This is a round slab about an inch thick and six inches in diameter. Any rechargeable device that is set on the slab and left for a minute will be 100% charged, and the battery will be optimized for maximum lifespan. Nothing bad happens to your device if you leave it on the charger for longer than a minute.
For a particularly large device such as a vehicle, the Universal Charger can charge it as long as it’s touching the device in some way. In the case of a vehicle, setting the charger on the hood would work fine.