On Jul 31, 1:46 pm, Harold Climer <Harold-Clli...@utc.edu> wrote:
> I have three HP-25 Battery Packs I want to rebuilt. Does anyone know
> what the specs are for the Nicads I need to replace the originals.
> Also a source of the batteries.
> Harold A. Climer
> Dept,Of Physics,Geology & Astronomy
> U.T.Chattanooga
> Room 318 Grote Hall
> 615 McCallie Ave
> Chattanooga TN 37403
> 423-425-4546
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
Hello:
The below may save you some time even though it is not
well organized.
I am going to post in the third segment below "result
sentence fragments" that I retrieved from doing
google searches earlier this year with keywords like the below:
site:www.hpmuseum.org +{"hp-25" OR "HP-25c"
OR "HP-21") As you can see most of it comes from the below:
"The Museum of HP Calculators" at http://www.hpmuseum.org/ "
There are some recommendations in the fragments for which
NiCads to use if you wish to maintain your original power
source. For most I did not capture the URLs.
However, in the second segment I am posting some URLs they
can get you started.
Please note, none of these are my writings and therefore
take them with a "grain of salt." They did help me to decide
which battery technology to use in going forward. I am using
Nickel-Metal Hydride batteries instead of the original NiCads
in both of my HP-25c and HP-25 calculators. I take them out
and charge them with a wall charger ... and never charge the
batteries in the calculator. . You too may conclude that
that is a safer way to go.
Good luck, watch the polarity, and make sure you have good
contact . This is a high level conclusion that I got from my
readings.
Regards
{fdw}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
URLS
HP Battery Information
"Woodstocks (HP-21, 22, 25, 25C, 27, 29C)"
http://www.hpmuseum.org/batts/battery.htm
Nickel Cadmiun vs Nickel Methal
"Nickel Cadmiun vs Nickel Methal
Message #1 Posted by Edwin Morales on 30 July 2001, 2:30 p.m."
http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv006.cgi?read=3D10358
"The Museum of HP Calculators"
http://www.hpmuseum.org/
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
RESEARCH SENTENCE FRAGMENTS
SUBJECT: HP Museum HP-25c Power Wisdom
FOCUS QUESTION: What is the best battery technology
to use in the 21st century for my HP-25 and HP-25c calculators?
RESEARCH: Google Site Search:
KEYWORDS: site:www.hpmuseum.org +{"hp-25" OR "HP-25c"
OR "HP-21")
DATE: Wednesday 31 January 2007
NOTES:The "20" series actually had two stages: in the first,
the HP 21, 22 and 25 were launched, and none had the
so-called "continuous memory", nor CMOS circuitry. / In the
second, the HP 25C, and the 19C & 29C do had "continuous
memory" (CMOS battery-backed RAM).
////////////////////////////////////////////////////////
SOURCE: HP Museum Posts
----------------
Keywords: +{"hp-25" OR "HP-25c" OR "HP-21")
About HP-25c Power Requirements
-----------------------------------------------
The Woodstock battery pack consists of 2 AA NiCd batteries
providing 2.5 volts.
Batteries are Rechargeable: The batteries act both as a
filter to smooth the half-wave pulsating voltage, and also
as a regulator to ensure no more than some 2.6 V are applied
to internal circuits.
..=2E. HP-25C IMPORTANT WARNING
DON'T run it from ANY AC adapter, at least before being 100%
sure that: 1) you have good (new) rechargeable batteries in
the battery pack AND 2) batteries make good contact (oxide
free, etc) with the battery contacts of the calculator.
The chances of permanently damage your calculator (almost
with no hope of repair) are VERY high if any of the
abovementioned rules are not obeyed.
..=2E. Better To Not Use The AC Adapter
P1>Please NEVER use your HP25's AC adapter without a
rechargeable battery pack inside. The rechargeable batteries
serve as both a filter and a regulator for the AC adapter.
Without the rechargeable batteries, excessive voltages will
be applied to the calculator, causing severe and
irrecoverable damage.
P2> the battery contacts...which need to be scrupulously
clean in a woodstock. Also be sure the all contact points
within the battery pack are clean also. And be sure not to
connect the calculator to the AC adapter unless you're
positive you have a good battery pack with good contact
installed (this could over-voltage the machine). I
personally never connect my machine to an AC adapter,
period.
P3> Please don't ever think about running your HP 25 from an
AC Adapter if you are not 100% sure that GOOD rechargeable
batteries are in place and doing GOOD contact with the
calculator terminals! Else, you may irreparably damage your
calculator.
..=2E. Reasonable Approach To Long-term Use
a) Clean the contacts and keep them clean
b) Rebuild the battery pack with new rechargeable two 1.2V
NiCads(NICD) AA-size cells with a flat top or with 2 AA 1.2
V, 1900 mAh or 1500mAH NiMH cells with an industrial cap (a
bit shorter than the button-topped consumer variety)
c) Recharge the cells outside the calculator (i.e.: don't
use the calculator as a charger);
d) Always run your calculator from batteries, never from AC
power, to avoid the excess voltage the AC adapter may cause
(more than 8 volts, which certainly will fry the chips
inside; particularly the "continuous memory" RAM chip, even
with the calculator turned off);
e) Risky but probably okay to run the calculator
from alkaline batteries. The voltage (3 volt) will be
somehow higher than from rechargeables (2.5 volt), but
within tolerances (there should be no problems, but this one
is at your own risk, no warranties).
About HP-20 Series Battery Pack Construction
--------------------------------------------------
HP made Woodstock battery packs in two different ways. In
one type of pack the cells are not joined together except by
the spring. This allows the cells to float around and make
contact to the calculator pins if they are uneven. It also
caused the cells to not make good contact with the spring.
The other type of packs strapped the cells together with a
weld strap. This type of pack did not have the problem with
bad contacts at the spring... it did cause bad contacts at
the calculator if the calculator contacts were not even
(usually the case).
in continuous memory models, where the RAM chips receive the
battery voltage directly, even when the machine is off; if
the batteries are not present, the chips get the full
unregulated 10+ Volts that comes out of the charger.
2007 DigiKey Flattop NiCd Replacement Said To Work
--------------------------------------------------
Part numbers for nicad batteries.
DigiKey: See http://www.digikey.com.
DigiKey Part #: P092
CALCULATOR: Woodstock series
HP Battery Pack: 82019B
Battery Construction: 2 x AA 700mah
Qty of Batteries: 2 (individual cells)
2005 Price: $ 4.42
Note: Individual cells (with tabs, e.g. P092T) and the end
cells used in packs come with solder tabs on them. For the
82019B, 82109A and 82052A battery packs remove the end tabs
before inserting into the plastic pack. This will leave a
flat end to the battery that matches the original HP battery
pack's design. Part number P092 (no T suffix) are flat-top
cells without tabs.
About Battery Pack Polarity Required
-------------------------------------
Many 20 series calculators have been damaged by replacing
batteries with the polarity reversed so be careful.
The required polarity is shown inside the battery holder I
have. It is on the angled section on the long side of the
bottom of the holder.
If you need to rebuild a Woodstock pack and are not sure of
the battery polarity you can find it marked inside the
battery holder (a small raised outline of the battery)
About AA NICD Rechargable Batteries As Replacement
---------------------------------------------------
..=2E. Preferred For Continuous Model Rebuild
P1> Rebuilt battery packs with NiCd AA batteries are OK. In
the meantime, it is possible, at your own risk, to try with
regular alkaline AA cells. The voltage will be somehow
higher than NiCd, indeed, but the difference may be within
tolerance. P2>Alkalines/dry cells/primary-type ZnCl
batteries all put out 1.5v nominal PER CELL, while NiCads
and NiMH are 1.2v/cell. Not a great deal of difference, but
my personal rule is that you should not act as if alkalines
and NiCads are equivalent if there will be more than 3 in
series (an excess of > .9v). Most devices have regulation
circuitry, true: but most regulation circuitry consists of
converting excess voltage to heat, not great for reliable
small devices.
..=2E. Will 1.2V NICDs Be Okay
Q> 1.2v NiCd ? I would like to rebuild my battery pack, but
have two question: 1. The information from this web site
says the HP-25 is designed to operate on 2.5V battery power.
The only NiCads I have seen are 1.2V. Will two 1.2V NiCads
supply the correct voltage for the HP-25? 2. When I
disassemled the battery pack, there was a small metal bar
connecting the batteries. Is this bar necessary for the
HP-25 to operate properly? If so, where does the bar go?
Against the springs in the battery pack or at the other end?
A> 2 NiCd cells is precisely what the HP-25 is designed to
operate upon. The metal bar is not really necessary, the
springs connect the two cells just fine. If you want to do a
real nice job, however, buy NiCd cells with solder tabs; you
can then solder the two cells together to ensure a good
connection, and rip the solder tabs off the other ends.
I replaced my batteries with Radio Shack's AA nicad
rechargeables.
In my opinion, the battery packs for the HP-21 and HP-25 are
the easier to rebuild, you just have to buy 2 AA NiCd
batteries and put them there, if you don't want you don't
even have to solder them.
About Lithium Batteries As Replacement
--------------------------------------------------
fdw: voltage is too high
Take two cells already together, in a battery holder case
(or a regulated power supply, if you have one, set to 2.4
volts. I have lithium AA cells in my every-day HP-25 that
have lasted for months, will store safely for over ten
years, and won't leak. They do put out about 3 volts, which
can be a little hard on the circuits, so I'm told. I've had
no problem with this, and the calculator never needs to be
charged.)
About Battery Pack Modification
---------------------------------------------------
..=2E. Rebuilder Recommendation
P1> Try the NiCad Lady http://www.nicdlady.com/ or TNR
Technical http://www.tnrtechnical.com/. They both have
extensive experience rebuilding HP battery packs. P2> Please
have a look at http://www.hpmuseum.org/repair.htm#batteries
if you haven't yet; if you go through the entire section,
yuo'll find some references to build or buy battery packs.
..=2E. Why Spot Wielded Strap Better Than Solder
You really are better off with the strap... if the spring
alone does not make good contact, the pack open circuits,
the internal machine voltage rises to potentially
destructive levels when connected to the charger.
Some early HP packs just use the spring. Later ones added a
tight fitting strap. Finally they went to a strap with some
play in it. The cells need to be able to move independently
to assure good contact with the potentially uneven battery
contacts in the machine.
One should NEVER solder to the batteries... it can damage
them and a damaged battery can lead to a dead machine. Best
to get them spot welded together at a battery store.
..=2E. Doing It Yourself
P1> It is easy to rebuild the battery pack yourself.
Just neatly cut through the plastic bar near the contact end
of the batteries and lift them out after making a note of
the polarity ( I made a red dot on the plastic holder at the
+ side with a permanent marker). Clean the spring and fit
new NiCad or NiMH battries, carefully sliding them in under
the plastic bar, dont bend it up more than is necessary.
Then refit to the calc.
P2> I just cut the center divider out of my shell, so I can
snap rechargable batteries in and out of it (I marked the
shell for + and -). It works OK, and I don't have the cells
soldered together, as in the original pack. Occasionally I
will lose power to the calc, and I have to reposition the
battery pack to get it back...I think soldering the cells
together would cure this problem, but it's something I can
live with.
..=2E. Corrosion Battery Acid Removal
P1>I have an HP-25, which had similar battery leakage
damage, that I brought back from the dead; basically the
bottom third of the PCB was damaged by corrosion. I
carefully immersed the damaged portion of the PCB in a
baking-soda solution. After the baking soda had done its
work for a few minutes, I scrubbed at the corroded area with
an old tooth brush, particularly trying the work the
bristles under the chips where additional corrosion was
visible. I repeated this process a couple of times, using a
fresh baking soda solution, until no evidence of corrosion
was visible. Last, I carefully rinsed and dried the PCB.
P2> Thanks for the advice. It does have slightly corroded
contacts on the two prongs that touch the batteries, so I'll
clean them as best I can. (I'm thinking Q-tips moistened
with Windex.)
P3> Also gently sand the contacts with fine grit sandpaper.
Another useful trick is to put a small wad of aluminum foil
on top of each battery contact.
P4>The battery contacts required additional work. I cleaned
them with a fiberglass brush from Radio Shack (Cat. No.
64-1986). This brush reaches into pits and cracks you can't
get at otherwise, doing a much better job than scraping, and
not taking away as much of the underlying good material as
sandpaper.
..=2E. Batteries Do Double Duty
P1>The batteries act both as a filter to smooth the
half-wave pulsating voltage, and also as a regulator to
ensure no more
than some 2.6 V are applied to internal circuits.
Replacement of batteries with Capacitors may work as
filters but NOT as regulators: replacing the batteries with
capacitors will result in excess voltage applied to internal
parts and, almost sure, irrecoverable damage. If capacitors
are used in place of batteries, the applied voltage will be
around 8 Volts, absolutely out of 2.6V +|- 10%
tolerance range!
On the HP 21, 22, 25, 27, the batteries voltage will be
applied to a switching power supply circuit, which will
furnish appropriate voltages to internal components (+5
Volts, -12 Volts, I guess). On the Continuous Memory models,
batteries voltage will be applied more or less directly to
ICs, in order to keep memory contents while the calculator
is off. So the 25C and 29C are more sensitive to
overvoltages and polarity errors. I think (no warranties)
that the alkalines are acceptable even on those models.
P2> if you don't have a good battery contact, I
wouldn't risk driving the unit from the mains transformer.
The batteries act as a filter (as really huge capacitors)
and as a stabilization for the input voltage. If there's the
possibility, they have no contact to the rest of the
calculator, you could fry your calculator by driving it
directly from the mains circuit. You should check the PCB
traces (calc - battery contacts) first.
P3> I'd like to add that because the memory is constantly
powered in the 25C/29C, these machines tend to die even if
they're turned off, and the battery momentarily loses
contact (or has a bad or corroded contact) during charging.
Another reason why I charge my Woodstock batteries in a
broken HP-25!
..=2E. Tin Plated Contacts
Issue with: P1> More likely in my experience is a bad
contact between the battery terminals and the connectors
inside the battery bay. This problem is troublesome (those
tin-plated contacts tend to oxidize, hence the problem) and
it's also dangerous, because if you ever turn on the
calculator while the charger is hooked up and the battery is
not connected properly (and cannot perform its power
regulation function) you can destroy the calculator in a
matter of seconds. P2> Getting good battery contact in the
Woodstock series calculators can be tricky. Sometime I put a
drop of solder on the negative side of the battery so it
will stick out as far as the nipple on the positive cell.
P3> My favorite way (recommended to me by HP in the dark
ages) to get good contacts on Woodstock, HP19C and HP10
machines is to make a small folded square of about three
layers of aluminum foil and place one over each battery
contact in the battery pack. Insert the pack into the
machine and voila... works every time for me.
Also, make sure that the spring is well polished or sanded
if you haven't wired the ends of the batteries together on
the spring side.
P4>If the calculator works with the external supply, then
you need to work on making sure the battery pack makes
contact when the springs are extended in the case. Thus, my
luck with a small ball of solder on the negative battery
face. Tighten the springs, also, by stretching them some.
..=2E. Battery Pack Springs Weaken With Age Solution
P1> Viktor is likely on to the solution as the battery pack
springs weaken with age as well. Go to Radio Shack and buy a
small package of the flat "matchlight" solder. taking two
strips, fold each in half individually, and slide in front
of the battery contact in the pack just past the plastic
housing so it will stay in. Reinsert pack and test. Try two
strips if needed but can make the pack hard to get out.
Note:The batteries used in the HP 25 battery pack can
easily be replaced by someone that knows how to do it and
that has the right equipment. They're a standard size
battery cell so you should always be able to get
replacements. I strongly recommend that you get the
batteries from someone that can weld the straps on them and
not buy off the shelf batteries and simply solder a strap to
them. The NiCad batteries have a vent in them that is sealed
with a plastic ball. Soldering them can easily overheat them
and melt the ball and cause it to plug the vent. If that
happens then the batteries can explode if they are over
charged. OR the ball can melt and leave the vent open in
which case the battery electrolyte can leak into the
calcualtor and cause corrosion. I can rebuild any of the HP
batteries. Contact me directly if you need any fixed.
About Using AA Alkaline Batteries
--------------------------------------
Q1> Are AA Alkaline okay for continuous HP-20 series
models like the 3 cell HP-29c or 2 cell HP-25c?
A> P1> Truthfully, I would be skittish about alkalines in
the 25c in particular, mainly because this is VERY EARLY
C-MOS, and I think it is more picky about the voltages it
tolerates. I would try to keep it to NiCads.
Because Alkalines/dry cells/primary-type ZnCl
batteries all put out 1.5v nominal PER CELL, while NiCads
and NiMH are 1.2v/cell. Not a great deal of difference, but
my personal rule is that you should not act as if alkalines
and NiCads are equivalent if there will be more than 3 in
series (an excess of > .9v). Most devices have regulation
circuitry, true: but most regulation circuitry consists of
converting excess voltage to heat, not great for reliable
small devices. P2> Regarding alkalines, I've heard horror
stories about the 3.2V output voltage of a pair of new
alkalines being sufficient to ruin the early C-MOS
circuitry. A pair of slightly used alkalines (<1.5V per
cell) should not cause any problems, since fully charged
NiCd cells can put out that kind of voltage as well.
Q> I know that the HP-21 was designed to have only a 2.5 V
Ni-Cd battery pack for power. How badly could the calculator
be damaged if someone replaced the 2 Ni-Cd cells with a pair
of 1.5 V AA Alkaline batteries?
A> P1> I have used alkalines in my woodstock (and spice)
series units with no ill effects. At the suggestion of
someone here, i use used batteries that test nearer to the
1=2E25v of a fully charged ni-cad than the 1.5v of a new
alkaline. I don't know if that is necessary but it can't
hurt. P2>I have used alakline batteries with the HP-21,
H_25, and HP-29C with no problems. P3> I have used alkalines
in my Woodstocks during 6 years and disposable Lithium AA
batteries since the 3 last years without any problems.
Q> 2 alkaline batteries would be at least 3 volts or more
(if new). Is 2.6 volts an absolute maximum?
A> P1) In my experience, 2 alkalines (under load) will give
about 2.8 Volts; 2 NiCd when charging may reach about 2.6
Volts. It will be within a 10% tolerance, a reasonable
value. Many years ago, I used my HP-25 with alkalines
without any problem, there was people at this forum telling
similar experiences. It is very important NOT to use the AC
adapter with alkaline batteries, since a charging current
will be applied to the batteries, what may cause overheat
and (eventually) make them explode.
P2) On the Continuous Memory models, batteries voltage will
be applied more or less directly to ICs, in order to keep
memory contents while the calculator is off. So the 25C and
29C are more sensitive to overvoltages and polarity errors.
I think (no warranties) that the alkalines are acceptable
even on those models.
I remove the center plastic strip in the battery pack and
insert (with a bit of force) AA alkalines. Works just fine
for my 21,22,25C,and 29C. Woodstocks don't care so much
about length but are incredibly sensitive to both bad
connections and batteries when connected to a charger.
Q>I was given an HP-21 and the previous owner had replaced
the batteries with standard 1.5 volt batteries. Does anyone
know if this may have damaged it and can you tell me which
pin is +ve and -ve. A>according to the archives that does no
harm (if you have any doubt then use used batteries) . the
pin on the same side as the enter key is pos.
Q>Does anyone know if an HP-21 will operate on alkaline
cells, or will the too-high voltage damage it? A>I had used
alkaline batteries on a HP 25 (not C) wihout problems. It
seems that the voltage, while a little greater than NiCd, is
within tolerance. The HP 21 should be the same (no
warranties!).
Never charge alkaline batteries in the calculator using the
HP AC Adapter
Running from 2 Alkaline AA is acceptable, even with a little
higher voltage than NiCd (many people reported this OK, but,
as always, NO WARRANTIES!!)
Assuming you have the battery holder and simply want to
rebuild it, my advice is to get rid of the charger and use
ordinary alkaline cells. You can remove the plastic piece
between the two batteries to allow easy replacement and
carefully mark the holder with the correct polarity. E.g.
About Troubleshooting:
------------------------------------------------------
AA Alkalines And Troubleshooting On Non-HP-25c Models
P1>Just as an emergency option, you may use two standard
alkaline AA cells as a replacemnent for the rechargeables,
but NEVER use the AC adapter with alkalines, since a
charging current will be applied to the alkalines with
concrete risks (even explosion). P2>try with two alkaline,
standard AA batteries. VERIFY POLARITY and work without the
battery charger. If the calculator does not turn on, we
should assume internal damage... You may still open it up,
clean corrosion and contacts, etc., and try again.
Problem Batteries Not Good OR Not Making Good Contact
Q> I have an HP-25 whose display runs unstoppably when the
calc is switched on. (I'm running it from the charger
because the battery pack is shot, and I have no other.)
Pressing any of the buttons makes no difference, nor does
switching the w/prgm switch.
As far as I can tell, the display is cycling through all the
digits wildly, much as it might while a program is running.
(Except for no blank-outs.)
A>
P1> First off, you need a GOOD battery installed in the
machine... without one you get the display that you talk
about and often times a burned out machine.
P2> My HP-25 does the same as yours if the battery is not
connected properly, or if there is an old bad battery
fitted. The power adapter relies on the battery to smooth
the voltage. BEWARE... it may cause damage if there is not a
good battery fitted.
P3> Corrosion would cause this. Thanks for the advice. It
does have slightly corroded contacts on the two prongs that
touch the batteries, so I'll clean them as best I can. (I'm
thinking Q-tips moistened with Windex.)
P4> The flashing digits in an HP-25 display are almost
always a symptom of a bad, missing or disconnected battery
pack. DO NOT, repeat DO NOT turn on the calculator if you
see this. It is very easy to fry these little puppies
without the battery. The battery pack is used to filter and
regulate the power to the circuitry and without it the
innards can be destroyed in an instant. Your charger cord is
probably OK. My bet is the battery pack or the conenction to
it. Check for corrosion first. Lightly sand the little
rivits that toch the battery terminals and the terminals
themselves.
About Using NiMh Batteries
--------------------------------------
Q> Can NIMH batteries be used on the HP-25c in the
place of NICD batteries?
A> P1> If NiMH is charged in a NiMH charger, you DO get more
power capacity, and you DO get more cycles, but since HP
chargers weren't originally designed for NiMH, the way to
use them is to convert your calc battery pack to allow you
to change batteries in it easily, then keep a set of NiMH
in your charger, and a set in the calc, and switch them out
RATHER THAN trying to charge them IN THE CALC.
By "converting your pack to allow you to change batteries
easily", I mean you can, for instance, use the NiMH cells
with an industrial cap (a bit shorter than the button-topped
consumer variety), and use a spring-metal contact in each
side of the pack, and "seal" the pack after each change with
scotch tape. It works, and even someday, can be easily
reversed to a plain ol' NiCad pack again.
Be picky. Power is the only thing a calculator needs to do
its work-- and on such a diet, the best you can do to keep
your calculator healthy is provide it with the RIGHT stuff.
Q> 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=2E2V 2300 mA/h NiHM batteries. My questions are:
1=2E 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?
A> P1> 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. P2> 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., 1=BD 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 =BC 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.
P3> 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.
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Harold
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