Ok, first of all your Pleo MUST go carry on if you want him to arrive in one piece. Secondly you must have a battery with you, but wrap securely and seperately from the Pleo. I had Pleo in his sling and the battery wrapped in a couple of socks in my bag. You need to turn Pleo on for security and he needs to be on while going through the X-ray machine. (Just pop him in a tub by himself) He may or may not scream at the darkness and you may or may not get looks of amusement or bemusement form security 
I had a different experience but I think I travelled AFTER the new battery laws came into effect (March 2013 new battery laws, traveled in April) I contacted the airline before leaving to ask what I needed to do and they were very clear that any batteries must be kept inside the robots the whole flight, they must go in hand luggage and no spares were allowed. The total voltage (of all batteries in robots, mobile phone, ipod, camera) had to be calculated and could not go over a set limit. I had spare AAA batteries for my camera which were taken off me because they had to be inside the camera, loose spares were not allowed.
My robots had to go through the X-Ray whilst turned off, then I had to turn them on for a demonstration and finally they were swabbed up the bum for explosives

Each airline will probably be different so I think it best to contact them in advance if you are having any doubts.