HP41 Battery idea Lipo

[Freeclimber77]

Hi, I'm experimenting with a new battery pack for my 41cx
In the battery compartment there is just a lipo 103450 of 3.7v and 2000mAh

If I have a little luck, a 5v (tp4056 module) DC DC step up that allows both USB C charging via the originale connector hole should also fit... The 3d printed cap...

[rprosperi]

(05-22-2023 08:31 PM)Freeclimber77 Wrote:  Hi, I'm experimenting with a new battery pack for my 41cx
In the battery compartment there is just a lipo 103450 of 3.7v and 2000mAh

If I have a little luck, a 5v (tp4056 module) DC DC step up that allows both USB C charging via the originale connector hole should also fit... The 3d printed cap...

Strange, it seems I've heard this elsewhere, and recently too....

Oh wait, here it is:

https://www.hpmuseum.org/forum/thread-19...#pid173046

Sounds suspiciously similar...

[Freeclimber77]

It sounds similar .. honestly I had not seen.( Im new here) but my project is much more modest.. i use existing boards.. . and works at 5v

[FLISZT]

@Freeclimber77

Aosta… I've been to La Thuile several times.
A long time ago. Very... the 70's!

Good memories!

[rprosperi]

(05-22-2023 10:00 PM)Freeclimber77 Wrote:  It sounds similar .. honestly I had not seen.( Im new here) but my project is much more modest.. i use existing boards.. . and works at 5v

First, welcome to MoHPC, if you're a 41 user I think you'll find this a fun and interesting place.

I was only kidding, no worries... 4-5 folks have posted about doing this is in the past year, but it appears to be harder than expected as none have announced complete success. Hopefully you work it out, and even if not, Diego's will be coming soon.

[MeindertKuipers]

(05-22-2023 08:31 PM)Freeclimber77 Wrote:  Hi, I'm experimenting with a new battery pack for my 41cx
In the battery compartment there is just a lipo 103450 of 3.7v and 2000mAh

If I have a little luck, a 5v (tp4056 module) DC DC step up that allows both USB C charging via the originale connector hole should also fit... The 3d printed cap...

This is not a very complicated project, I have made several myself and this really works great. I use 3D printed battery holders (open source available) for the HP41 and copper tape for the battery contact. Main challenges I see:
- take care to use batteries with protection or use a charger circuit with protection, otherwise the battery will empty completely and may not recharge anymore (I found out the hard way)
- when charging inside the calculator, the port on the side may not fit just any USB cable
- no detection of low battery to give the BAT indicator

I am not near my calculators now, will post pictures sometime later

[Diego Diaz]

Hi all,

@Freeclimber
As Bob said, welcome to the community.

Clearly a coincidence, the general idea is pretty straight forward and I know for sure I'm not the only one working on this.

Good luck with your project!

@rprosperi

Bob, I really appreciate your confidence in my success! ;-)

Let's see how long it takes for this idea to become a real thing.

@MeindertKuipers

I'm not planning to go the 3D housing path for now, just naked PCB's.

Look forward to see your implementation (your design ideas are always inspiring).

From the three main problems, inherent to these type of power packs, that you've pointed out; I think I've sorted out the first two: USB-C type connectors can easily fit in the standard charging port of the 41; and by all means only protected batteries are considered for this project.

Working on a solution for the third: the BAT indicator. It requires some sort of "emulation" of the discharge curve of the alkalines, and I think it can be done by tweaking a bit the DC-DC converter with some 'amplified negative feedback loop' (just an idea for now), and yes I know it may sound kinda contradictory since it's a stet-"up" converter... :-D

Ordered the parts, let's see how it works.

All he best from the (heavily rainy nowadays) Caribbean.

Diego.

[Hans Brueggemann]

(05-23-2023 05:11 PM)Diego Diaz Wrote:  ...
Working on a solution for the third: the BAT indicator. It requires some sort of "emulation" of the discharge curve of the alkalines, and I think it can be done by tweaking a bit the DC-DC converter with some 'amplified negative feedback loop' (just an idea for now), and yes I know it may sound kinda contradictory since it's a stet-"up" converter... :-D

Ordered the parts, let's see how it works.

All he best from the (heavily rainy nowadays) Caribbean.

Diego.

i think what you are looking for is "feed forward control" of the output voltage. have a look at my Hurricane II project, there is also the schematic hiding somewhere down on page 2 of the thread.

[Diego Diaz]

Hi Hans,

Thanks for stopping by and share your solution.

Quite similar to what I was thinking, though implemented in a different functional way.

Still in the drawing board so I'll give it a go once I receive the parts.

The reference of 3.7v you give to the BAT annunciator ON is consistent with the voltage I've measured in my CX: some 3.8v; and what is described in the bipolar IC specs.

The Service Manual on the contrary is rather misleading on this subject.

Did you also check at what voltage does the 41 stop turning ON?

I tested in mine and it happen to be <2.9v. It would be interesting to gather some more data on this to confirm consistency.

Have someone else tested it? If so, please share your findings.

All the best.

Diego.

[Harald]

(05-23-2023 05:11 PM)Diego Diaz Wrote:  Working on a solution for the third: the BAT indicator. It requires some sort of "emulation" of the discharge curve of the alkalines, and I think it can be done by tweaking a bit the DC-DC converter with some 'amplified negative feedback loop' (just an idea for now), and yes I know it may sound kinda contradictory since it's a stet-"up" converter... :-D

I think all you need to do is detect low voltage on the lipo, and when you detect it set the DC/DC converter to an output voltage that makes the 41 turn on the low voltage indiction.
So basically just a comparator that detects when the LiPo drops below a threshold and then tweaks the DC/DC converter feedback path.