The story of the "Baghdad Battery" is a staple on many paranormal, "alternative history" and "ancient astronaut" websites. According to the lore, the "battery" indicates that ancient people in the Middle East knew about and used electricity some 2,000 years before the Europeans supposedly discovered it. But how solid is the evidence for this?
A diagram of the "Baghdad Battery"
Here is the story as it is usually given in paranormal books and websites: just before World War Two, a German archaeologist named Wilhelm Konig found an odd object in Iraq, near Baghdad, consisting of a small clay jar about five inches tall, a plug made of asphalt (natural oil tar) that sealed the opening, and inside, a hollow tube of copper with a solid iron rod inserted into it. When assembled and filled with an acidic liquid like grape juice, lemon juice, or vinegar, Konig realized, the device could function as a very crude battery, with an electrolyte and two electrodes. Reproductions made by Konig and later others were capable of producing from half a volt to two volts of electric current. Konig published a paper on the possible "battery" in a German science journal, but with the coming of the war, it was forgotten. In the 1960's, Konig's paper was rediscovered during the wave of "ancient astronaut" writings triggered by Erik von Daniken's best-selling book, Chariots of the Gods, and claims soon appeared that the "Baghdad Batteries" were used for everything from electroplating jewelry and coins, to generating a seemingly-supernatural electric shock when connected to an idol or temple statue, to powering electric light bulbs inside the pyramids and the lighthouse at Alexandria. And, according to the flying saucer fans, since the ancient Mesopotamians did not have the knowledge to make a battery or to use electricity, they must have obtained this information from "someone else", perhaps some technologically advanced race of extraterrestrials who visited Earth in ancient times.
So, how accurate is this story?
Right at the start, we run into problems. There really was a German archaeologist named Wilhelm Konig; he worked at the Baghdad Museum in the 30's and took over as Director in 1934. And he did publish a paper on the "Baghdad Battery", in a 1938 issue of the German journal Forschungen und Fortschritte. But here is where the problems begin: that journal ceased publication in 1967, and has no online archive. So the original paper has apparently not been available since then, and none of the people who write about the topic today seem to have actually read the original paper (several skeptical investigators mention that they have tried, unsuccessfully, to find a copy of Konig's original article). So today we are left only with the accounts drawn from the article in the 1960's, nearly all of them written by flying saucer fans. Sadly, those "researchers" seem to have embellished their stories, and as a result we today have several different and contradictory versions, and no way of knowing which, if any of them, is accurate.
For instance, we don't know where or when the "battery" was found. Some accounts have Konig himself finding the battery during a dig at the site of Khujut Rabu, near Baghdad, in 1937 or 1938. Other accounts have Konig finding the battery in storage at the Museum in Baghdad after taking over as Director, in 1934 or 1937, having been dug up by someone else and placed in the museum's vaults. Many accounts have the battery as having been found in the ruins of a Parthian village from 250 BCE (the Parthians were an ancient Middle East people most famous for having successfully resisted the efforts of the Roman Empire to conquer them). Other accounts, however, cite unnamed "authorities" as classifying the clay jar as typical of the Sassanid period, about 250-650 CE, several hundred years later. It is not even clear how many "Baghdad Batteries" are supposed to exist--many accounts mention just one, but others assert that as many as a dozen may have been found.
The construction of the presumed "battery" also presents a number of problems. None of the pieces are particularly exotic or technologically advanced. Clay jars have existed in the area for thousands of years, and were common implements at the time. Asphalt is naturally available in the area as tar and oil bubbles to the surface, and it has long been used for waterproofing material. Copper tubes were often used as protective covers for papyrus scrolls. And iron was a common material for the time, used for weapons and tools. So there is nothing particularly unique about the materials used in the "battery". Archaeologists who have looked into the putative "battery" have concluded that it is nothing more than a now-decayed papyrus scroll wrapped around an iron rod and placed inside a copper tube, which was sealed inside a clay storage jar with asphalt to protect it from water and weather.
Nevertheless, replicas of the "battery" have been built by several researchers (including a recent Mythbusters TV show), and when they are filled with an acidic liquid like grape juice, they do indeed, whether intentionally or not, produce a small electric current, usually between half a volt and one volt. That has led to several speculations about how the "batteries" could have been used. One hypothesis has been that the battery was connected to small iron statues inside temples, where, if touched by a worshiper, they would produce a seemingly-supernatural tingling that would show the power of the gods or spirits. It is known that ancient Greek temples used technological tricks to produce effects such as doors that opened by themselves, or statues that moved, to awe worshipers. But there is so far no archaeological evidence that this sort of thing was done in either the Parthian or Sassanid cultures.
Another popular hypothesis is that a number of the "batteries" were connected in series and produced a high enough voltage to be used in electroplating, in which metal objects are placed in a liquid through which an electric current is passed, thereby depositing a thin coating of another metal, like gold or silver, onto the object. We do know that the Parthians and Sassanids did have gold-plated jewelry, but we also know that they used a process called "mercury gilding" to do this, in which a mixture of gold/silver and mercury was applied to an object and heated, turning the mercury into a vapor which then deposited a thin layer of bonded gold or silver onto the intended object. All of the gold or silver plated items found from this period show the chemical signature of mercury vapor, and none of them exhibits the characteristics of electroplated coatings.
The most fanciful hypothesis has been that huge banks of "Baghdad batteries" were used to power electric lightbulbs ("ancient astronaut" fans point to some Egyptian hieroglyphics which, they claim, depict electric light bulbs in use inside the pyramids). Perhaps, they claim, even the famous Alexandria lighthouse was powered by an electric lightbulb. There is of course not a shred of evidence at all for any of this.
So, the consensus of archaeological science is that the "Baghdad Battery" is in fact not a battery at all, but simply a storage jar for a scroll.
But the "Battery" continues to be a source of myth and story. In the latest mythology, the original Baghdad Battery found by Konig was said to be stored in the archives of the Baghdad Museum. But, in the looting and destruction that took place after the American invasion of Iraq, the Baghdad Battery is said to have now disappeared.