The following blog post is about our progression of our Li-ion cell protection board. As I’m sure you know, Li-Ion cells need to be handled with care and caution so as to avoid thermal destruction and hot carnage. For this reason, a protection IC is essential in order to cut the power in and out when the cells are in their safe operating range 3.0v – 4.2v. To add to the complication, we needed to satisfy a few extra criteria; these included: over discharge protection, 5v output, 3.3v output and over-current protection in the form of a fuse. I must admit that, during our initial testing, we were forced to use the cells without protection however this was only for a few short minutes. After evaluating our requirements, I decided that making our own circuit board would be a good learning experience and would provide a reliable, effective protection board for the batteries. With this in mind, I checked prices of a well-known PCB manufacturing services and began drawing up a schematic.
This one was drawn up using Easy-EDA which I chose because it was easy to progress to a fully fledged PCB design and, since it was my first ever PCB, I wanted to have the best possible software. After just a few hours of track adjustment and routing, this was the layout I came up with.
Finally happy with my design, I clicked on the convert to Gerber button and was given the opportunity to upload my PCB to JLC PCB for manufacture; where, after yet another last check, I spent an entire £2 on 5 copies of the circuit board. While this was, I think, a promotional offer for first time customers, I was still blown away by the incredibly low price. Next, I visited LCSC to get the required components and ordered them too. Fast forward a week or two, I was greeted with this.
Five perfect copies of my board, the required components and even a free keyring - which meant the next job was, of course, assembling the parts, which, would be easier said than done considering the size of some components. Despite their small size, I began soldering all the required components; the first one of which was this tiny SMD capacitor.
This seemingly mundane capacitor represented a huge turning point in my electronics experience as this was literally the very first SMD component I ever soldered – I would say not too bad for a first attempt. This continued for a little while as I slowly made my way through the other components – only trashing around 3 microchips in the process. The final result was very impressive; at least I thought that it looked so. Here it is in all its glory.
As you may notice, in the above picture, the voltage regulator is housed in its own 3D printed clip – not on the circuit board as you may have expected. This is because the original regulator I ordered did not function as expected so it had to be replaced with a different one which, unhelpfully, had a different pinout. Having blown up a Pi With too many volts due to crossing over metal pins, I just added some wire to extend the regulator off the board. After getting fed up of the board just flapping around in the breeze, I eventually designed and printed a holder for it that incorporated a special clip for the regulator. If you have another look at the image you will notice the blue relays; these are controlled by a DW01 protection IC that will cut the power if any one of the two batteries goes below a safe level. The control level for these is in series with a manual switch which allows the relays to be switched off easily and thus the circuit can be de-powered. Incidently, this is not done by the large black switch in the middle; this controls the motor power – it provides a quick and easy cut off for the robot without resetting the processor.
In conclusion, while I am, overall, extremely impressed with the board, there are still a few things I would change if I was to remake the circuit. Number one would be to use MOSFETs as oppose to relays since I shied away from them this time as I’m not that familiar with them. Number 2 would defiantly be the battery connection method: proper screw terminals would have been a far better choice as oppose to these puny little solder pads. Once again, thanks for reading this post, catch you next time!
