Several years ago there were numerous reports here on TS of JB3s overheating. You can use search for more details but iirc generally folks recommended not having it sit on a battery or other device that produces heat, don't keep it in an enclosed space and don't leave it in direct sunlight. If in daylight cover the JB3 with a towel, t-shirt or some other white/light colored cloth. I never had a problem with any of mine overheating but I also took the faceplate and battery compartment cover off so there would be just a little more ventilation for the JB3 and I never ran it in a bag, always set it on top of the bag or some other surface (tabletop, chair, etc.).