Undercharged Batteries explode after recharging

even if undercharged batteries seem ok and balance up again after being pulled down too low... dont

What happened:

My neighbour borrowed one of my 'in progress' bikes while i was away, before it was finished. I got a msg saying the bike is dead and wont ride anymore, i asked what voltage the CA says and he said 40v... it was a 96v pack They were sitting around 2v and i tried to gently balance the packs up over a few days, and half of them held charge and seemed ok, they even did 3-5 cycles no problems... until this this is definitely not the batteries fault, but a great lesson as to why not to try 'recover' undercharged and abused cells