental and brand-related material will be time to compile all of this and write a good, concise battery-related article.

Best regards, folks.

Luiz C. Vieira - Brazil


Hi Luiz,
1. I am using the original HP Wall adapter with the original built in (printers, drive, ASO).
2. So, you are right I have to charge about 12h for fully charge a battery pack. BTW the 5h is only correct for the 82161 and 82162.
The NiMHs do have a memory effect that is negligible to the older NiCds. Newer NiCds should not have a noticeable memory effect, because the engineers of the battery manufacturers learned to make the area of the negative plates a bit bigger than the positive one and 'voila' the memory effect was a thing of the past! So, there should not be any noticeable memory effect when using fresh produced NiCds or NiMHs.
About your leakage concerns I can say: Charge only to 90% of the rated capacity, or better use a good(often expensive) external charger that really uses the -dV/dt method and/or the -d²V/dt² method to find the 100% charged point and you should not have problems with leakage anymore.
The Sub-C cells are used for power tools, RC-Models ASO. In the RC-Model scene you will find battery packs made from Sub-C cells f.e. 20 cells in series to deliver 60A(not a joke!) to the motor for a very short time(1 to 2 minutes). So, the Sub-C types are really rugged.
Don't hesitate to ask your questions. That's what a forum lives from, IMHO.


Hello Andreas,
I am very interested in your usage of a 3300 mAH
NiMH cell. Do I correctly understand that to be AA ??

If the original NiCd cells were 700 mAH and what
you suggest is 3300 mAH then that is a wonderful
improvement. Especially if the charger circuit
can be used without modifications.

However, you did not give me a mfr. or a part #.
Could you provide a link, a part #, or a suggestion
about where to buy them?

My passion is not HP-25C but it is HP-34C and I will
pretty soon want to re-cell some battery packs.

Also in this thread, we are wanting to nail down
a part number for a Zener diode for HP-25C. Perhaps we
can have part numbers both for the better AA cells
and for the zener diode before we are finished .

- Norm


The 3300maH cells are a sub-C size. The highest capacity AA cell you are likely to find is 1800maH. Works great in a spice series machine with 15+ hours run time on a full (36 hour) charge.


Hi David,

you can even get 2100mAh AA-cells from GP or Sanyo these days. They are brandnew!



Hi Norm,

what David sais is correct. The 3300mAh NiMH is of Sub-C type NOT of AA type! So, the improvement is from 1200mAh to 3300mAh.
Here -


you can get info about a variety of cells.


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