There will be 2 heavy wires on the top.
The one from the battery will have 1 or 2 smaller wires connected to it.
One will go through a fuse then on to the ignition switch to power the rest of the mower.
If there is a second power wire it will go into the engine and be connected to the rectifier to recharge the battery when the engine is running.
the single small terminal will have the blue wire attached to it.
IT should go + 12V when you turn the switch to the start position.