This morning my 8 month old M pass didn't work at the first 3 booths. The guy had to put it against the window on the booth and suggested the battery is dead. However, it worked at the last booth i entered.  So are the batteries changeable, or do i have to take it back and get a replacement?

It's a sealed unit as far as I know. Take it back for a replacement.

 

That said, ours (EasyPass) is still going strong after around 10 years.

 

Thanks. I will see if it works on the way back tomorrow before getting a replacement.