How can I tell if the batteries are fully charged?

You can either use the trickle charge or fast charge capability with the Robop supplied charger. Using the normal or “trickle” charge requires approximately 8 hours to charge, so it is best left to charge overnight. The charger’s fast charge facility runs for 1 hour then cuts out. If you push the button again it may well charge for another hour and cut out again. Charging for a third or even fourth time might be necessary. The only way you can tell that the battery is fully charged is when the cutout occurs after less than 1 hour, say 20 minutes.