Re-Build HP-25 Battery Pack - Hewlett Packard

This is a discussion on Re-Build HP-25 Battery Pack - Hewlett Packard ; 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 ...

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Thread: Re-Build HP-25 Battery Pack

  1. Re-Build HP-25 Battery Pack

    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

  2. Re: Re-Build HP-25 Battery Pack

    On Jul 31, 1:46 pm, Harold Climer 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


    ================================================== ==========
    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/cgiw...cgi?read=10358

    "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.

    .... 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.

    .... 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.

    .... 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
    ---------------------------------------------------
    .... 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.

    .... 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
    ---------------------------------------------------
    .... 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.

    .... 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.
    .... 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.

    .... 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.

    .... 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!

    .... 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.

    .... 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
    1.25v 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
    1.2V 2300 mA/h NiHM batteries. My questions are:
    1. 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 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 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.

    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~



  3. Re: Re-Build HP-25 Battery Pack

    On Tue, 31 Jul 2007 13:46:34 -0500, Harold Climer wrote:

    > I have three HP-25 Battery Packs I want to rebuild.
    > What the specs are for the Nicads I need to replace the originals.


    If you remove the center strip that imprisoned the original cells,
    you will be able to pop in any size AA cells, flat or button top;
    you would even be able to exchange cells at any time,
    and use some other independent charger.

    Be sure to note the original polarity and permanently mark it somewhere,
    before first removing the original cells.

    In a similar HP22, I have used various standard NiCd, NiMH,
    and even alkalines (don't charge the latter on AC power).

    As mentioned elsewhere, never use the AC power unless fresh batteries
    are ready to absorb the pulsed, completely unfiltered over-voltage
    from the "wall-wart," which is actually meant only
    to charge the batteries, and can fry the calc circuitry in no time
    unless "clamped" by the battery itself, like an auto satellite receiver
    hooked directly to the alternator, instead of to the car battery.

    This same horror afflicts all the HP2x (Woodstock) and HP3x (Spice)
    series; brilliantly engineered internally, and left to die
    from lightning bolts if the batteries are ever removed,
    loosened, or dry out and no longer absorb all the current pumped in
    (apparently there's not even a zener diode to guard the gate).

    [r->] [OFF]

  4. Re: Re-Build HP-25 Battery Pack

    On Jul 31, 2:46 pm, Harold Climer 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


    Back in the day, I had some success with plain old NiCd AA's.

    And don't run the calculator from the AC adapter without batteries.



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