Casio FXCG50 Rechargable Battery Mod

Features:

• Allowable for use in exams conditions!*
• Uses built-in USB port for charging. No visible modifications seen on the outside
• Minimal soldering
• Reversable mod. Can easily be removed without permenant damage (if you choose to reroute wires and not drill a small hole. This hole is not seen from the outside)
• Discreet mod. No changes can be seen on the outside.
• Ultra low quiescent current
• If ordering from with the HASL finish from PCBWay, the total project cost is about $25-30.

* Officially allowed by most UK a-level and gcse exam boards, AQA, EDEXCEL, OCR and most others. I personally contacted these organisations through my insitution. Unchecked by university boards. Make sure to inform your professors/teachers about this. Use at your own risk, you may risk exam disqualification, although that is unlikely. This project does not use a microcontroller, does not interface digitally with the calculator, and has no EEPROM/Flash (and similar) memory types. All chips and components are designed for battery management only. 

Disclaimers: 

- Handle Lithium batteries with caution. Risk of fires and damage to calculator. 
- Before use, test the circuit externally. Charge the battery with an external power supply through the two input pads. Check battery voltage with a multimeter. Allow the battery to discharge through a 500 ohm resistor over the span of a few days. When the battery is dead, measure the battery again and check for overdischarge. Charge the battery again, and install in to the calculator. The output should be between 5 and 6v.
- The calculator will no longer report a % remaining as the output voltage should remain constant, and not drop like with direct battery use. 
- Carry spare 1.5v batteries for emergancies. Recharge before examinations.

Project Overview

The soldering pads are large and labelled clearly. Messing up soldering the wires should be impossible.

I suggest checking out the video about the project, but to summarise, these calculators go through batteries relatively quickly. Typically, it takes 6 months or so for 4 batteries to die, but it takes about 2 months for my use. The cost of constantly replacing these batteries adds up over time, and i thought a project like this could potentially save money. If not, it would still be a cool project, without the hastle of constantly buying new AAA cells.

This mod makes no external changes, and uses the on-board USB port for charging. The only intrusive change needed is to solder two wires to the calculator pcb. The power gets injected back in to the calcultor through the battery compartment springs.

Attach the mod board to the input USB by soldering wires to these points below. Solder the other ends to the “IN+” and “IN-” pads on the right side of the board. 

Adding a small dot of glue on the pads really helps preventing the wire from getting ripped out.

Bill of Materials:

Labels are contained in the attached BOM Zip file:

4.7uF 0603 Capacitor: 1x
100nf 0402 Capacitor: 4x
10uF 0603 Capacitor: 1x
4148 SOD323 Diode: 2x
4.7uH Inductor 3.0x3.0x1.5 (height of 1.5 or less): 1x
FS8205A: 1x
1M 0402 resistor: 1x
510k 0402 resistor: 1x
30k 0402 resistor: 1x
1k 0201 resistor: 3x
2k 0201 resistor: 1x
100 ohm 0402 resistor: 1x
MCP1640CT: 1x
TP4056: 1x
0402 led: 2x
DW01A: 1x
Small spring: 1x (make sure it is made for soldering. Regular springs do not stick)
803040 lithium battery: 1x

Assembly tips:

• Use a hotplate or reflow oven. It is not possible to solder the top side of the pcb with a soldering iron. Only use a sondering iron for the spring and optional 1206 PTC fuse.
• use a stencil. These are $10 from PCBWay for a non-framework, and $15 for a framework one. The framework is not important for DIY PCB assembly. That is mostly for medium to large scale productions.
• use slighltly less solder paste than youd expect for the boost converter. That way the chances of shorts is greatly reduced.
• Use plenty of flux. make sure it doesnt turn rock hard, and is actually easy to clean!
• DO EXTENSIVE TESTING BEFORE DOING THE FINAL INSTALL IN TO THE CALCULATOR

Sponsor:

This project is once again sponsored by long time sponsor, PCBWay. This is the first time I have used their panelization service for the project. I feel that heir V-cuts are very underutilised by makers, and underrated as a whole. 

With regards to service, it was outstanding, like always. I submitted a file with just a single PCB and a few lines on the document layer. PCBWay's engineers handled panelization by themselves. 

I initially submitted revision R1.3E for manufacuring, but shortly noticed that I had not connected the enable pin of the boost converter to Bat+. PCBWay allowed me to change the file through a quick email. (You can only change the file before manufacturing has started!)

I have always found the quality of their PCBs to be of really high quality, even with their cheap $5 prototyping service. Their soldermask and silkscreen placement is crisp and sharp. Letters at 0.8mm can easily be distinguished. One thing I appreciate about PCBWay is the fact that their PCBs either have no scratches at all, or barely visible imperfections. Less established services often have patchy silkscreen, damaged or warped boards, and large scratches from mishandling. PCBWay is a company that I trust with delivering reliable boards.

Also, I suggest you try out PCBWay's V-score panelization, especially for (relatively) large batches, and especially for small boards. 

If you would like this board but arent able to assemble it yourself, PCBWay also offer an assembly service. I have used it before without a sponsorship, and it came out perfectly. I cannot overstate the quality you get from ordering with PCBWay. I have attached the pick and place files to this project page.

Upload the file titled “…GERBER” to PCBWay to proceed. This is the manufacturing file. If you want PCBWay to assemble these boards for you too, upload the pick-and-place and BOM files too. Default settigns can be left as-is, but I suggest changing the soldermask color and possibly the surface finish (the default is HASL with lead, but I used the golden “ENIG” finish)

Set the battery settings as alkaline, although it doesnt matter much in the end. the battery setting determines cutoff voltage, but when power goes from 5.5v to 0v in the blink of an eye, it doesnt reslly matter if the cutoff is at 1 or 0.8v (example)