Fish with electric organs
Some fish like the electric ray or torpedo ray have electric organs composed of electroplaques. The electric organ has vertically stacked cells that resemble a pile of coins. Each cell produces a voltage, a series of stacked cells can generate a much higher charge. The fish uses this peculiar organ to create a jolt of up to 200 volts to kill prey.
The torpedo ray inspired robot designers to develop the "Raybot". A robot that lives independently in the seas. It forages krill or zooplankton and eats algae and kelp. It transforms its food into energy and stores it in a series of batteries. Once all batteries are fully charged it swims back to the puller. There it clicks in the docking station to transfer the stored energy.
The robot is initially invented to explore the unknown depths of the sea. A task they still fulfill.