Copo

Hi all. Desperately trying to find a forum where I can get some help with an electric quad issue. Hoping this is the place. I have issues with the power on my kids 36v quad. It's been in the garage for a while and when I brought it out, it wouldn't work. Did a bit of electric testing a realised the barrel ignition needed replacing due to a connection issue. Apart from that the 3 batteries appear to be pushing out 36v, the motor turns under no load, the throttle seems to work, lights all working, forward and reverse working and the two speed setting ok. The issue is when there is any load on the bike, it doesn't move. I've checked the power cable going to the motor and it's got 36v. I have no idea if it could still be the batteries or the motor. Can anyone suggest some diagnostic I might be able to do, to find out which post is at fault?