Is there a kind of thermal sensor that only changes resistance if it is above 50'C and that can be wired in series?
I want to wire 50 thermal sensors on 50 batteries so that i can see if any one of them gets too hot.
A re-settable thermal fuse, such as this, could probably do what you're looking for.
Wire them all in series and place one on each battery. If any battery gets above 50C the fuse will trip and the circuit will break past the fuse. These are usually available in many different temperatures to suit your needs.