Help! Red light <10v on electrical panel

[MagicalArmchair]

We've just set off on our summer cruise. First trip an overnight passage, nav lights on, we chug to the end of the Medway. Two hours in, we put the sails up and turn the revs down on the engine. The instruments die and the red light on the main panel comes on. The fridges are dead (one a new 12 V cigarette lighter powered one). I put the sails down again and power the revs up again and the red light goes out.

The shunt tells me the batteries are full. I tried killing the nav lights with the revs low (thinking it's a short??) but it persists just less bright red.

Any thoughts? We are limping back to port. Not the start we hoped for!

[MagicalArmchair]

So we limped back to port, went through the lock in the pitch dark at midnight. On stopping the engine all circuits went dead. Strange, I thought.

Turns out the first mate had managed to bonk the electrics key out when moving the fridge!!

Key put back in, boat flipping perfect again. We left the lock five minutes later in the dark and are now headed for Belgium once again .

[Yngmar]

You found a key difference!