There are subtle changes between cars depending on age, model, country etc.
Remove both +ive and -ive leads from the battery and clean up both the terminals, and the connectors.
Then follow the negative battery lead down to where it attaches to the chassis. There it may stop, or carry on down to the gearbox. Make sure all connecting points are clean and fre of rust.
If this lead just stops at the chassis, (car body), then look for another strap that connects the engine/gearbox to thr chassis and make sure it is clean and intact.
When I first got my velocopede, the -ve battery connection was broken and just lodged on. That may be your problem.