Creating a Battery Pack Using Capacitive Discharge Welding

In my previous project video, I showed you how to create a battery pack out of 18650 lithium-ion batteries using a different method. However, this time around, I wanted to try creating a bigger battery pack and was in dire need of a battery spot welder. Instead of buying one, I decided to attempt a DIY project to create my own battery spot welder.

When it comes to spot welders, there are two common methods: using a big transformer in combination with power electronics or creating a CD welder (capacitive discharge welder). The first method uses a big transformer to generate small energy pulses, while the second method utilizes capacitors as the main power source. In this project, I decided to use capacitors and create a capacitor bank.

The capacitor bank consisted of six super-capacitors connected in series, with Kapton tape used to create a poor man's capacitor bank. The capacitors were charged up to 15 volts, resulting in an energy storage capacity of around 1875 joules – more than enough for spot-welding purposes. To control the capacitor bank, I used IRL 2505 N-channel MOSFETs, which can handle up to 104 amps. One MOSFET was used for charging, and three were connected in parallel to discharge during welding.

For the control electronics, I chose an Arduino Nano as the brain of the operation, along with a 128x64 OLED display to show the current pulse time and capacitor charge status. A rotary encoder was used to adjust the pulse time, while a pedal switch was added for sewing purposes – literally! The pedal switch was connected to the circuit through an audio jack.

To create the PCB (Printed Circuit Board) layout, I used the free online circuit design software called EasyEDA. This allowed me to connect all components to the correct Arduino pins and package with included PCB footprint. Instead of connecting the components to a perfboard, I clicked the "convert to PCB" button and arranged the PCB footprints. Once happy with the layout, I added copper traces for the power electronics and created a solid copper region for the ground potential.

After exporting the Gerber files, I ordered five PCBs from JLC PCB, which arrived in just one week. Although the PCBs looked good on paper, there was a problem – I forgot to correct the footprint for the DC jack. Enlarging the holes worked out eventually. Now that the hardware was complete, it was time to program the Arduino.

Programming the microcontroller involved using an external interrupt for the rotary encoder, pin change interrupts for the push button and pedal switch, and a simple delay function to create the welding pulse. After uploading the code, I connected the capacitor bank and DC voltage of 15 volts to the circuits – it was time for testing.

The rotary encoder control worked beautifully, charging the capacitors and creating precise pulses according to the set value. However, the capacitor bank was always connected to the welding terminals, regardless of whether there was an impulse or not. This resulted in a missing reference potential from the discharge MOSFETs – a problem that I couldn't add as an afterthought because it would have shortened out the main power supply.

Unfortunately, this project turned out to be a big bummer for me. Despite the success of creating a capacitor bank and control electronics, the missing reference potential proved to be too much to overcome. Welding projects seemed to be my kryptonite. However, I hope that by sharing my failed attempt, others can learn from my mistakes and create their own functioning CD welder.

For those interested in fixing this problem or seeing a working CD welder, simply check the video description. In the meantime, thank you for watching, and don't forget to like, share, and subscribe if you enjoyed watching my failed attempt.

WEBVTTKind: captionsLanguage: enIn a previous project video I showed you how to create a battery pack out of18650 lithium ion batteries.Back, then I utilized a rather riskier and not recommended methods of connecting the battery cells together.By soldering the nickel strips directly to them.This can be dangerous because you heat up the battery quite a bit which according to its data sheets can lead to problems.So even back then I tried an immediately failed at doing some DIY spot welding.With the two salvaged microwave transformers which I had lying around from another failed wedding video.But since I want to create a bigger battery pack in the near futureI'm in dire need of a battery spot welder and truth be told I almost bought a cheap one for around 200 euros.Which seems to be capable of doing the job without a problem.But before buying I wanted to try the DIY route and see whether I could create my own battery spot welder.Which is the subject of this DIY or by episode.And spoilers I will fail terribly.Let's get started!When it comes to spot weldersThen we can either use a big transformer in combination with some power electronicsTo create small energy pulses with the length of a couple of millisecondsOr we can create a CD welder aka a capacitive discharge welderThis method still uses power electronics to create millisecond pulses of energyBut the main power source are this time capacitors.As you might know if capacitors get connected to a voltage source then they get charged upand can also hold this charge for a whileand if you short the capacitor leads with a nickel stripthen we get an energy burst which in this case is not enough for welding thoughSince the 220 microfarad capacitor only held an energy of zero point zero two four seven five joules.But if we instead utilized super capacitors with a capacity of whopping 100 faradsconnect six of them in seriesUtilize Kapton tape to create a poor man's capacitor bank and charge the pack up to 15 voltsThen we successfully stored around1875 joules of energyWhich should be plenty for spot-welding.That means it was time to create the control circuitry around the capacitor bankAs electrical switches I went with IRL 2505 N channel MOSFETs,which can handle up to 104 ampsThus I used one for the charging process of the capacitors and three in parallel to discharge the capacitors during welding.Additionally I utilized four TCO four 4420 MOSFET driver ICsWhich already ends the power electronics part for this project.For he control electronics I went with an Arduino Nano as the brains of the organization,a 128 by 64 OLED display so that we can see what pulse time is currently used and whether the capacitors get charged upa rotary encoder to adjust the port's time and initialize the charging of the capacitorsand finally a pedal for sewing, to which I soldered one channel and the ground cable of an audio cableSo that I can connect it to the circuits through an audio jack in order to activate the discharge pulse.To those main components I then added some complimentary onesand started turning this theoretical composition of components into a proper schematicthrough the free on-line circuit design software called Easy EDANot only did I make sure to connect all the components to the correct Arduino pin and counterpartBut also make sure to utilize the correct package with included PCB footprintBecause once the schematic was complete. I did not feel like connecting this big number of components to one another on a piece of perfboardInstead I click the convert to PCB button and started arranging all the PCB footprints of the componentsOnce I was happy with the layouts I then started tracking the copper traces on the top and bottom layerat the end I placed thicker copper traces for the power electronics andcreated a solid copper region for the ground potentialand with that being done, I exported the Gerber files and ordered five of the PCBs through JLC PCB.Which was not only affordable, but also only took one week for them to arrive.Now even though the PCBs look like they would work without a problem, there still was a problem.Which was that I did not use to correct footprint for the DC jack.So I had to enlarge the holes, which did work out eventually.That means it was time to first all the SMD capacitors to the PCBand afterwards all the through-hole components which all and all took roughly around 30 minutes.And if you're wondering for what the big 10 ohm power resistors is usedThen let me tell you that it is used to slowly charge the capacitors while providing feedback voltage to the Arduino.So that the microcontroller knows when to stop charging.Only problem was that the feedback voltage can easily exceed 5 volts,which the 5 volt Arduino certainly does not like.So as an afterthought I interrupted the feedback copper traceand added a 2 kilo ohm/1 kilo ohm voltage divider to lower the voltage.And with that being done the hardware was complete, and it was time to program the microcontrollerNow I will not go into too much detail on how I created the .Since I already talked about interfacing each one of the utilized components in previous videos.Let me just say that I used an external interrupt for the rotary encoder,pin change interrupts for the push button and the paddle button and a super simple delay function to create the welding pulse.So after uploading the code, connecting the capacitor bank and a DC voltage of 15 volts to the circuitsIt was time for testing and as you can see the rotary encoder control works.Charging the capacitors is possibleand even the pedal switch created a precise pulse according to what we have set with the rotary encoderBeautiful, right?Well here begins the depressing part, since the capacitor bank was always connected to the welding terminalsno matter whether there was an impulse or notThe problem was, which I found out after too many hours of troubleshootingthe missing reference potential from the discharge MOSFETsWhich I could not add as an afterthought because that would have shortened out the main power supply.So all in all this project was a big bummer for me.But it shows beautifully that buying a product can often saves you a lot of nerves.Now for me going back to the schematic and fixing this problem is not worth it.Since welding projects seemed to be my kryptonite.But if you want my project information to fix it or see a functioning CD welderthen simply have a look in the video description.As always, thanks for watching.If you enjoyed watching my failed attempt, then don't forget to like, share and subscribeStay creative, and I will see you next time! With a functioning project...hopefully.