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Adapting cell battery to HP 41
01-10-2020, 02:15 PM
Post: #21
RE: Adapting cell battery to HP 41
(02-26-2019 12:04 PM)Artur - Brasil Wrote:  I just have one doubt about my project: what will happen when battery discharge too much...
Best wishes!

Artur, did you find out what happens when the battery is discharged to a low voltage? From looking a lithium polymer battery pack specifications on line, if they have protection circuitry, they have a low voltage shut off.

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01-10-2020, 04:37 PM
Post: #22
RE: Adapting cell battery to HP 41
(01-10-2020 04:40 AM)Craig Bladow Wrote:  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.

I liked this quote from the description in the second link you've provided

"Unfortunately, the designers of the HP-41 didn't consider the usage of an USB cable to charge the calculator."

If they had, the designers would have been remarkably prescient! Smile

Who decides?
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01-10-2020, 04:48 PM
Post: #23
RE: Adapting cell battery to HP 41
(01-10-2020 04:40 AM)Craig Bladow Wrote:  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.

Very professional! The second link is what I've done using two individual boards. He did put in only one board the charger and the voltage step-up. Very nice!

Thanks
Artur

ARTUR MARIO JUNIOR
BRAZIL
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01-10-2020, 04:51 PM
Post: #24
RE: Adapting cell battery to HP 41
Once purchased an TI-58C modified with a Nokia phone battery pack. Worked beautifully.

HP-28S (1988 US model), DM41X (2020)
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01-10-2020, 04:52 PM (This post was last modified: 01-10-2020 05:31 PM by Artur - Brasil.)
Post: #25
RE: Adapting cell battery to HP 41
(01-10-2020 02:15 PM)Craig Bladow Wrote:  
(02-26-2019 12:04 PM)Artur - Brasil Wrote:  I just have one doubt about my project: what will happen when battery discharge too much...
Best wishes!

Artur, did you find out what happens when the battery is discharged to a low voltage? From looking a lithium polymer battery pack specifications on line, if they have protection circuitry, they have a low voltage shut off.

Hi, Craig!

I really don't know. In my HP41 I don't let it get too low, or I'll loose my programs! Good Li-Ion batteries last much more than NiCd or NiMH.
But I believe the devices will just not work.
I saw on Ebay some small charge indicators, very nice, but here I couldn't find them.
Many thanks
Artur

ARTUR MARIO JUNIOR
BRAZIL
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01-18-2020, 04:04 AM (This post was last modified: 01-18-2020 04:06 AM by Craig Bladow.)
Post: #26
RE: Adapting cell battery to HP 41
I've acquired an MT3068 DC-DC converter. It will hold the output voltage as set until the input voltage falls somewhere below 2 volts. This is not ideal for use in an HP-41 as many lithium-ion batteries have protection circuitry that shuts off their output at 3.0 volts so the 41 would see its input voltage drop from a good level quickly to 0 volts with little or no warning in time to change the battery.

The MT3068 senses the output voltage via a two resistor voltage divider connected to its feedback input. One would need some way to drop the feedback input so the output voltage would go just below the 41's low battery threshold when the lithium-ion battery was mostly discharged, say to 3.3 volts.

I've also printed out the battery cases I linked to in a previous post in this thread. There is not a lot of room in the cases so some smaller DC-DC converter modules are on the way to try out.

With a removable battery case the charging circuit could be external which would save room.

Check out NQ41!
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08-02-2020, 05:29 PM
Post: #27
RE: Adapting cell battery to HP 41
(01-10-2020 04:40 AM)Craig Bladow Wrote:  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.

The owner of the askrprojects web site has kindly corrected the two design file links at the bottom of the page (hardware and mechanics).

Enjoy!

-Pat (KG5YPQ)
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08-02-2020, 06:25 PM
Post: #28
RE: Adapting cell battery to HP 41
(01-18-2020 04:04 AM)Craig Bladow Wrote:  I've acquired an MT3068 DC-DC converter. It will hold the output voltage as set until the input voltage falls somewhere below 2 volts. This is not ideal for use in an HP-41 as many lithium-ion batteries have protection circuitry that shuts off their output at 3.0 volts so the 41 would see its input voltage drop from a good level quickly to 0 volts with little or no warning in time to change the battery.

The MT3068 senses the output voltage via a two resistor voltage divider connected to its feedback input. One would need some way to drop the feedback input so the output voltage would go just below the 41's low battery threshold when the lithium-ion battery was mostly discharged, say to 3.3 volts.

I've also printed out the battery cases I linked to in a previous post in this thread. There is not a lot of room in the cases so some smaller DC-DC converter modules are on the way to try out.

With a removable battery case the charging circuit could be external which would save room.

Here is an example of what I have made for my HP-67. Charging circuit is external and no modifications to the calculator are required.
       
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08-04-2020, 12:47 AM
Post: #29
RE: Adapting cell battery to HP 41
That is a very nice setup you have there TW!

The 41c solution needs a dc-dc boost circuit in order to use lithium ion batteries. Hmm, a fresh set of 4 N cells starts at around 6.4 volts output, maybe two LiFePO4 cells, if they made them small enough, would do the trick for a 41c.

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08-04-2020, 01:13 AM (This post was last modified: 08-04-2020 01:14 AM by Craig Bladow.)
Post: #30
RE: Adapting cell battery to HP 41
Turns out that LiFePO4 cylindrical cells (3.2 volts each) are available in the same size as an AA battery. I quickly popped out my HP-41CX battery back and two alkaline AA cells fit in to the compartment with room left over.

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08-04-2020, 03:24 AM
Post: #31
RE: Adapting cell battery to HP 41
HMM...
That looks very interesting. I will have to order some and play around with 3D printing a holder. I already have a charger that will charge LiFePO4 cells.
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08-04-2020, 10:56 AM (This post was last modified: 08-04-2020 10:56 AM by Hans-Peter.)
Post: #32
RE: Adapting cell battery to HP 41
Quote:Turns out that LiFePO4 cylindrical cells (3.2 volts each)

Be careful with two LiFePo cells. These have 3.6V per cell at full charge. That would be 7.2V and probably too high to survive.
I have connected two LiFePo4 cells of Soshine AAA size in parallel and added a 5V step-up converter. These have only 2x280mAh but they are suitable for lower temperatures outside.
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08-07-2020, 07:26 PM
Post: #33
RE: Adapting cell battery to HP 41
(08-04-2020 10:56 AM)Hans-Peter Wrote:  
Quote:Turns out that LiFePO4 cylindrical cells (3.2 volts each)

Be careful with two LiFePo cells. These have 3.6V per cell at full charge. That would be 7.2V and probably too high to survive.
I have connected two LiFePo4 cells of Soshine AAA size in parallel and added a 5V step-up converter. These have only 2x280mAh but they are suitable for lower temperatures outside.

Thanks for the cautionary note. I have various dc-dc converters on hand so have ordered two 14500 LiFePO4 batteries with a charger.

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