When striving to create a small, disposable test for various disease such as diabetes, scientists were pushed to find an equally small, inexpensive, disposable battery. (someone ironic, seeing as you’re not supposed to throw out batteries)
What they discovered is that many tests rely on urine to determine test results. Since urine is acidity, scientists have developed a battery powered by the very urine it is testing. An acid based battery has been around since you could buy kits to make batteries out of potatoes or lemon water.
“In order to address this problem, we have designed a disposable battery on a chip, which is activated by biofluids such as urine,” says Ki Bang Lee, one of the researchers responsible for the innovation.
Now we can talk about environmental benefits. Urine is a renewable resource. Just ask any pregnant woman. Although slightly toxic, it’s no more so than vinegar. Urine is virtually ubiquitous and the batteries are essentially biodegradable.
So how do they do it? It always seems batteries are made by sandwiching different types of metal together in different ways and orders. This is no exception. The developers of the battery sandwiched paper soaked in copper chloride between layers of magnesium and copper. The whole thing is then wrapped in plastic except for a small hole. When urine is dropped onto the paper, it creates a chemical reaction (like all batteries) and produces electricity.
So how much juice are we talking about here? (no pun intended) About 1.5 volts for 90 minutes. That’s about the same as a AA battery. Imagine this scenario: “Hey Joe, my phone is dying. Let me give you a call back after I chug some coffee”. Source