What is the reason for the discharge?
I built an extra battey for me and my son.
The battery is connected via XT60. The original battery is via the outdoor XT60.
So that when it discharges the battery on the board, it pulls out the connector and connects the XT60 additional batteries through it. Simple.
Son has now informed me after the winter break that the voltmeter on battery shows nothing.
Before the winter, I told him to keep it about 70% charged.
I opened it at home and each parallel group about 0.15 - 0.2V.
What the hell could he do about it?
Something in your system is imposing an idle drain on the batteries. What motor controller are you using?
Why does your bms say 12s 
Sorry, wrong picture. I made two battery packs, this is my for Zealot, 12S5P.
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Was the battery connected to the board and drained bothe the board battery AND the top box battery?
Or has just the top box battery drained?
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Son used the board over the winter only on a stock battery. He claims that he had an additional battery stored and disconnected somewhere. Additional battery is KO.
Does the bms have a power switch? Was the fuel gauge left on?
BMS don’t have power switch. He claims the voltmeter indicator was off.
All i can suggest is that the bms itself is the culprit. Kinda disappointing given that it’s job is to protect the battery. That sucks dude.
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I’m with @glyphiks, from what you have described the only possible culprit is the BMS.
Those cells are now garbage, unfortunately. Lithium ion cells are not meant to be discharged below 2.5V. Below that voltage, the cells start to become damaged internally, and become extremely unsafe to charge/discharge.
Since we dont how long the cells have been sitting below 2.5V, we cant say for sure how damaged the cells are. All we can say is that damage is likely, and those cells are no longer safe to charge, use, discharge, etc.
Some people might tell you that it’s ok to “recover” over-discharged cells by charging them up very slowly. I disagree. I feel that it’s incredibly dangerous, and give the massive danger that’s inherent in Li-ion batteries even in the best condition, “recovering” damaged cells is a bad idea.
Take the battery pack to a responsible Li-Ion recycling center. There are lots of places that do this, including several big-box stores all across the country. I recommend going to this link to find what’s near your zip code.
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Sidenote, what boxes are those? They look nice sized for range extension pack.
You can try to revive the cells. Charge them verrrrry slowly to about 2.6v, and then see which p-groups drop off. Better yet, disassemble the pack and treat each cell individually. There’s a good chance that many cells are still ok, although they all lost a lot of cycles.
When trying to “revive” very, very badly over-discharged cells like these how would you recommend they check for possible internal copper dissolution and the random replating that can happen during charging? This replating can eventually causing internal short-circuits in some cells.
If they have been damaged enough to have lost a lot of their cycle life can we say that many are still “okay”?
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Depends on your definition of okay, doesn’t it? 
If they can still perform, I’d consider this “okay”.
You’d notice this in the change of IR I think. (Correct me if I’m wrong. ) I’d say if they don’t blow up, leak or get really hot while recharging, they are probably okay. That is, if they can hold the charge. It all depends on your willingness to accept risk though.
Some would argue that is a really important thing to mention to someone you tell that they can “revive” severely over-discharged cells.
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