I’m in the process of wiring a 10s2p lipo pack. I connected all of the balance leads and without thinking attempted to join the two 10s1p packs in parallel. In case you didn’t know, it makes a big spark!! And will also turn your fingers black! Any chance this killed a pack?
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If it was momentary contact it should probably be fine, the quick charging of the battery with lower voltage from the one with higher voltage might have caused some damage to that battery.
If there is a large voltage difference between two batteries and you connect them in parallel you effectively end up dumping charge from the higher voltage one into the lower voltage one, this is okay if there isn’t a big difference but in this case it must have been pretty substantial. Would check both batteries with a voltmeter or if you have it a balance lead tester (can also do that with a voltmeter but a bit more tedious trying to keep on the contacts).
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That must have been quite the startling experience haha 
I would imagine the pack is fine, just got a big spark from the high voltage, I would check each cell and see if they are still balanced.
This is how I destroyed my first bms…
Yeah it’s a pretty common problem when people are hooking up quadcopter batteries for parallel charging. I have a parallel charging board but never really used it somewhat out of fear of frying something (think I understand this all well enough to use it now but just haven’t done it yet).
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Those bullets look a little small… you should be using 5.5mm bullets…the spark will be like a nuclear reactor, but shouldn’t do to much… just gotta make sure the bullets you use can carry the ampacity of the inrush current. but really, wire in an antispark xt90 loop key
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It was certainly a startling experience! The tow batteries were about 4% apart in voltage. I was expecting a small pop when they equalized but damn! I am using xt90 connections for the final wiring. I was simply touching them to release this spark.
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And it has work 
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Haha it definitely worked!!
Next time bottle the spark
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was it just a big voltage difference with the two paralled packs or maybe two balance leads were connected and you completed a circuit with the leads connecting? that’s a big black spot
The balance leads for the two packs are connected in parallel.
yea I was thinking you could possibly put the balance leads in the wrong parallel order and not notice and then you’d be making a circuit with possible every cell in series when you put the mains in parallel.
here’s my guess about cells put in parallel and hope someone can correct me. when two cells are put in parallel the voltage of the lower cell will instantly rise, and the higher cell instantly fall, the current will follow the voltage and balance as quickly as it can with only the pack resistance to decrease the possible amp flow. The math would be the voltage difference between the packs divided by the resistance of both packs would give the current flow. What would determine how long it would happen?
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I’m 99.9% sure all the balance wires are correct and I have already soldered the main negative leads in parallel. Will certainly be connecting the positive leads with a bit of reservation!
I would suggest you double check the balance leads from 1 to 10 with a volt meter just to be sure. Other than that, use an ct90s to connect the parallel packs. That’s what I did anyways 
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I have this problem a lot. I end up soldiering balance lead and xt90 plugs all the time. The xt90 positive and negative connectors often melts when I attempted to connect my batteries in parallel. I end up resoldering it or have to disassemble the battery. That is why I am thinking of using a antispark xt90 plug on all my parallel cables
What can I do to prevent my balance lead from spark and melting.
Are the parallel packs at the same voltage?
I’ve never encountered issues hooking up parallel packs, always check that they are balanced. I combined 2 7S80P 18650 PowerWalls without a single spark. On a side note I have about 10Kwh of energy storage in a PowerWall made from salvaged 18650s
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Balance leads are paired by order as well as color.really no way to get that part wrong. They are however not connected to the BMS. I will be placing a xt90 anti spark going into the vesc.
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These packs are 5x 2s 5ah 30c each. I have just connected them in parallel and have not had any additional sparks.
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You are doing something wrong. This shouldn’t happen. Thats the spark of a short, not just slightly different voltage in parallel cells.