HP 41 Plugin Module Problem


My HP-41CX is having problems with one of its plugin modules. Extra memory in slot 1 is okay, but when I put the Math Pac into slot 2, it usually causes the calculator to go crazy on startup (garbage on the screen, upper half of the display completely blank, etc) and then it won't turn off until I remove the battery.

I thought my Math Pac must be bad, but when I tried it one last time everything was fine. Since then I have left it in place and have not seen any more bad symptoms.

Can anyone provide some insight into the probable problem? Are the connectors unreliable? Might they need to be cleaned?


Usually the contacts have to be cleaned. This can be done using a Q-Tip or some tissue.

As a precaution I'd suggest to remove the batteries before the cleaning.

Apart from that, have you tried putting the Math module into another slot, other than slot 2 ?




Are the connectors unreliable?

Not a chance. The connectors in a 41 have more gold on them then you'll find in some jewelry stores :^) With 41's, it's always batteries, batteries, batteries.

Try rapidly removing and reinserting the modules several times with power off. Many times this is all that is needed to remove the surface crud that grows due to the battery outgassing - as the cells age and begin leaking all over your 41.

Remove, discard and replace cells every year without fail and chances are very good that your 41 will outlive you.


Before I couldn't get the module to work. Now I can't get it to fail, in any slot, so it's likely that a few insertions did the trick in cleaning off the contacts.

I wasn't aware of the problem with leaking batteries, but after reading about them here I found some nice pictures showing just how bad it can get.

Just a few days ago I had rebuilt one of my two rechargeable battery packs by cutting out a dead cell and soldering in a (slightly) leaking but still live cell from the other battery pack. Now I'll have to rethink the wisdom of this repair.

There seems to be a lot of useful information like this that HP 41 owners should know. Is there an online document that summarizes this collected wisdom, or do I need to read through the HP Forum archives to get up to speed?


Mixing batteries of different brands/ages/capacities/types is a perfect recipe for disaster. It will just about guarantee cell rupture and leaking faster than any other method.

The chemistry doesn't matter, be it rechargeable, alkaline or otherwise. Always replace batteries as a matched set as it is not worth loosing a calculator to save a few units of your local currency.


Using or applying heat to a NiCad battery or any other type be it alkaline, lithium, silver oxide or the two NiCad types is a BIG NO NO.

Ever notice the rebuilt packs or any pack you recieve from the battery store for any purpose is ultrasonically welded. The reason is the heat application from a solder gun alters the conductivity of the internal chemical creating a resistance at the end cap under the solder joint. This heat also creates a chemical reaction which can accelerate leakage and premature death of the cell.

My advice is to remove all the nicads from the pack and then carefully remove the two plated contact strips for reuse. Take the whole kit and kabudel down to your local electronics or battery store and have them re weld ultrasonically the strips into place and the batteries in series.

Cheers, Geoff


Even if that's true, ultrasonic welding is still welding, but it uses mechanical vibration to raise the temperature to melt the metal, right? And raising the temperature is how all electronics stuff is soldered, so there's no need to worry about resistance increasing. When soldering to batteries using a gun or iron, I use a very high temperature so the job can be done in a second, so there's no time to heat up more than just the end cap on the battery. I've never had any trouble this way. The battery cell itself where the chemical reaction produces the current hardly gets warmed up at all. Granted, I've never soldered to any of the button cells.

Edited: 30 Jan 2008, 5:43 p.m.


Battery packs are built with spot welding, not ultrasonic welding which is used for plastics. The energy used in spot welding does not raise the temperature of the surrounding material enough to affect whats inside.

Directly soldering to the ends of batteries is just asking for trouble. Yes, they can and will explode. It is a pressure vessel that does have venting ability but push things too far and ka-blam, somebody gets hurt.

The amount of heat needed to solder directly to cell can only damage it. Maybe not right away but it will affect it. If you're not soldering to tabs that have been spot welded to the batteries, you're doing it wrong, regardless of how fast you heat the area.

BTW, what exactly did you mean by this?

And raising the temperature is how all electronics stuff is soldered, so there's no need to worry about resistance increasing

Resistance of what? The ESR of the battery? Of the metal itself?

Edited: 30 Jan 2008, 9:35 p.m. after one or more responses were posted


I have purchased Nimh cells with long welded tabs that make great packs. Check all-battery,com Sam


hey randy; if you use that low temp solder; is that less bad, or is it not low temp enough?


And raising the temperature is how all electronics stuff is soldered, so there's no need to worry about resistance increasing
Resistance of what? The ESR of the battery? Of the metal itself?
I see I misread it. He did mean the chemistry in the battery, not the metal itself. However, the endcaps themselves of the batteries I've taken apart only have a small contact area to the battery parts, and the endcaps are not involved in the chemistry.

With a hot enough iron, the internals of the battery should stay so cool you could handle them with bare hands. I use an iron near 1,000 degrees F, which allows the job to be done in an instant if you do it right. Often the production people even in our own little company find that hard to believe until they see it demonstrated. The lead lady got started at another company with low-temperature irons and the idea that somehow that will avoid damaging things, and then she melts plastic rocker switches because the iron is too cool and it takes too long to get the lug up to soldering temperature.

Edited: 31 Jan 2008, 1:38 a.m.


I use an iron near 1,000 degrees F, which allows the job to be done in an instant if you do it right.

Operative phrase: "if you do it right".

While the technique seems to work for you, I'll stick to buying my batteries with tabs welded to them.


Yes, never use a soldering gun, no matter how many watts it is. You can quite readily solder NiCd and NiMH cells with a sufficiently large soldering iron. The difference is that the iron has a huge mass near the tip, so when you touch it to the cell, the cell top heats up instead of the soldering tool cooling down.

I've been soldering directly to NiCd and NiMH cells (Sub-C and AA sizes mostly, and some smaller sizes like N when they were still available) for years to make packs for my electric powered model aircraft. It's a two stage process. First clean the area to be soldered (really clean) and tin it. This usually takes under 1/2 a second of soldering. Then once the cell has cooled, you can solder to the tinned area (usually less than a second).

In my early days of doing this, I tried using a 140W soldering gun. When it worked, it usually took 5 to 10 seconds to get the cell top hot enough, by which time the rest of the cell was hot too. When it didn't work, the gun ended up soldered to the cell.

I now use a 100W iron, which has such a high thermal mass that it takes about 15 minutes to heat up after I plug it in. I have soldered NiCd and NiMH packs that are now 10 years old and still going strong. (Most of these are end-to-end soldered, meaning each cell is soldered directly to the next one, with no intervening wiring. This requires a special hammer-head tip, rapidly heating two already tinned cells at once, pulling the iron out, and pushing the cells together.)

Having said all that, for the average calculator repair, cells with welded tabs are probably better and easier to work with. For electric powered airplanes, the tiny spot welds have too high of a resistance (even a few milliohms matters when your plane is drawing 40+ Amps from the battery).



even a few milliohms matters when your plane is drawing 40+ Amps from the battery

and 16 gauge wire is about 4 milliohms a foot. And then there has to be a switch... and some connectors... and 40 amps? Just two feet of wire alone is ~300 mv of drop.

I'll be the first admit I don't know models but the numbers just don't add up for me. Assuming a 12 volt battery, you're talking close to a half-kilowatt of power there. Just how big is this plane?


Electric models vary in size from little 50W models (generally around 16oz or so and 2sq.ft of wing), to well over 1000W (size and weight varies, depending on what the goal is). We tend to use 14 or 16ga wire in the small models, and 12 or 10ga wire in the larger ones. And we usually keep the wire lengths to inches, not feet.

Yes, there are resistive losses throughout the system, but we try to keep them to a minimum.

A typical F5B class aircraft from a few years ago used 27 1200mAh Sub-Sub-C cells, and pulled around 80A. With the voltage depression that comes at those currents, that's about 2kW. Full throttle run time would be under a minute, but such planes were typically only climbed at full throttle (vertically, to speck height in seconds), and then flew a race course in a high speed glide. Repeat until time is up. The person who flew the most laps wins.

(If one used cells with tabs in this application, either the tabs or the spot welds would melt in seconds.)

A more average sport aircraft like I fly now has the following specs:

Weight: About 45 oz or 1.4 kg

Wing Span: 48" or 1.2m

Wing Area: 3 sq.ft or 28 sq.dm

Battery: 8.4V (7 cells, 4000 mAh Sub-C NiMH)

Current: 30A

That's only about 250W (1/4kW) for 8 minutes at full throttle, much longer at cruise throttle.

It's a whole different world from collecting calculators.

You can learn more at http://www.stefanv.com/rcstuff


Edited: 31 Jan 2008, 3:21 p.m.


It's a whole different world from collecting calculators.

Indeed it is... at those discharge rates, the heat has to shorten the life of the cells. Any idea how hot they actually get when in use under these conditions?


It depends on a lot of factor, such as how long you run them, how good your cooling vents are, etc. In my sport planes, the battery is generally cool enough to hold comfortably in your hand (and warm enough to warm your hands after a winter flight). In extreme cases, they can reach 60 Celsius (140 on the old Fahrenheit scale). In very extreme cases (e.g. too large of a prop for the motor, poor cooling, etc.), you can cook them, which can get catastrophic if the shrinkwrap on adjacent cells melts.

Battery life time is typically around 500 charge/discharge cycles if you use them a lot. Less if you tend to fly infrequently.

Now days, many modelers have switched over to Lithium Polymer batteries, which have a higher power-to-weight ratio, but are also much easier to damage by overcharging, overdischarging, being banged up in a crash, or just plain bad luck. I've stuck with NiMH for now.


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