Battery causing MP3 to have no power and make odd noise. What could it be?

[Capedbandit]

So my son recently assembled/fixed up a Local Motors Verrado Drift Trike that uses a GM MP3 in the front. He had a 14S4P battery built with Samsung 30Q cells. Things were going great for about 2 weeks until one afternoon whilst drifting, the motor cut out. Mine does this too, but all I have to do is turn the key off and on and it will be fine (I believe this is a known issue, though no known cause). His, not so much. The power lights on the throttle light up and we checked it and it shows 50.9V. But, when giving it full throttle, it goes very slow and makes an odd clunking/grinding noise. I took a video - https://youtu.be/9Mfk61qbA38. If little throttle is given (1%-75%) it still goes slow, but no clunking.

We swapped batteries and his motor ran fine and mine clunked away. So I know it is the battery...but what is it about the battery? Does this point to the BMS? Bad cell(s)? I'm not sure where to begin? I've spoken to the guy that assembled the battery, but he's basically asking the same questions and will just have to tear it down and check individual cells and BMS function. If someone has seen this before, perhaps we can reduce the troubleshooting time and help someone else out in the future. Thanks all! 

[Bikemad]

It looks like that battery pack is almost completely discharged, a "no load" battery voltage of 50.9V is very low for a 14S pack (that's an average of 3.64V per cell), and this voltage would be a lot lower under load. 



Charge it fully and it will hopefully cure the problem, when fully charged it should be ~58.8V (4.2V per cell).

If the parallel cell groups are slightly imbalanced, it may simply be that under load, the voltage of the lowest group of parallelled cells is low enough to cause the BMS to cut the power and cause the lumpy running.

Alan