Am I Stupid? Why Can't I Get The Battery Back In

When pulling the battery out you have probably pulled the negative lead up from its original routing. Take the battery out again and see whether the earth lead will go down and run close to the battery so that the angle of the connector just fits over the top of the battery.
Best of luck!

 

Wot he said ^^^^ easy done.

 

I am sure will sort it.....but why take the batt off to charge it?

 

Looking at the negative clamp in the video it appears it has been bent, personally I would just bend it back to fit, and re attaché it, don't make it any harder for yourself than that.