When my family went sailing one summer, I helped my sister into her life jacket and struggled to show her how to use the emergency lights. How could a little kid do this when stranded without adults? To solve this problem, I looked for ways to create self-activating battery for lights. The two main challenges I faced were:
1. Activation: when to activate it?
Ideally the light should light up immediately when the occupant has fallen into the sea. This can be detected in many ways, like a smart sensor or radar, but the incidental nature of the device's use and the presence of water demanded a lower-tech solution.
2. Fuel: how to power it?
whatever battery I chose must be waterproof and functional in salty sea conditions.
To address both problems at once, I looked for a way to make a light that responds to water and uses no conventional battery. I remember learning about the conductivity of salt water in chemistry, so I conducted more research on compounds that conduct highly in water. My final battery design was a perforated box containing magnesium and carbon. When the box submerges, paper strips stuffed through the perforations absorb salt water into the chamber to catalyze the ionization of magnesium and carbon. Those ions generate electric current for an LED strip. Air is expelled through a hole on top.
When I tested my solution, I discovered the voltage was insufficient to power the LED: I had to connect three of them in series to generate sufficient voltage. I was thrilled to find that my sister is happy as a clam swimming with her safe, glowing life jacket.