Well it's a bit puzzling actually. Think I need to look into it.
In the manual it says it's an AC 12V 40/60/80W power outlet that must only be used for lighting and not to connect to a battery.
The OB is fitted with a lighting coil plus what Yamaha call a regulator/rectifier so I'd have thought it would be 12V DC suitable for a battery.
Here is the diagram in the workshop manual. I've found a comment on the net to charge a battery you need to use the currently unconnected red wire plus an earth.