Setting: In a fantasy setting much like World of Warcraft (where huge liberties are taken regarding the level of technological sophistication), an evil emperor rules from a castle in which there are massive channels of flowing water. These channels are decorative, although the throne room contains a pool big enough to swim in.
Scenario: One day, for one reason or another, this emperor decides he needs a way to store massive amounts of electricity for use in one of his dastardly plots. He brings in the world's brightest alchemist and burdens him with the task.
I would really like it if the alchemist could basically turn the castle's decorative channels into a giant battery. He'd fill them with the necessary chemicals and components, then wire up a giant lightning rod on the top of the castle, which, when struck, would channel the electricity down into these pools where the electricity gets stored.
Question: Using the materials available in this quasi-advanced fantasy world, where the concept of a battery might already exist in some form, what would it take for this Giant Castle Battery to be even remotely "believable"? What would this alchemist have to do?
As should be clear by now, I have no real idea how batteries work. To wit, as an added bonus I would like it if the evil emperor could throw his enemies into the toxic pool as a means of painfully killing them.
Thanks for your consideration!