Making sure the rover is not too hot or cold through thermostats and heat switches
Thermostats
During the day, the rover actually might need to release heat if any excess heat is generated by the rover electronics or electrical heaters. Just as a light bulb gives off heat or a car engine produces excess heat that can be felt through the hood, the rover can run the risk of working too hard and overheating.
The rover has a "thermal switches" that turn the heaters on and off, depending on the "open" and "closed" set-points of the thermostats, which are similar to common thermostats used in homes. The thermal switches would automatically activate or deactivate heaters to maintain electronics and battery temperatures above -40 degrees Celsius (-40º Fahrenheit), even during the martian night, while the rover is "asleep."
The batteries inside the rover body need individual attention because, like a beating heart, they ultimately keep the rover alive. The baseline plan is to pack the batteries with six RHUs, survival heaters, and two heat switches that each connect to separate radiators mounted on the walls of the rover body.
Heat switch
The heat switch is another autonomous device that moderates a heat transfer path in order to maintain a specific battery temperature. If the batteries start to cool below -20º Celsius (-4º Fahrenheit), the heaters will turn on. If the temperature of the batteries approaches 20º Celsius (68º Fahrenheit), the switch would increase the heat transfer path to the radiators so that the excess heater could be convected and radiated away to the martian environment. (Convection is heat transfer between sources that have different densities.) When the excess heat becomes necessary to maintain battery temperatures (that is, when daytime turns to nighttime), the switch decreases the heat transfer path to the radiators in order to retain heat. The radiators are similar to the heat fins on the back of a stereo amplifier, whereby a relatively larger surface is in contact with the environment to reject heat by convection (and heat radiation in the case of the Rover).
