Both battery compartments open electrically via a bar-mounted button, and the batteries simply slot in and out. No plugs, no cables, no latches.
If you’re using one battery, it always goes in the primary battery compartment. This is the silver rectangular section in the center of the bike which opens to the right-hand side. If you’re also using a second, optional battery, it lies horizontally in the top compartment or “fuel tank”. The batteries for these compartments are exactly the same, so you don’t need to be concerned with which one goes where.