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The Baghdad battery is a collection of artifacts found in a village near Baghdad, Iraq, in the 1930s. Apparently dating to the Sassanid era, the "battery" consists of a fired ceramic container, some rolled sheet copper, a rod of iron and a bitumen bung. The separate components of a simple galvanic cell were within the technological means of the local artisans of its time, but the question is why they would ever have wanted to build such a thing.
History of the "ancient battery" theory
Wilhelm König found the objects in the collection of the National Museum of Iraq, or possibly dug them up himself at a place called Khujut Rabu, depending on which account you're reading. Certainly there is no reliable documentation of the archeological dig, nor enough information about the stratigraphy to date the finds from their archeological context. For no well-documented reason, they were declared to date from the Parthian civilisation, and are sometimes known as the Parthian batteries, but the outer jars are in the much later Sassanid style. The possible date range for the finds is therefore from 250 BCE to 650 CE.
König wrote a book in 1940, Neun Jahre Irak, hypothesizing that the objects were primitive galvanic cells, or electric batteries. He speculated that the "batteries" were used for electroplating precious metals.
Hypothesis versus evidence
The König hypothesis, as a hypothesis, isn't quite as far out there as it sounds. His speculation was based in part on the observation that the iron rods appeared to have been corroded by contact with an acid. In modern times, electroplating was discovered multiple times independently within a few years of the invention of the galvanic cell. If an ancient inventor happened to assemble something that functioned as a battery, there is no particular reason why the discovery of electroplating could not follow. However, there is no evidence whatsoever that this actually happened, and no ancient electroplated artifacts have been identified. Ancient items that were thought by König to have been electroplated have since been shown to have been produced using the fire-gilding method.
There is also the fact that the objects, as found, are not a complete "electroplating kit". No evidence of accompanying wires or conductors has been discovered. Neither has any evidence of other technology that might have required electricity as a power source.
Based on experimental archaeology, it is generally agreed that the objects discovered in Iraq, with some modifications and the addition of an electrolyte solution, are capable of functioning as a crude battery. (So is a lemon.) It is also generally agreed that several such batteries would be required for electroplating. However, the important point about experimental archeology is that proving that something was feasible does not equal scientific proof that it ever happened.
Experimental archeologists, not to mention the Mythbusters, have created functional, if extremely weak, galvanic cells based on the design of the "Baghdad battery". The most frequently cited experiment is that made by Dr Arne Eggebrecht during the 1970s. Dr Eggebrecht reported success in electroplating with silver using replicas of the Baghdad objects, but this experiment was undocumented.
Another hypothesis (as seen on Mythbusters) is that the low voltage battery was used to generate a sort of religious experience, with the "buzz" of touching an electrified object being perceived as proof of supernatural or divine forces at work.
Another theory is that the low voltage batteries were used medicinally. A startling new scientific discovery might be seized upon by the credulous or the unscrupulous as the new panacea, as happened when electricity caught the public imagination during the 19th century.
An alternative theory that doesn't involve battery technology at all suggests that the jars are a form of protective document storage, with the decayed organic matter of the original papyrus documents accounting for the acidic residue and corrosion that the battery hypothesis attributes to the presence of an electrolyte solution. There isn't any overwhelming proof of the "scroll jars" scenario either, beyond the conclusion that very similar (but jar-less) objects discovered nearby had this function. Strangely, there is rather less media and pseudoscientific interest in the suggestion that people used to store stuff in pots.
Assuming that they're batteries...
So, let's assume for the sake of argument that the objects are batteries. What can we extrapolate from their existence before we strike pure woo? Any one of the Baghdad batteries is capable of producing up to about 1 volt at low current. To put that in context, two of them would be capable of powering a digital watch. For any prolonged use, you'd need to keep topping up the electrolyte solution; the design and small size of the batteries makes this tricky. It would take several of them to power the electroplating process, and they would need to be connected together somehow.
No evidence for additional components has yet been discovered, nor is there anything to suggest the existence of any technology that required electrical power, so let's go with the "religious experience" idea. Let's guess that there was a slightly electrified metal icon, with the battery hidden out of sight, and that people were invited to touch or to kiss it to obtain a religious or a novelty experience. Many schoolchildren today try out the lemon-battery experiment and feel the tingle of electricity on their tongue, while learning about how an electrical circuit works. That distinctive tingle must have been a pretty impressive, even "magical" experience in a pre-industrial society, whose only other experience of electricity would be uncontrolled, in the form of static charge and lightning strikes. A priest or showman able to deliver the miracle on demand could have commanded considerable awe.
It's fun to speculate, just so long as nobody gets confused and mistakes the speculation for hard evidence. Which brings us to the woo.
The woo arrives with the notion of the out-of-place artifact, and the belief that the ancient world could not possibly have produced such technology independently. A number of authors, notably including Erich von Däniken, have decided that the König interpretation is evidence that Ancients Possessed Advanced Technology. Depending on the author's favorite brand of woo, the source of the advancement may be Atlantis or aliens. Rather than conclude from the available evidence, "wow, those ancients might just have been experimenting with chemical reactions and discovered the battery; that's pretty damned incredible!", the pseudoscientist leaps all the way from, "possible crude battery" to, "obviously the secrets of an advanced civilisation, used to power transportation and lighting".
The Baghdad battery keeps company in woo circles with pseudoarcheology favourites such as the Coso artifact, the belief that the Pharos lighthouse was powered by electricity, the Dendera lamp, the Ark of the Covenant, the Abydos helicopter, and the construction of the Giza pyramids.
The interpretation of the Baghdad objects as out-of-place artifacts rests upon the assumption that the Parthians or the Sassanids could not possibly have built a battery because the technology is too advanced. In reality, the objects are nothing more than an earthenware jar, some copper, some iron and some bitumen, with a possible acidic residue. Our hypothetical ancient inventor does not need to understand that copper and iron form an electrochemical couple that reacts with an electrolyte solution in order to observe the effect, nor have any concept of the potential uses of controlled electricity in order to conclude, "wow, that tingles".
- Neun jahre Irak by Wilhelm König (1940). Rudolf M. Rohrer, 184 pages.