The so-called Baghdad Batteries are clay jars that were found in modern day Iraq in the early 20th century. They seem to be date back to the Parthian Empire (200 BCE-200 CE).
The batteries are small terracotta jars with a copper cylinder (rolled sheet) which houses an iron bar inside. The copper and iron are separated by a plug. Since the copper cylinder is not air-tight, if the jar were to be filled with fluid, it would surround the iron bar as well. If the fluid were acidic (e.g. lemon juice or vinegar), then the mechanism would create an electric potential - like a modern day battery.
What was the purpose of the battery? Some think that it was used in electroplating or even acupuncture. Could it have been used as for security? or religious ceremonies? The batteries could have enough charge to electrify an object like a statue to give a small but impressive electrical shock.