Spot Welding Process

Owned by Eric Zhao
Last updated: Jan 10, 2023 by Eric Zhao
5 min read

Introduction

To connect individual cells together, we need a method to form a solid electrical connection while minimizing the damage to the cell's inner structure (regular soldering requires applying heat for too long of a duration,).

The most common method is to weld conductive (nickel) strips across them using resistance spot welders.

The working principle of spot welding is to push electrical current through the metal strip and battery tab. As the metals have an electrical resistance, this current causes the material to melt together. The molten metal should only occupy a small area to prevent damage to the battery, which is accomplished by using a high current, allowing for very short pulses.

Our current spot welder is the kWeld, operation manual here:

https://www.keenlab.de/wp-content/uploads/2018/07/kWeld-operation-manual-r3.0.pdf

Operation Steps

  1. Read through operation manual.

  2. Check charge of lithium-polymer battery, charge using iCharger 306B BALANCE charge, if necessary (voltage < 10.8V).

    1. Battery: https://hobbyking.com/en_us/turnigy-graphene-6000mah-3s-75c-lipo-pack-w-xt90.html?wrh_pdp=2&___store=en_us

    2. Connect leads of battery to iCharger with alligator clips, and connect balance port to the top middle port (should be the only port that it fits into).

    3. Change settings to LiPo BALANCE CHG, 3-6A (0.5-1C, lower current ideal if you have the time to wait), 11.1V(3S)

      1. To start the charge, long press the start button

      2. If you are powering the iCharger with one of the old power supplies, the power supply output will be limited to 1A. For fastest charging, adjust the current limit on the iCharger so that the POWER draw (Voltage x Current) from the power supply is maximized (may be a lower current than 3A)

    4. Make sure light pressure is applied on the battery during use (should be squished between two wooden blocks right now). If unsure about how much pressure to apply, ask Micah (or Eric eventually lol) or read a bunch of forums on LiPo pouch cell battery packs.

  3. Connect and keep fan blowing across battery, cables, and electrodes at all times during welding.

  4. Connect various components to welding system if disassembled (manual foot pedal, electrode probes)

  5. Connect battery to welding system.

  6. Calibrate system (if it hasn't been used in a long time, or if the electrodes are changed)

    1. Follow operation manual, make sure to apply strong pressure to the electrodes and keep them together during the short test)

  7. Select manual mode (use foot switch to trigger welding pulses)

    1. Do not press down the foot switch when you connect the welder to the power supply

    2. Or select manual mode from the 'Mode' menu item (turn dial knob, and press and release knob to toggle between manual and automatic trigger modes)

    3. NOTE: Automatic mode, on the other hand, will automatically perform a weld as soon as it detects a connection between the two electrodes (after waiting a set amount of time designated in the settings). We usually don’t want to use this since it might automatically trigger pulses when we don’t want to (still adjusting electrode position for example), and we have to be more careful with where we put our electrodes while in automatic mode.

      1. The system should beep every second while in automatic mode (but from my last check the sound was either not there or really quiet, so be careful and double check that you’re on manual mode whenever you’re unsure).

      2. When in automatic mode, a rising tone will sound when electrodes are shorted, indicating a pulse is about to be set. Take away electrodes during this time to stop the process.

  8. Wait for weld energy screen to appear, and adjust energy with dial knob

    1. A good weld energy will likely be different between the positive and negative ends of the cell as well, start at a lower weld energy than expected, and increase up until good welds are being formed.

    2. Basic starting points welding to 18650 type cells (from manual)

      1. 0.1mm pure nickel strip: 20J

        1. From previous experience, the MSXII modules used 0.1mm pure nickel strips and welded them all at 15J, which may be a better starting point (the lowest energy required to form a solid weld is best)

      2. 0.2mm pure nickel strip: 50J

      3. 0.3mm pure nickel strip: 100J

      4. Higher energy will typically result in better welds (or less failed welds), but will heat up the spot welding system quicker leading to longer wait times between welds. A weld energy that is too high will also heat deeper into the battery, reaching some of the active material and vaporizing it, leaving behind residue that could decrease capacity or lead to internal shorts in the future.

  9. Clean the surfaces of the metal pieces (isopropyl alcohol and wipes)

  10. Firmly push both electrodes down to the weld material at a slight angle. Make sure that the electrodes do not touch each other, or you will likely weld them together.

    1. A good distance between the probes would be just under 1cm, the closer the better (not touching). The actual distance is not too important, just focus on getting them roughly close (not touching).

    2. Make sure not to spot weld in the center of the negative side can, only on the edges.

    3. Make sure that the two metal pieces are in contact (push hard or adjust pieces until there’s no air gap)

  11. Trigger a pulse by stepping on the foot switch, and keep the foot down to listen to the sound from the spot welder.
    a. One short beep signals success.
    b. A higher tone, followed by a lower tone, signals something went wrong

  12. Check for relevant information displayed on LCD if necessary (keep foot down)

    1. Some possibly relevant info:

      1. Calculated average resistance of weld spot (mOhms)

      2. Power switch temperature (degrees Celsius)

      3. Weld current (Amps). A low weld current (<1200A) means the battery is starting to get low. Consider stopping soon to charge it.

      4. Weld time (ms). A low welding current means more time is required to put the same energy into the weld. High welding time is also a sign that the battery needs to be charged.

    2. If there was an error message, check operation manual for explanations

  13. Release foot from switch, check for weld quality. If the weld wasn't great, you can repeat the welding process one more time with the probes over the first weld spots.

    1. If performing a test weld, check connection by tugging on the tabs. If they come apart without any metal ripping, there probably wasn't a solid connection. This could be due to contamination on the metal surfaces, too low of a weld energy, not enough pressure, or (least likely) the probes being too far apart (more resistance).

    2. Indicators of a bad weld

      1. Lots of sparks

      2. High weld resistance (good weld should see something around 1.21mOhm)

      3. What the weld looks like - watch the weld in progress as well - does it get red-hot, are there heating marks on the nickel when the weld is done?

      4. I watch the weld current as well. Its typically around 1400A with a fully charge battery and when it gets lower than 1250A or so charge the battery again, even if the voltage alarm hasn't triggered yet. Higher current = lower time, more concentrated heat, better weld.

  14. Repeat process for rest of welds. Monitor temperature of system and take breaks in between if system is getting too hot.

    1. Battery connectors and cables usually get hotter fastest due to the high currents passing through each weld (not the displayed temperature of the power switch), so monitor these by feel

    2. No specific temperature to check, mainly go by feel.

      1. Warm is ok.

      2. Too hot to hold for 3+ sec or so is too hot.

 

Spot Welding

kWeld - Next level battery spot welder
kWeld - Operation Manual