Adapting cell battery to HP 41

[Artur - Brasil]

Hi, guys!

Last week I concluded my plans to adapt a cell phone battery to my HP-41.

Using just a small 850mAh battery, one small charger board, another small step-up board and it's done!!!

The circuits themselves use only 85nA;
The HP in standby mode I think something around 1mA.

So, I believe 850mAh/1.085mA --> 780 h --> 30 days in standby mode!

Red - charging
Blue - charged.


   
   
   
   
   

[mfleming]

Nicely done! What did you use for the battery cover, and how did you connect with the battery contacts?

~Mark

[Artur - Brasil]

(02-25-2019 03:55 PM)mfleming Wrote:  Nicely done! What did you use for the battery cover, and how did you connect with the battery contacts?

~Mark

Hi, Mark

I had one old hp41 rechargeable pack, whose batteries were dead, so, I cut it, leaving just small borders to insert in the original place.
I soldered two wires in the cell battery and soldered them in the recharger board. From it, another two wires to the step-up conversor; finally, other two wires soldered directly on the battery contacts of my HP41.
Very easy to do, realy! Also, you can use the calculator while charging. The step-up converter keeps exactly 6v on output.
2US$ solution!!!
Card reader supported like a brease!
Next works, my tape recorder and the printer ....

[Jlouis]

Impressive! Congratulations!

[grsbanks]

Nice job!

[Dieter]

(02-25-2019 03:31 PM)Artur - Brasil Wrote:  The circuits themselves use only 85nA;
The HP in standby mode I think something around 1mA.

So the power consumption of the voltage converter (which I assume is what you call "the circuits") is negligible compared to the calculator in standby. In sleep mode it seems to be 10...30 µA which still is two magnitudes more than the 85 nA you stated. So this seems to be no problem.

(02-25-2019 03:31 PM)Artur - Brasil Wrote:  So, I believe 850mAh/1.085mA --> 780 h --> 30 days in standby mode!

In am not an Electrical Engineer, so please excuse me if this is a stupid question, but... the battery has 850 mAh times 3,6 V = about 3000 mWh. Since there is a voltage converter with 6 V output the calculator consumes 6 V times 1 mA = 6 mW when in standby. So the expected battery life should be more like 3000 : 6 = 500 h or about 20 days.

Which is about the same as what four NiMH N-cells deliver, and about half the capacity of N-Alkalines. But then again it's 3x what you can expect from a rebuilt 82120 pack with 1/3 AAA cells. So this cell phone battery seems to be a good choice.

Dieter

[Artur - Brasil]

Hi, Dieter!

Your calculus seems to be more precise than mine. I should have measured with my multimeter the currents envolved, I'll try to do that to be more exact.
Anyway, N cells here are hard to find by here. I have some rechargeable, but it's time consuming put then to charge, find the charger and, after all, they will oxidate even more the contacts of calculator, which are not so good.
So, the mainly porpous at the end is to have my HP-41 working back again.
The same for the cassete drive and printer: their big batteries, and very problematic too, the old charger, will go soon, soon! I'll take batteries from a portable charger of Pineng if they fit on battery compartment.
It seems that step-up converter can handle up 2A - I hope this is enough for these peripherics.

[Hans-Peter]

Have you the links to the charger, battery and the step up module?

Hans-Peter

[Artur - Brasil]

Hi,

Here the code of boards I used for this project.
They are used in Arduino projects and easily found in market.

03962A battery charger - this one has separeted outputs for battery and to the circuit to be energized;

MT3608A step-up converter

You can put such names on Ebay and it will bring tons of sellers.

I just have one doubt about my project: what will happen when battery discharge too much...
Best wishes!
   
   
   

[cruff]

Will the step up converter handle the current load of the card reader?

[Artur - Brasil]

(02-26-2019 12:45 PM)cruff Wrote:  Will the step up converter handle the current load of the card reader?

Yes, easily. I did this test, although my card reader (without use for years) only returned Error or checksum error.
I tried the bar code reader too, worked fine.
I tried many cards without any loose in power.

This battery has much more power than N cells. Any way, you can connect the cell charger and help it!

[Craig Bladow]

A very creative solution! I believe the card reader is the largest typical load a 41c will see.

Now if there were a 3d printed battery pack to hold these components and spring loaded battery contacts.....

[mfleming]

https://www.shapeways.com/shops/wants_beer

Someone else may have more details.

[burkhard]

(02-27-2019 03:48 AM)Craig Bladow Wrote:  A very creative solution! I believe the card reader is the largest typical load a 41c will see.

Now if there were a 3d printed battery pack to hold these components and spring loaded battery contacts.....

I agree. Nice work here! I like that the solution doesn't require hacking up the calculator itself and is completely drop-in/drop-out reversible. Hopefully that cell phone battery is a fairly common one that will not become unobtanium in 5 years.

Good job!

[Artur - Brasil]

(02-27-2019 03:48 AM)Craig Bladow Wrote:  A very creative solution! I believe the card reader is the largest typical load a 41c will see.

I agree with you.
About the cell phone battery, there are plenty of them out in market. Anyone that feets inside the HP-41 can be used.
The idea is: as much mAh you can get, go for it!

(02-27-2019 03:48 AM)Craig Bladow Wrote:  Now if there were a 3d printed battery pack to hold these components and spring loaded battery contacts.....

About the idea of a battery pack printed in 3d process, it would be the best.
In my solution, the board of charger was inserted in the side hole for AC adapter.
I had to make a small recess on HP to fit the board. I also had to cut in the board too.
I removed almost all the side of battery pack allowing to keep the boards in 41.
I also covered the space of battery pack in HP41 with tape to prevent lackage get inside it. This would be solved using a new sealed case, the one printed by 3d proccess.

   
   

[Massimo Gnerucci]

(02-27-2019 01:30 PM)Artur - Brasil Wrote:  This would be solved using a new sealed case, the one printed by 3d proccess.

Like this one, maybe, as mfleming already pointed out...

[Artur - Brasil]

(02-27-2019 03:10 PM)Massimo Gnerucci Wrote:  

(02-27-2019 01:30 PM)Artur - Brasil Wrote:  This would be solved using a new sealed case, the one printed by 3d proccess.

Like this one, maybe, as mfleming already pointed out...

It would be better a completed sealed box, up, down, left and right. One of it's cover should be removable, hold by screws, in order to replace dead battery or bad circuit.
The mini-USB conector, which should be accessible by hole in the HP41 case, can be placed on side of that case, aligned wth HP41 hole.
   

[Craig Bladow]

Some space could be saved by using a combined charger / step up voltage module. However you may not be able to charge at the same time as using. Both backup portable lithium battery phone chargers I have had this limitation.

[Artur - Brasil]

Finally I adapted the same charger circuit and a pair of Lithium batteries to my Hp 82162a and Hp 82161a, HP-il printer and cassette drive.
I lose a lot of time testing my tapes and trying to use a step-up circuit too, but, at end, it could not fit inside devices...
The simple charger and batteries in parallel did a good job.
   
   
   

[Craig Bladow]

I found 3D printed designs for the HP-41C battery case here: https://www.thingiverse.com/thing:123278
They look similar to the design mentioned here: http://www.askrprojects.net/hardware/hp41bat/index.html
Note that the design file download links from the second website didn't work for me.