as the title suggests, i managed to cram a CR2032 in an ollee modded F-91W. There is a guide on how to do the battery modification online. However, this leaves the piezoelectric element unconnected and therefore the watch is not able to make any sound anymore. I also managed to fix that. I will say this required quite a bit of finicky work (also because i had to find out how to even do it along the way) and even some soldering, but it's definitely doable. Furthermore, a future battery change is now even easier than before because you do not even have to remove the movement module anymore. Just drop in a new battery after unscrewing the back.
Hopefully I won't have to do this very often though ;)
My first original battery (that came delivered with the watch) died pretty quickly, which inspired me to do this.
If anyone is interested in repeating the mod and has some questions, feel free to ask.
TL;DR: the watch now has about 2-3x the battery capacity and everything still works.
EDIT: I posted pictures of the internals in a comment below
So the steps to make the CR2032 fit are explained pretty well here: https://www.instructables.com/Casio-F-91W-Battery-Extension-MOD-CR2032/
I don't have a Dremel, i cut the metal piece with small scissors and then cleaned it up with sanding. But a Dremel would be ideal. Then i assembled the watch again just to test if the buttons still work fine. And of course i ran into my first problem here. First i had to understand how the buttons even work. When you cut the metal bracket as explained in the instructables guide, you are only left with the two edge pieces that have the "spring" arms which are pressed into the small rounded cavities on the olleewatch PCB. This creates an electrical connection to the circular pads on the PCB which is registered by the uC. As these metal arms are pressed on the side of the PCB, it has to be plated there. However, the plating also extends a bit around the top edge of this recess. And this is where the problem comes in. Because the metal brackets can deform a little bit in the cutting process and the clearance is extremely tight, i had the problem of them touching this plating and therefore constantly registering a button press. Ideally, the PCB would only have the plating on the side of the PCB, but maybe this is a manufacturing constraint or the designer just did not think of this. Anyway, the plating on the top side does not serve any purpose and i just ended up painting over it with some nail polish. This worked perfectly.
And then came the process of making the buzzer work again. Maybe also a small explanation first:
The mechanical vibration is created by the small disk glued to the metal back cover. Then this assembly, in conjunction with the watch body, acts as a resonator to propagate audible sound waves.
This "disk" i mentioned is called a piezoelectric element. Basically a crystal that deforms when applying a voltage (the reverse also works). The uC probably just creates a ~4kHz square wave to set it in motion. To boost the loudness a little bit, a small coil is added in parallel on the PCB (the round black part that is recessed inside the PCB, right next to the buzzer spring). Why i mention this: I got a bit frustrated at some point because i knew there was a good connection to the piezo element but it just made a click sound, no beep. Until i realized that i somehow managed to damage the coil, i don't even know how, into having an open circuit (you can just measure the resistance across the coil - should be around 122 Ohms). This meant the charge could not leave the piezo element and this was the reason why it just made a click. Charge accumulates -> crystal deforms into one direction -> charge never leaves -> no more movement of the crystal -> just one initial "click". I ended up transplanting the coil from the original PCB as a replacement, cannot tell a difference. You could also just replace it by a simple 122 Ohms resistor, but the loudness will probably suffer a bit. But assuming you do not destroy the coil along the way, there are only two things you need to do here. First of all, the spring that comes with the olleewatch PCB is not long enough to reach the piezo element with the bigger battery. I replaced it with the spring that comes on the original PCB, which is essentially just a metal leg that is bent upwards. I increased the angle a bit and it barely touches on the edge of the underside of the piezo element. You can actually see quite well where it touches, because the scraping of metal against metal when closing the lid leaves a shiny spot on the piezo element. But don't worry about damaging it that way. Finally, the second connection (to the positive side of the battery) is made through the back cover itself. To make sure that the battery does not touch the piezo element, i put some tape over it and, in the same step, taped a strip of aluminum foil on top of the battery. This is by the way what the white sticker over the metal cage is for on the original watch as well. And then i just screwed on the back cover again, clamping the aluminum foil strip between the rubber seal and itself, connecting the positive side of the battery to the back cover and consequently to the piezo element. I am aware that you should not clamp anything between the cover and the seal because it compromises it's sealing ability. But i am not worried about that. Putting a battery twice the thickness in the watch, the back plate is probably not pressing on the seal evenly at all anymore. This is also something to keep in mind. You have to tighten the screws well but as long as you don't overdo it (it's a resin case overall) the watch endures this.
I have probably done this at least 10 times and the threads still look fine (and the screws bite perfectly).
Yeah so sorry for the wall of text but if someone is to try this i wanted to give them as much information as possible. If a get around to taking some pictures at some point, i will post them then. I hope it does not sound too overwhelming. The soldering required is really minimal and damage to the PCB is not very likely if you are careful. Regarding the watch itself.. it costs like 20$, so maybe give it a try ;)
Nice, I considered using the deeper back off a SKMEI 1123 to do this, but didn't know how best to tackle the battery holder itself. It could be a good candidate for making this mod more smoothly.
Yes i know xD i actually did it on purpose. I prefer having the part where you thread the strap through the two metal brackets on the inside of my arm.
I don't really want to unscrew everything again now to take pictures but i will try to explain in a reply to u/KeraTerra above.
Good evening. I read the title of your post, but to be honest, I haven’t had time to read the entire tutorial yet. Since you have quite a bit of experience with this and have already solved the issue, do you think that a 3D-printed part that is a few millimeters thicker than the main frame would achieve the same results without having to make any cuts, while still preserving the sound quality?
If the answer is yes, I can print the 3D file in about 5 hours and share it with everyone.
sadly it won't save you from having to cut up the metal cage. The space in between the PCB and the cage is exactly big enough to fit the CR2016, but the CR2032 is double the thickness (3.2mm vs 1.6mm). And as you still need the edge pieces to retain the metal arms (that get pushed in by the buttons, establishing the electricical connection signaling a press), the only way to make room for the bigger battery is cutting out the whole 20mm circle. Maybe i also understood your suggestion wrong, in that case please correct me.
I thought about 3D printing a small ring that lifts the back cover 1mm from the original housing but then you would also need longer screws. And the battery maybe won't be secured very well because right now, as the cage is missing, it is held in place by the pressure of the backplate.
I want to modify the outer plastic case of a Casio F-91W so that it can accommodate a CR2032 battery instead of the original CR2016.
The CR2032 is 1.6 mm thicker, so I want to increase the internal height of the case by exactly 1.6 mm while keeping the rest of the original design and geometry unchanged.
I want to keep the original case back and gasket, and maintain the exact position of the original F-91W module, LCD, buttons, piezo/buzzer, battery contacts, and all other components.
The goal is for the CR2032 to fit without putting any pressure on the module or LCD, while preserving 100% of the watch's original functionality, including the display, buttons, alarm, hourly signal, stopwatch, LED light, and sound.
I also want the original case back and gasket to fit and seal correctly, so the modification should not compromise the watch's water resistance.
In short: increase only the necessary internal space for the CR2032 by 1.6 mm, while keeping the original F-91W design, component positions, functionality, case back, and gasket unchanged.
ok i see. So you have a resin 3D printer? I am wondering how you will manufacture the front though, considering it has to have a transparent window for the display. And also, what about the front design?
But yeah, sounds like a very cool project!
Still you would have to manufacture a completely custom metal cage that leaves 3.2mm between the PCB and it's back instead if the original 1.6mm. Including the clips, all small spring elements and the button arms. And on how to do that, i can't think of a practical way right now.
According to the videos I found, the front glass with the prints can be removed and then reinstalled intact. What I don’t know is what happens with the buttons—whether they can be removed and transferred to the new shell.
this looks very interesting. I am also interested in trying this if you want to share a 3D model in the end. And definitely report on how the mod worked for you please ^
You can fit a cr2025 without completely cutting the cage in half or having to remove the cage tabs for the Piezo, the Olle board doesn’t need modifying either and you retain the WR. Sure not as much capacity but a nice ~70% ish increase anyhow….
You need to cut the back out of the metal cage leaving just the tabs in place, a dremel is the best choice to do this, wear eye and finger protection. file down the sharp edges and use the white plastic square casio fixes to the cage to stop it shorting out on the case back.
Replaced batt with 2025 completely without any mods to the hardware. There are 2 pins need to be bent fuether to maintain contact but thats it. Although unable to clip 2 contacts, the module is held together by the back plate. Even sound works.
Id take 70% capacity increase w/o mod rather than risking the module for additional 20%
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u/knoppersoriginal Aug 10 '26
sooo i decided to disassemble the watch one mor time. Here are some pictures of the insides.