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Spot-welding Li-ion packs - charged or discharged? Answered

I'm making a large 18650 pack for my onewheel-like hoverboard build. It's my first attempt at spot-welding (DIY spot welder powered with a MOT, scary beast can melt entire M8 nuts on longest impulse time), so I'm afraid I'll mess it up. I was wondering... is it safer/more convenient to work on charged or discharged cells? Does it make any difference at all?

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Downunder35m

13 days ago

Get some cheap AA batteries and try what setting works best to make the connection without burning a hole.
Then go down until you struggle to get a lasting and good contact.
Take the middle of both values as a starting point.
Then you have bunrt some useless and dirt cheap AA batteries but not your expensive rechargeables ;)
Since you have both welding contact on the same pole (spot) of the battery it does not matter if they are full, empty or anywhere between.
Just never try to weld from one pole to the other or you will see a really bad and quickly burning fire that water won't get out.
If in doubt keep a powder fire extinguisher at hand ;)

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MichałK83Downunder35m

Reply 12 days ago

I've already done some practice welds on dead 18650s from a dead laptop battery, and I was able to adjust the timer for a much-better-than-chinese-factory quality joints. Terminals were nowhere near getting hot, so I should be safe. The ribbon was the first thing to rip when tugging on it with a considerable force, welds stayed attached. I decided to weld fully charged cells after all, for the sake of BMS and its poor balancer - better to start with 70 or more equally full cells than a random mess of unknown voltages. Wish me luck!

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MichałK83liquidhandwash

Reply 12 days ago

I was mostly wondering about their resistance to heat in different
charge states (just a precaution - the test welds I made on dead cells
were perfect and nowhere near getting the terminals hot), but I came up
with another compelling reason to packet them in fully charged state -
it will be much easier for the BMSs inbuilt ballancer to work with
equally charged cells than having a messed up mosaic of different
voltages.