If a spare lithium battery or a power bank is discovered in your checked luggage, the TSA or airline security will remove the item from the bag before it is loaded onto the aircraft. In many cases, you will find a notice inside your suitcase explaining that a prohibited item was confiscated for safety reasons. You will generally not be able to recover the battery unless the airport has a specific "hold and collect" policy, which is rare. The reason for this strict enforcement is that lithium batteries are prone to "thermal runaway"—a chemical chain reaction that causes them to overheat and catch fire. If a fire occurs in the cargo hold, it is much harder for the crew to detect and extinguish it than if it happened in the cabin. If the battery is built-in (like in a laptop or phone), it is usually permitted in checked baggage as long as the device is completely powered off, but spare batteries and power banks must always be carried in your carry-on luggage where they can be monitored by the flight crew.