Rad Rover 6 step thru Won't start

[Ed7]

Semi Integrated Battery is 90% charged. When put into bike it will not start.  Spare battery powers on fine. There is no current measurement in the socket but there is none in the spare which does work. Also, there there doesn't appear to be any loose wiring to the socket.  Could there be an internal problem with bad cells, faulty internal wiring, or BMS system??
Any ideas on repair??