I've been tossing some ideas around that would add unnecessary, but neat features to my TI-84 Plus CE!

Finished: My first one would be to triple the CE's battery life. How? By add two additional batteries! With some preliminary measurements, I found out the shell is *just* deep enough (5mm) to add at least one battery in over the ASIC and flash if I file down some structural bits in the shell (squared in red). I'm not sure if I can fit another over the LCD. The shell is 3mm deep and the foam is 2mm which theoretically means I can fit a battery there, but I'm not sure if I can take off the foam without damaging the LCD? I'll also need to find a way to reliably connect the batteries together without the connections being loose.

(click for full size)

Finished: My second idea would be to add a wireless charging receiving pad to the inside of the shell. I haven't really done any measurements for this yet but it looks like even with the extra batteries I could still cram in such a thin device. Since the shell is just plastic, there shouldn't be much interference. All I would need to do is cut off the micro USB end and solder the power leads to the mini USB connector of the calculator. I wonder if there would be issues though if I plugged a cable into the calculator while it was wirelessly charging?


Finished: My third idea would require quite a lot of work and frankly I'm not sure I want to go though with it. It's such a good idea though, back-lighting the keys!

• I'd be fitting the LEDs between the plastic key and the rubber membrane
  
• I would need to find some low power LEDs that are small enough to even fit in the top 5 keys and arrow keys, but still bright enough to illuminate my denim colored plastic.
  
• The battery hit from turning on all those LEDs may require the battery mod from idea one.
  
• I would need an off switch somewhere because I'd be hooking the LEDs directly to the battery terminals. Probably removing one of the charging dock contacts at the bottom would be the best place.
  
• I need wire thin enough it doesn't interfere with the keys.
  
• I need to be sure the wire is insulated somehow so it doesn't accidentally short and cause big issues.
  
• Due to the size of the led inside the plastic button, I may need to file down the rubber membrane to compensate and avoid false button presses (can I even do that?).

Finished: My fourth idea was to add a bluetooth speaker to the calculator. Since I now have the second battery inside the calculator, I can take out the original battery and use the battery bay for small, isolated projects. I want to be able to make a phone call through my calculator!



So, yes, lots of work. I'd love to hear your input though! Someone always has knowledge I don't about hardware modification and I'm willing to hear even the worst news.

That sounds like a lot of fun!

On the topic of thin insulated wire for the backlighting, enameled wire is not noticeably thicker than the same gauge of bare wire, and you just scrape the enamel off to solder it.

And I would just solder directly to the battery contacts of you are brave/skilled enough

OK, small update but I think I'm going to settle with just adding one additional battery.
I originally tried putting them in horizontally, but it turns out the LCD connector is just too in the way for me to fit it within the shell. I might be able to make it work, but I've decided to just add one battery vertically which should be much easier.

1 horizontal + 1 vertical? .

Sounds like fun - I second the idea of enamel wire or other similar offerings.

You seem to be doing well with the batteries, just remember to hook them up in parallel, positive terminal to positive terminal and vice versa. This will give you the same voltage but double the amps, which is useless (unless you overclock the calc like DrDnar) but fun. I actually had a little idea a while back, although I haven't been able to mess with it, but it theoretically works: nuclear calculator. There's a substance called "tritium", an isotope of hydrogen with a half-life of 10-15 years (depends on supplier, I just assume 10). It's radioactive and glows brightly, but not dangerous unless ingested or breathed in as a dust. I saw a video a long time ago of someone making tritium batteries with some highly efficient solar panels, so there *theoretically* is a way to make a battery. Although within 5 mm is pretty tight, I've considered having it stick out of the case in a custom pack, but as I said, it's all just been a plan. Equally stupid and intriguing!



Again, just hook that up in parallel with the existing USB port terminals and you'll be fine. The only possible problem would be some sort of electromagnetic interference with the calc, but that shouldn't be an issue as far as I know.



Another idea I though of! This could maybe be done if you put the LEDs behind the circuit board, cut holes through the PCB for light without ruining the copper tracings, and got yourself some special buttons (maybe clear as well?). You could try moving the PCB farther from the front of the case, although the buttons wouldn't stick out as much, and there isn't much clearance to that either. However, if we could 3D-print our own case, that would solve everything, and we could make extended buttons and push the board back farther into the case, leaving clearance for LEDs, and more batteries. I've done some searching, but I haven't seen any TI-84 Plus CE case files online (although some for other calcs) although I feel like there must be something out there. All in all, neat project! Good luck, don't shred your case too badly!

I finally got started and filed out the structural pieces that were in the way on the back shell and I cut out part of the black foam. It took longer than expected but I ended up with, as I measured, just enough space for an extra battery! Unfortunately, there's no way I can fit another even if I make the other one horizontal as tr1p1ea suggested. Oh well, 2,400mAh is still more than enough for a couple months of charge!

(click for full size)

I'm going to make sure both batteries are the same charge because I think I heard somewhere that different charge levels could cause issues. I know I need to hook up the positive and negative terminals, but is there any reason to hook up the middle contact (which apparently reports battery temp?)?

Next, I need to connect leads to the extra battery. I would really love if I could do it without soldering onto the battery. Not only does heat kill batteries, I don't have any experience soldering to batteries and I don't want to ruin it. If someone has an alternative way I would love to hear it!

Sadly I don't have any enamel wiring in-house but I'll be sure to order some! King Dub Dub, I'll leave the nuclear powered CE to you, feel free to make your own thread for that!
Thanks for your support guys! I really appreciate it!

Putting lithium batteries directly in parallel is generally not a good idea. You want a BMS to balance the cells:

At best you won't get the best possible performance, at worst you have a fire waiting to happen.

Adafruit have an accessible overview of safely building a multi-cell pack, which mostly involves being able to switch the cells out to charge them. There exist ICs that can do all that automatically (for instance, a BQ25886) but they tend not to be very hobbyist-friendly. Another seller of charger modules suggests you can parallel two cells but need to be very vigilant that your cells are matched well (which Adafruit also note). There do exist battery management systems in module form, but mostly not appropriate for this application because they're meant for much higher-power applications (like batteries of 8x 18650 cells).

Temperature measurement is a good way to cut off the battery pack if something is going wrong- you should connect at least one of them. Unfortunately this is usually a thermistor so you can't just connect both. Given the calculator can't control the two cells anyway, trying to do something fancy doesn't seem worthwhile but leaving the monitoring disconnected completely is a good way to ensure it doesn't charge because it thinks something is wrong.

Since you can't monitor both cells I would not consider this battery system "safe" and would strongly consider building some kind of hard switch that can disconnect one battery when the system is left unattended. Unfortunately this would kind of defeat the purpose of your mod..

In manufacturing, battery contacts are usually spot-welded. I think some kind of conductive glue might be a good choice.

That's quite an informative post! I should've researched this more before trying this. I saw some guy do this exact mod with an old Android device and I guess I fell victim of the Dunning Kruger effect and didn't bother looking into it more.

I'm going to need quite a bit of help if I need to add any extra hardware because I'm quite inexperienced with it. The charger you linked looks small enough to work, but I have no idea how I can wire it up so the batteries can both be charged and discharged normally.

If I connect the batteries only to it, it's not going to power the calculator. If I try to connect them back to the 3 pin connector, whenever I plug the calculator in, the board and the calculator's built in charger are both going to try and charge them. Any help?

EDIT: Is charging the biggest issue? If I add an on/off switch like you suggested, I can remove the original battery, turn on the spare battery, then charge the spare. Once that's done charging, I put the original battery in, turn off the spare and charge the original. That wouldn't require any BMS right?

Separating the batteries to charge them is probably not a good option because it becomes much more difficult to ensure they're balanced when you reconnect them.

A reasonable way to protect the batteries from catastrophic failure might be to put a fuse on each one. You can estimate the maximum expected current flowing in our out of each cell (probably while charging), add a little headroom and put a fuse in that will disconnect the cell if it draws more than that much current. That should sufficiently mitigate the risk of catastrophic battery failure (fire), but won't protect against cell damage due to overcharging or overdischarge.

OK, so I got a fast act 750mA fuse to prevent fires! Unfortunately I couldn't find any that were smaller than 5mm x 20mm which just barely fits within the shell. I was really hoping to get something small like the 3A fuse you find in Christmas lights but I think I can make this work.

I soldered the positive wire to the battery and only melted the casing a tiny bit. I'm avoiding soldering directly to the fuse because if it ever blows, I want to be able to replace it without needing to break out the soldering equipment. I've run into another problem, the only place the massive fuse will fit is only a few millimeters away from where I need to connect it to the battery. Not only is the wire I'm using super short and difficult to work with, I can't get the solder to stick to the 3 pin battery connector. I'm probably going to have to get that conductive glue that was mentioned earlier.

Small progress so far:

(click for full size)

In other news, I took apart a burnt-out motor and salvaged the enameled wire out of it so I can use that if I ever do the back lighting mod! Now that I think about it, maybe I can try that wire instead of the 18 gauge I've been using...

Double Battery Updates:
That was mostly a good idea, me! I tried that enameled wire and it works with the shell closed, fuse and all! Unfortunately it seems CS never leaves my projects, because: without the shell, it doesn't work and I don't know why. With the shell on, it works and I don't know why. ¯\_(ツ)_/¯

No really, it was working normally until suddenly it wasn't. I checked with the multi meter, the circuit is perfectly fine but the calculator refuses to turn on unless I put pressure on the fuse. I bypassed the fuse but it didn't make a difference. Maybe someone has an explanation for this?

Final Product:
Eye candy!


I freaked a few of my friends out with this one


I just remembered I forgot to put the reset button back in, whoops.

Please use kapton tape next time

Note: I have all my questions TL;DR-ed at the very end of this post.

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Good suggestion! The scotch tape was only temporary, unfortunately I have no kapton tape however I have plenty of electrical tape that should work just fine!

Wireless Charging Updates:
I've ordered a wireless charging receiver coil, it was a pain figuring out what the size of the pad of the charger was since sellers counted the cable extension at the end part of the product. I believe I found one that was properly sized and should be able to charge even through the thick plastic. It's a shame it can only do 5W charging but the calculator can't take much more than 5W so I guess it's no loss.

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Backlighting Keys Update:
I've taken everything apart again including breaking the heat melted plastic parts that hold down the PCB so I could document a ton of measurements of the keys. Not counting the keys that have rounded corners, there's only 4 different types. The function keys, arrow keys, number keys, and the rest. Thank god I didn't need to measure 50 different types!

(Click for full size)

With the measurements, I've learned for most keys I need an black-light LED that's at most 3mm x 4mm x 2.5mm. As you'd expect, finding a light that's bright at that size is a little difficult. Or at least I think it's difficult. It's incredibly hard to tell what brightness I actually need to shine through the plastic.

Side rant:
Since Digikey measures the brightness in millicandelas instead of lumens I have to convert each one to see how bright it actually is (to find actual brightness using millicandelas requires a viewing angle. So, unlike lumens, higher millicandelas does not necessarily mean a brighter LED. For example, a 110,000 millicandela LED with a viewing angle of 15° is the same brightness as a 1900 millicandela LED with a viewing angle of 120°!). Also, I have no reference or way to test what one lumen looks like so who knows if the brightest in stock LED I could find (~6 lumens) is even remotely bright enough to shine through the keys! [/rant]

Back to Backlighting Keys Updates:
Phew, anyways, I believe I narrowed the results down to either these 10 results, or these 38 results. The top ones I'm considering are:

• Perfect size but very dim at 1.6 lumens.
  
• A bit wide and small, but one of the brightest at 5.9 lumens.

They require around 3.3v while the CE's battery peaks at 4.2 so I may need a resistor somewhere. Both draw 20mA which, with my 2,400mAh battery mod, will grant me 2.4 hours of battery if 50 LEDs are installed. I'm pretty worried that drawing that much current on top of what the calculator needs to function may beyond the limits of the CE's battery. Will I need to tone down the LED current draw? So the battery life will be pretty terrible but I only expect to use it while charging (which shouldn't be an issue, right?) and an off switch is definitely being connected to it.

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Question TL;DR: I'd appreciate it if someone could help me out with understanding how bright 6 lumens is so I know if any of these are even close to working. Also, is drawing 1Ah from the battery beyond its capabilities? Lastly, I'd like to be sure that using the back-light while charging shouldn't be an issue.

No no no. No. Never do that. Electrical tape is neither heat resistant and it will leave gunk all over the pcb.

After quite a massive struggle, I successfully added wireless charging to my calculator!


The Choice:
I started by looking at wireless charging recievers and eventually settled on the Nillkin Qi Reciever. I chose it because one of the images showed nice, large, flat contact areas for me to solder on. Also, it was very thin at only 2mm and one of the reviewers mentioned that its reception worked even through thick phone case plastic.

The Build:
When I received it, I immediately cut off the micro USB end and exposed the two power strips in the cable. It took quite a long time to figure out which strip was the positive side and which was the negative because my multimeter wouldn't pick up any voltage! It turns out there was a coating over the copper that looked very similar to copper, but when I put it under a microscope it became clear that it was blocking any conductivity.

(Click for full size)

After I scraped away the sudo-copper, I easily found which strip was positive and negative and soldered two thin wires to the strips. That was the only easy part of this mod. My next step was to solder the other ends of the wires to the mini-USB port. I sorta set myself up for failure here. My soldering iron tip was way to big, my solder was too thick, and I had no flux. Therefor the port structure itself was in the way and it took me over 45 minutes and dozens of tries to get the wire even remotely soldered to the port! Thankfully I finally get both connected and successfully made the green charging light turn on!


Fitting it all together:
This was also quite tedious since the charger was a bit bigger than I expected. The second battery was already using up the 5mm of free clearance between the shell and the PCB, stuffing an additional 2mm put the calculator at the limits of being able to even close it at all. In fact, the plastic snaps alone couldn't keep it closed, I have to keep the screws in or it all pops apart. I already cut away even more shell to accommodate for the extra size but it still barely fits. (Fun fact, so far I have cut away a whole 7 grams of the shell) Also, I added the reset button back in which caused space issues with my existing wire. I still don't have any Kapton tape, but I've gone ahead and used electrical tape in some places. Sorry Mateo, if I continue with the back lighting mod, I'll certainly order some.

Final Product:
In the end, the wireless charging receiver is way higher than I wanted, right now it's just behind the LCD. Unfortunately there's no way for me to bring it lower with my current internal setup. The calculator feels quite heafty now, I decided to weight it and it's now 195 grams! Nevertheless, it still works very well! Just as the reviewer mentioned, the receiver worked through the plastic just fine. The calculator can still be charged either wirelessly or via wire! Although not super useful since I don't have a wireless charger of my own, it's quite a neat trick and I'm still pretty pleased with it!

This is really cool! I assume the calculator can still be charged from the bottom contact points as well? What happens if you use all 3 charging options or even just the 2 charging options at the same time?

I think you should build an external battery sleeve of some sort next, with several batteries, that way your calculator can run forever

Seems cool! Sounds like an awesome project to do. Too bad I'm too lazy to even think of doing these things.

I'm not sure really since I haven't tried it. Everything is hooked up in parallel so the voltage is the same. I guess the calculator will just stuck amperage from whatever power source it feels like. ¯\_(ツ)_/¯

Haha that'd be fun! I'll just stick to a normal battery bank though

Thanks bio_hazard! Really all you need to do is be crazy enough to think of random features you don't even need, then implement them anyways.

Bad news guys. In my impatience to wait for proper equipment and overconfidence in my skills, I've screwed up my USB port. I connected it to my computer and Windows informed me that the device was malfunctioning. I tried my other CE and it connected just fine and I tried to run a USB self test but it ended up failing (I successfully tested the cable on two working calculators) so the problem is certainly my calculator's USB port. I also tried disconnecting the wireless charger but that didn't help.
Here's a video of it failing:


Mateo thinks I've fried the USB chip. How do I test this and if it turns out to be the issue, why did this happen? The explanation I was given sounds like it only applies if there's two power sources at once.

OK, there's a chance that there's no ASIC damage and the problem could potentially be I screwed something up inside the port itself. Fingers crossed that's the issue.

What I'm going to do next is wait for my new soldering equipment to arrive then remove the wireless charger and fix the USB port the best I can. Next, I'll just solder the wireless charger to the charging dock pads on the bottom of the calculator which I should have done in the first place!

I'll leave you with this mysterious behavior: oh, well then common behavior:

(I apologize for the god-awful quality)

For the first time I'm experiencing the calc allowing me to power it solely off of the USB port. It dims the brightness below what the normal limits are and it's about 10% slower at doing everything. The LCD also turns off whenever you try and do anything like press a button or run a program until the operation is complete.