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TI-59 LiPo Battery Pack

A modern replacement for the TI-58/59 Calculator BP1

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The first programmable calculator I ever used was my mother's TI-58 in the late 1970's. I still have it. I have a few TI-59 calculators, too - they're neat. Of course, the vintage NiCd battery packs ("BP-1A") are all shot by now, often times damaging the calculator due to battery leakage. It is possible to rebuild the BP-1A pack, with care and patience. I've rebuilt a number of these old packs with NiMH cells, but they are starting to fail now, also. Based upon the success and utility of my HP calculator LiPo battery pack, I decided to try to do something similar for my TI calculators. It's a bit more involved, because the TI battery pack is the 'back' of the calculator, too, rather than being contained within the calculator as in the case of the HP calculators.

This is my attempt at a LiPo-based battery pack for Texas Instruments Calculators that use the BP01 battery pack,  specifically the TI-58, TI-58C, and TI-59. I thought it would be fun to design a 3D printed housing and re-use the circuitry from my rechargeable HP LiPo battery pack. This is a follow-on project to my Classic HP Calculator LiPo battery pack.

LiPo chemistry has very specific care-and-feeding when it comes to recharging, the original TI AC adapter and built-in charging circuitry will not suffice.  For this reason, I include a USB receptacle, which allows for charging when removed from the calculator.

The basic idea is to use a gold-flashed (ENIG) PCB as the contact substrate, and mount the LiPo, USB micro B receptacle, and charging circuitry on the opposite side. A charging LED is placed on the same side as the contact pads, but in areas where there will be no mechanical interference. A 3D printed plastic holder ties it all together, and mates with the TI calculator case. 

The Housing 

By far this was the most challenging part of this project. I  have several original TI BP01 battery packs, so I was able to carefully measure them and create a 3D model using OpenSCAD. This is essentially a 3D solid 'programming language' which (for me) is much easier to work with than an interactive 3D CAD program. It took several iterations to arrive at a working holder design that would 3D print (FDM) reliably. Note that the holder is best printed "flat side down", but that this orientation is not ideal for the retention tab; it wants to break along the FDM layers. I have had good luck with PLA+ material, and even better results with PTEG. Care when inserting and removing is still recommended, however - these 3D holders are nowhere near as strong as their injection-molded predecessors!

The PCB 

The PCB holds the USB micro B connector, the charging circuitry, the protection circuitry, a connector for the LiPo battery, and most importantly the pads that mate with the contacts in the calculator.  The overall size of the PCB is 53mm x 28mm. The material is 1.6mm thick FR4. So that the contact pads do not  tarnish over time, I chose a gold-flash treatment (ENIG). 

The schematic and PCB were designed using KiCAD.

The LiPo Battery 

I wanted a 1S (3.7V) LiPo battery in the neighborhood of 1200mAhr that would fit within the confines of the original pack envelope. This led me to a size "103040" LiPo battery. 

Putting It All Together

Pretty self-explanatory. I used 0805 size components to allow for easy hand-soldering. Note that D1 (LED) and Z1 (Zener) are both polarized, so pay attention to that. Soldering the USB connector requires a fine-tip iron and some patience, or a hot-air solder station. I use the latter with very good results. 

Connect the battery to the PCB, double-checking that the polarity of the LiPo is matching the "+" and "-" marks on the PCB. Place the battery into the holder in between the two pairs of screw bosses, and then press the PCB into place above it. Four #2 self-tapping stainless steel screws secure the PCB into the 3D printed holder.

Charge It

Connect a micro-B cable to the board and charge the LiPo battery. The LED will stay lit during the charging process, and extinguish when it is completed. The charge current is about 220mA (0.2C for a 1200mAhr battery). You can adjust R1 to change the current (decrease R1 will increase charging current) if you need to. It took about three hours to charge my battery.

Use It!

Install the pack into the calculator. Be very careful with the delicate latch retention tab when inserting and removing the holder, because it is fragile and prone to breakage. If it does break, you can usually re-attach the tab with Krazy Glue. Enjoy using your vintage TI programmable calculator!

  • PETG to the Rescue!

    tomcircuit02/07/2021 at 19:00 0 comments

    Printing the holder from PETG makes a world of difference! This material has a lot more 'give' than PLA and the fragile retention tab really benefits from this. I've even had a few holders fabricated through the Voodoo Manufacturing service and they are even better than what my modest 3D printer can generate - at a very reasonable price (less than $3 each holder).

  • 'Mechanical Stuff is Hard'

    tomcircuit01/31/2021 at 22:30 0 comments

    Still playing with the holder 3D model.  I had to adjust the latch tab (again) a bit, to correctly engage with the inside of the calculator.  I still struggle with the fact that I'm printing the holder "wrong".  That is, I'm printing the outside surface of the holder as the bottom, and building upwards - this makes perfect sense for everything except the latch. The latch wants to be printed "sideways" so that the FDM lines are running perpendicular to the arc of motion.  Unfortunately, on my printer, this would require a lot of support material and probably result in a really ugly holder.

    I've tried printing with "soft PLA" (SPLA) and geez is that stuff icky. What a mess! I don't have a heated bed on my printer, so I can't print Nylon or PETG which would probably be much better. 

    My gut feeling is that, long term, once I've got the geometry of the holder worked out, I'll send off an order to a place that can generate very high quality prints in a nylon-like material and have a dozen or so made (I've got quite a few Ti-58 and TI-59 calculators, and maybe others will want some of these, also).

  • Boards Arrived, More Holder Tweaks

    tomcircuit01/22/2021 at 04:30 0 comments

    The PCBs arrived today, and the first board went together well. I had neglected to order the correct USB receptacle earlier, so that’s not populated yet. Those will arrive next week, but for now I can charge the battery using my HP pack boards. 

    I was a bit overzealous on making sure the TI contacts would touch the PCB pads. The PCB was so far into the cavity that the attachment screws and the “charging indicator” LED collided with stuff inside the TI-59 and prevented lid closure. So, I quickly tweaked the holder model by reducing its height by 2mm and printed a new one. Perfect fit in two of my TI-59s and with the holder latched I was rewarded with a running calculator.

    Tomorrow I’ll print a couple more and check fitment in a few more calculators. I will also connect an AC adapter and ensure the ideal diode and zener protection is working as expected. If all is well I will upload the revised OpenSCAD holder model (ver 0.6 for those keeping track) and some assembly photos. Final release will have to wait for those pesky USB receptacles to arrive...

  • PCB's ordered!

    tomcircuit01/14/2021 at 22:14 0 comments

    Order sent off to PCBWay for fabrication, expected before end of January.

    1.6mm thick FR4, ENIG surface treatment, black mask, white silkscreen

    These boards will be used to check the 0v5 3D printed holder to see if all is well.

  • Mechanical Adjustments

    tomcircuit01/13/2021 at 00:57 0 comments

    I have made some progress on modeling the battery holder, using OpenSCAD tool and a 3D printer to iterate the design a few times. There are a couple of critical dimensions on the BP-1A battery pack, notably the amount of latch tab “engagement” and the depth of the “step” that fits beneath the calculator housing towards the top of the cavity. But, I think I’m there, now  I’ll upload the files to GitHub soon.

    It seems that PLA is not the best material with which to make living hinge features. It’s quite brittle. The retention tabs have broken off from most of my prototypes. I suspect the best solution is to find someone to print it in ABS - or perhaps send to a service and get even better quality.

    I also managed to enlarge the PCB dimensions a bit; now 30x50mm. This should help with the crowding I’m facing in layout, trying to avoid interference between SMD components and the LiPo battery pack.

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