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HP Forum Archive 13

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Testing an HP41CX
Message #1 Posted by Ben Moss on 2 Aug 2003, 1:58 a.m.

I've been using the V41 emulator for a long time and I really like it (kudos to the programmers), so I decided to get a real HP41CX off of ebay as a programming toy. Of course, it has a dead rechargeable battery pack, and I have no charger... (you know the story). I've been reading through the archives about battery options, but I still need a bit of clarification.

1. Apparently the battery pack uses an AC adaptor, and the voltage isn't constant. Is there some way I can try to charge the battery pack I have? I have access to fairly well-regulated adjustable AC and DC power supplies. I will be on the lookout for a real power supply in the meantime.

2. Apparently there is circuitry inside the rechargeable pack. This has made me paranoid. Can I test my calculator by giving it 6 volts DC from a power supply? If so, do I apply +6 to the leftmost gold contact and ground to the rightmost contact, leaving the middle two alone? (looking at the calculator from the back). I would like to know if it works.

I have an HP41CX, s/n 2851S21829. It has a rounded display, I think this makes it a halfnut or something, one of the later models? The rechargeable pack (82120A) has 2 pins for the charger.

Thank you very much. I know this topic has been beaten to death, and I apologize in advance for bringing it up again. If there's a website or FAQ that I've missed please post the link. -Ben Moss

Re: Testing an HP41CX
Message #2 Posted by Ernie Malaga on 2 Aug 2003, 3:15 a.m.,
in response to message #1 by Ben Moss


After suffering rechargeable batteries for years on older HP calculators and cell phones, I rebelled and decided to never use them again if I had a choice. You have to leave the batteries charging for hours, only so they can die on you when you least expect it.

The HP-41 was initially intended to use 4 N-cell _non_-rechargeable batteries. I know they're a bit expensive, but they will give you several months of use without having to worry about recharging them.

On the other hand, I still remember my HP-67. I had to recharge its batteries for 14 hours -- and then the charge lasted only 3 hours (without using the card reader).

Do yourself a favor and use non-rechargeable batteries on your 41CX.

Just my 2¢.


Re: Testing an HP41CX - additional info
Message #3 Posted by Vieira, Luiz C. (Brazil) on 2 Aug 2003, 8:25 a.m.,
in response to message #2 by Ernie Malaga

Hi Ben, Ernie;

just to add a few additonal info to Ernie's good advices.

About polarity: looking at the back, LCD in the upper side, the leftomst golden contact is the negative one (closer to the square hole used by the charger connector) and the rightmost is the positive: use ONLY 4.8 to 6.2 Vcc external power source. You should not care for the mid ones: they just close the circuit and are not used internaly.

I built a 6Vcc power source with a set of D-side alkaline batteries that I use when reading cards, barcoded data or IR output. Otherwise, I keep standard N-cells inside.

I agree with Ernie that alkalines are actually a good choice, but I still think that when you use extra power-consuming devices, a set of rechargeable would be fine. My thoughts. Anyway, I think that charging batteries for 14 hours and using the equipment for three, four hours tops, is something weird.

I've been using an HP71B with a set of four NiCad and I have other four always charged. I've been using the first set for more than one month and they are running fine, no Low Batt indication so far.

Add 2 to Ernie's contribution from my pockets.

Best regards.

Luzi (Brazil)

Re: Testing an HP41CX - yet more info
Message #4 Posted by Randy Sloyer on 2 Aug 2003, 10:01 a.m.,
in response to message #3 by Vieira, Luiz C. (Brazil)

Yes, there is circuitry inside the pack, a basic rectifier with current and voltage limiting devices.

The charger input to the pack is 8 VAC, 3VA max. I would recommend charging the pack outside the calculator so you can monitor battery voltage to see if the pack is any good. Connect the AC source to the two side pins and watch what happens on the battery terminals. Voltage that never rises is a shorted pack, voltage that immediately rises to 4 volts and above is an open pack.

While Ernie's suggestion of using alkaline N cells is good, you'll find it an almost impossible task to locate a battery holder. The best method is to buy the cheapest dead 41C you can find on eBay with a holder. You'll still pay at least $40 though. Average price these days is $50 to $60 for a decent shape unit. You can resell the remaining bits and recover maybe $20-$30.

All my rechargeable 82120A packs have been rebuilt with Gold Peak 160 mah 1/3 AAA nickel-metal-hydride cells. They run forever since they have almost three times the capacity of the original ni-cads. in Florida will rebuild your pack for $15 plus shipping. I buy replacement cells from them welded up in the proper orientation and rebuild my own. Takes some patience to split the case open, but it can be done very cleanly with a single-edge razor blade.

Beware of using alligator clips on the gold battery terminals (actually, don't do it), it is a flexible film and will tear. Clip onto a ball of crumpled tin foil and tape in place for testing. There is no current draw to speak of (< 100 ua) so the contact force need not be very high. Only when using accessories does the power go up.

Edited: 2 Aug 2003, 10:07 a.m.

Re: Testing an HP41CX - additional info
Message #5 Posted by Gordon Dyer on 2 Aug 2003, 10:03 a.m.,
in response to message #3 by Vieira, Luiz C. (Brazil)

You should all try NiMH batteries instead of NiCd. I find they are well worth the extra money for longer life and they are rechargeable.

Thank you! It works!
Message #6 Posted by Ben Moss on 2 Aug 2003, 5:12 p.m.,
in response to message #1 by Ben Moss

The calculator gives a "Memory Lost" message when I turn it on, so all is good.

The NiCd pack charged to 3.1 volts, so I cracked open the case and found the two dead cells. After "zapping" the cells with a couple of amps the battery pack was at close to 6 volts. It doesn't hold a charge for long, so it will have to be rebuilt, but at least I know the calculator works.

Again, thank you for the help. Regards, -Ben Moss

Re: Thank you! It works!
Message #7 Posted by David Smith on 4 Aug 2003, 4:16 p.m.,
in response to message #6 by Ben Moss

The 1/2N cells in the pack were only made by Sanyo. They stopped building them a few years back and are just about impossible to find anymore. Possible replacements are 1/3AAA cells. Nicads have less capacity than the originals. NiMH cells can be hard to find in 1/3AAA. Also the 1/3AAA cells will require some shimming because they are a bit smaller than the 1/2N.

Gold Peak does make a 1/2N NiMH cell, but you have to buy 1500 of them. I have never found a distributor of smaller quantities.

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