A thundercloud is a big battery made of gas. From memory, the light, positive ions move to the top of the cloud, and the heavy negative ions move to the bottom. The plasma discharge of the gas is the lightbulb flashing on for a moment.
Put two tanks and the fuel cell in a single box. You can't say from the outside what is inside, it doesn't behave differently than a chemical battery, so I am ready to call it a battery.