what is and where do you get a battery sensor =P
A battery sensor is just a voltage divider, that is 2 resistors in series connected between GND and the battery voltage and the middle connected to the analog input pin of the microcontroller. Why is needed? Because the battery voltage is greater than 5V that is the maximum voltage an analog input pin can take. Also is better if you adjust the values of the resistors so the monitored voltage range is better spread over the input range (0-5V).
I didn't had a chance to work on the battery sensor yet. Lots of things got in the way so the work to the robot is postponed for now. You can see the code for IR remote controll on my blog, because it is pretty long and can't post it directly here unless I make a zip file.
Thanks for the interest and the comments!