Battery welding

Lately I've been occupied with welding e-bike batteries. I ran into a ton of problems trying to get the process right, so rather than my usual blow-by-blow diatribe of how I put something together, I've just outlined my learnings, and posted a few photos of my progress. Hopefully this is helpful to others having trouble.

I had to make a new battery pack for Jenna before I could start on my board tracker. Her bike had just come back from being at MOTAT, and her pack had significantly degraded.  One of the things I struggled with was finding good advice for troubleshooting my particular issues. 

Where I looked first:

Equipment and parts I used:

Before starting I wish I'd known:

  • The best way to dial in your welder parameters is to do a test weld with two bits of nickel, and inspect the underneath side of the weld. If you can't see obvious weld marks, you're not using enough current and/or pulses.
  • The welder pen on the Sunkko 709 series welders is only good for up to 0.15mm pure nickel strip (I purchased some and verified this).
  • You can do thicker pure nickel with the fixed welding head (I used 0.2mm pure nickel)
  • The welding pins need to be pressed against the nickel strip with enough pressure to remove any air gaps between the pins, strip, and battery. Failure to do this will result in big sparks and unreliable welds.
  • To help guarantee good contact when using the fixed welding head, always trigger the welder with the footpedal.
  • If in doubt, use wider, but thinner strip (e.g. 8x 0.15mm over 7x0.2mm)
  • Use a battery load testing jig to test your battery performance, before fitting it to your bike.


My welder 

Panasonic NCR18650 cells. These things can discharge a crazy amount of continuous current (10A), and hold tons of charge (3.45Ah)

Test fitting the cells on Jenna's bike, before I realised I had major welding problems.

My load testing jig: Some big 4ohm resistors in a bucket of water.  I measured the voltage across each cell group at the BMS while the battery was discharging. Any groups measuring significantly differently from the majority, would point to problem welding.

Weld testing, pin side

Weld testing, underside. This is an example of good quality welds. Notice the visible weld marks.

The welded battery pack for my board tracker. I'll be writing up a seperate post for the battery enclosure.

The board tracker battery pack with the BMS fitted, and shrink-wrapped. This thing is a 52V 24Ah battery, with a 60A BMS. It's scary powerful.


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