Review on Engine Block Heater Heat Fluids by Richard Alemayehu

They stick well to clean surfaces. It's easy to get a thin layer of oil and dirt in engine compartments and even new batteries causing problems with most adhesives so they need to be kept clean. In case of doubt, it can make sense to secure the heating pad mechanically. They're simple devices and the dozen or so I've used all work pretty well. Note that they do not have a built in thermostat and can get very hot if not connected to a battery or other 'heating element'. Works the same as the more expensive brands we've tried (some of them don't even have glue)