For the record, you can't use the 12v dc outlet
to charge your batteries PRECISELY because it puts out 12v dc.Your lead-acid batteries, when fully charged, have a potential of about 12.8v dc, and as you discharge them, their voltage drops gradually, untill at about 10.8 volts they are fully discharged.In order to CHARGE them, you must apply a higher voltage in order to force current to flow towards the battery. In general terms, this is done at about 14 volts, which is higher than your generator will provide.If your batteries are at 50%, their voltage will be about 12.05 volts, which is higher than the nominal output of your generator. Luckily, the generator has a 'blocking diode' to prevent current from flowing FROM the batteries into the generator, which would otherwise be a messy and potentially destructive affair.

And just in case you are thinking you can just drain your batteries fully, then get SOME charging out of the 12v DC output of your generator, you face 2 problems: first, discharging your batteries all the way is NOT good for them, and second, your generator probably only provides 25 or 50 amps at 12 volts DC, and a fully discharged battery will want to draw much more than that, which will either blow a fuse in the generator, or overheat its wiring, once again with potentially destructive consequences. REAL battery chargers have circuitry that varies the voltage in order to avoid excessive current flow - your generator does not.Far better to go Stu's route.

Cheers,Bobs/y X SAIL R 8