Yes-ish it’s more about losing the voltage out of that series connection, drop from 10s to 9s and that also causes current spike so it all goes nuts You could have 1 series pack bottoming out in voltage and the others spiking in voltage, total voltage will look reasonable, but each series could be way apart
Are you certain that your batteries are genuine HG2’s? Those cells were extremely popular back in 2016 when LG couldn’t keep up with demand. As an obvious result, China started producing a lot of knock-offs that looked convincingly genuine and was sold to 3rd party wholesalers for a tidy profit. Have you tested them prior to your build? I would do that just as a safety counter-measure for every purchase, because even if you trust your source, they’re not perfect.
Oh, if you haven’t seen it, here’s a blog testing a batch to identify fakes.
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Wow those are very elaborate fakes
Voltage across all series is identical @ 40.32 V (I had went down the road and back since last charge). Even when I’m on the board, if I’m not drawing current then it is expected charge.
To me, this would lean more to possible a bad connection for 1 of 3 batteries in a series set. In this case it would still read 40.32V but could get massive SAG like I’m seeing… How I would test for this, I’m not sure…
Thanks for the site @Aeroquiv - It crossed my mind but i took the leap. Everything I can tell from the te site you sent me shy of doing discharge test is showing that they are legit.
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Yes total voltage could look normal, measure each series connection please.
Sorry - I didn’t type that right… I measured each series separately and it was 4.03V across all
All the series connections look good too?
Yea, I’ve disassembled and inspected. None loose… no burns. My last ditch effort might be to fire up the iron and just zap each one very quickly
If i were.u i would still check internal resistance. I think that should be good way to see where the cells stand… maybe some of them is messed up? What happens on cell level at end of discharge? Are all cell groups at same level or is it like lipo with different levels of state of charge?
Was that 4.03 after full charge? @okami with 18650 they should be within like 0.005 after full if they are same cells, same resistance
no there is about a mile on this charge. They get a good full charge and I use a BMS for charging.
@Okami : I suppose I could hook it up and discharge it down to see. Maybe it would show a problem child. I think I will try the internal resistance test as well. Maybe I can do that test on each series so I dont have to completely disassemble
Any chance you have a Foxnovo 4s or opus bt-c3100 charger? I would run charge/discharge cycle see if any heat up like crazy. I do think at this point it’s worth tearing it down, testing cells individually and rebuilding
I dont. I wish I had something to do this discharge cycle. Ill play around and see what I can come up with
Thanks for the tip - looks like something good to have on hand - I really appreciate you guys trying to help figure out my issues.
Every problem solved for 1 person, solves it for 100 more 
This is a unique one I would like to see solved
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I retouched every point on the battery with the solder gun in hopes it was something not visible. Similar results riding a few hundred yards.
I just ordered some of these so I can do some test to batteries other than voltage. I figure I only have to remove the outer connections of the pack vs completely taking it apart to isolate one cell.
https://www.amazon.com/gp/product/B0087ZDH7I/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
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I ended up getting this cheap battery tester which can also do internal resistance check.
https://www.amazon.com/gp/product/B0178P8H9U/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
After measuring the internal resistance for each cell… nothing looks too off… so i think…
I’m very close to buying a new battery from @barajabali but I will kick my ass up and down the street if I buy it and it still does crazy voltage drops (meaning that it was something else like VESC acting funny).
Anyone have any other ideas? -
On my agenda as of now… put the outter rim of the battery back on. Hook back up to wiring harness. Leave off the BMS since full charged… Test… Remove 100A fuse… Test… Switch BT HM10 to other VESC B… Test to see if readings are consistent with VESC A.
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Battery put back together - Same extreme sag. Fuse Removed - No Changes… Battery Meter Removed - No Changes. Motor B & Vesc B - By Itself (Was the Slave) - No Changes…
Are there any other options?
Just ordered a battery from @barajabali (Chi Boards) - If this battery comes in and solves the problem then we know there is some type of issue with the battery pack that Voltage and Internal Resistance checks couldn’t bring to light.
Mimicking a 30A-60A load @ 40V and checking the voltage might have brought some clarity to this.
Ive done some research on how to do this without going out and buying some expensive load tester.
…“Get a 1500 watt space heater”… which should give a decent comparison…
If this is a bad idea - Stop me 