Had two batteries costing $300-320 do that. Bad welds internal, is my guess. When drawing more than 50 watts, voltage would go to 7 and stay there on one, go to 11 and bounce back as soon as load was gone on the other. Got my money back for the first. Second I had to prove it wasn't the controller, so I took more than 31 days, warrenty expired. Don't use any watts over 40 would have made those batteries useful, but not for riding a bike. I cut the top off the 2nd battery, only one stack was even working at all. 6 stacks permanently not connected to the BMS board.
Battery costing $630, I'm still using it 4 1/2 years later. He is out of the 48 v battery business, so I can't recommend him. Wouldn't fit a voltbike anyway.
I load tested the 2nd battery with 15 ohm resistor rated 750 watts. And parallel DVM to read voltage when 3 amps was drawn.