Temp controllers are devices that enable control of other devices above/below a given temperature. Most controllers used in vans will be 12v, but 120vac controllers also exist.

• add low-temperature cutoff to chargers that lack that feature by either
  • driving the device's control circuit (examples)
  • or more crudely cutting power getting to the charger in the first place by disconnecting solar, alternator, etc
• add precise temperature control to crockpots or other devices that lack it

Note: controlling larger currents directly (like alternator or solar panel input) will use the temp controller to control an external and larger relay.