Battery charge before spot welding?

Batteries, Chargers, and Battery Management Systems.
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Themachine   10 µW

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Battery charge before spot welding?

Post by Themachine » Jun 17 2018 5:30am

I have a small collection of 18650 cells from old laptop batteries that I'm now testing & "refreshing" with my bt-c3100 charger. I will use them to build 2 10s4p e-bike batteries. When the "refresh" cycles are done, the batteries are fully charged. Is it perfectly fine, or rather a bad idea to start spot welding on fully charged cells? Should I discharge each one first or doesn't this matter?
(If it does: can the bt-c3100 charger discharge to a specific voltage? I know you can change the max charge voltage, but discharge, I don't have a clue...)

I built my previous battery with new cells which had a factory charge of 3,6V, which worked great.
The old batteries have little "spikes" on the ends where I pulled off the old nickel strips, so I think I'll just sand these away and start spot welding. I am now charging all the old laptop cells in small groups. I'm not sure if I'm going to be just fine or if the thing is more likely to explode in my face if start the welding process now :? Does anyone have any info on this? Thanks!

jonyjoe303   1 W

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Re: Battery charge before spot welding?

Post by jonyjoe303 » Jun 17 2018 12:17pm

you can do it either way but just make sure all batteries are at the same voltage. I spot welded them fully charged and also when they were at a lower voltage.
Just make sure you dont short them out as you putting them together I always cover the cells I'm not working on with tape in case I drop something metallic on them. This applies whether they are charged fully or not, even a less then full battery will give you sparks.

The btc3100 will discharge to a set voltage that you can't adjust. If your looking for a specific voltage like 3.85 volts you need to monitor battery and pull it off at the right time.

999zip999   100 GW

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Re: Battery charge before spot welding?

Post by 999zip999 » Jun 17 2018 3:19pm

If using a BMS to balance the pack before hooking up the tiny little BMS it makes it work too hard and take too long.

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Themachine   10 µW

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Re: Battery charge before spot welding?

Post by Themachine » Jun 18 2018 4:45am

thanks for the tips. So I will just charge all the cells and assemble/weld the battery with caution not to short anything.
PS: The BT-C3100 does take its time when refreshing the batteries, but if the readings are correct I believe it does "stretch" the cells a bit to their optimum capacity.
Another BT-C3100 thing I found (on the internet, not by myself) was that there is an internat switch to charge up to 3,7V if you'd like. This would be interesting for me if I could DIScharge to that point. Now I'd have to discharge all cells and then charge them all again to that point. That would take days and days extra...

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