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what would be the optimal voltage for an external battery for a HP-41CX? Is it 5v or 3.3v?Would ther be a maximum on the volt/amps that should be used? I have the MLDL2000 and when the unit is disconnected from the usb cable it does not function correctly. I was thinking of using an external battery to run the calc and the mldl2000.


I use rechargeable batteries, but when I need extra time and extra current, I use an external pack. This was my first set of external batteries for the HP41 - pretty ugly, I know - and this is how it connects to the calculator. Yep, it needs the sphere+spring set originally found in the earlier models. As strongly stated here, if polarity is inverted the circuits are fried, so I stressed the [+] and [-] signals in the upper part of the small PCB that connects to the calculator. After the old batteries got down, I rebuilt the battery set and this is it. I use it mainly to transfer data through the IR module from the HP41 to the HP48G, and because the batteries are rechargeable I think the set will last longer.

In time: AFAIK, the HP41 will not work (at least properly) with 3.3V, but as I use rechargeable batteries, I guess that something closer to 5Vcc will do (the battery set measures 4.9Vcc with no load).

Hope this helps.


Edited: 16 Nov 2012, 5:56 p.m.

Another thing: I use an HP41CX + MLDL2000 with rechargeable batteries (4 X 190mAh) and it lasts reasonably well with it. I built my MLDL2000 very carefully and took all procedures to make it work fine. In its manual, Meindert states that the MLDL2000 adds no more than 6-7mA to the whole set when it is running. Quiescent current is in the uA range.

I should rephrase, when I disconnect the MLDL2000 from USB the SRAM contents are not readable, Flash is ok. What does the Connection inside the battery pack look like?


In this case, I'd suggest checking if the VCC wiring is OK, i.e., the black, red and yellow wires in this picture. Meindert's are soldered straight to the PCB's and I decided to use the terminals, but I changed his color scheme: red for yellow. If the SRAM is fed by USB and not fed by the HP41 batteries, chances are one of these wires is not actually connected (broken/cold soldering, perhaps).

What does the Connection inside the battery pack look like?
You mean my external battery pack, right? The batteries are in series, so the green wire goes to the positive tab and the white wire goes to the negative tab. The white bands are just to avoid the terminals to touch something.


Luiz (Brazil)

Edited: 16 Nov 2012, 8:51 p.m.