Hack a day has a fascinating system for exchanging a robot's battery, to extend its useful life. The main advantage of this is that the robot has basically no downtime, remaining on duty almost continuously. Useful, when shutting down your robot would start costing you money, as in some factories.
The mobile robot probably has a battery for a power source. Batteries are portable chemical cells that can provide energy with no cords or cables, which a robot could easily trip over. The main limitation of batteries has been that they only have so much power. Some batteries are rechargeable, and can be provided with an energy source to charge them back up again. (When provided electricity, the chemical reaction that powered them reverses, setting it back to its original state.) Non-rechargeable batteries must be thrown away when they are out of power.
This station removes the old battery with an extractor arm, and replaces it with a new one. The entire process takes 30 seconds. If the robot has a capacitor bank, it doesn't even shut down. The robot is also held in place during the process to ensure that it doesn't roll away or tip over, which would confuse its systems.
I remember a similar system described for electric cars, in which the car would drive to a station, the owner would pay with credit card, and robotic systems in the station would extract the old battery, put in a new one, and go recharge the old battery to sell it again. The batteries would, of course, have to be rechargeable and identical in shape, but car batteries already are.
So if you have a system like this with rechargeable batteries, you have the continuousness of a wall-socket, and the mobility of a battery, at the same time. A major plus for any robotics system.