"The battery died again."
Toma shook the flashlight.
Kana laughed. "Do batteries get tired too?"
"Rather than tired, they're depleted," Rei corrected.
"What gets depleted?"
"The materials for chemical reactions. Zinc, manganese dioxide, etc."
Toma started disassembling the battery. "Let's look."
"Dangerous!" Rei stopped him.
"It's okay, it's a used-up dry cell."
He took out the contents. Black powder and a metal rod.
"This is what's inside a battery?" Kana peered in.
"The outside is zinc, inside is manganese dioxide and electrolyte," Rei explained.
"So what's happening?"
"Redox reaction. Zinc releases electrons, manganese dioxide receives them."
Kana wrote in her notebook. "Electrons move?"
"Yes. That flow of electrons is electric current."
Toma drew a diagram. "Zinc becomes Zn²⁺, losing two electrons."
"Oxidation?"
"Correct. Losing electrons is oxidation," Rei confirmed.
"And where do those electrons go?"
"Through the wire to the manganese dioxide side."
"Where reduction occurs."
Kana understood. "Receiving electrons is reduction."
"Exactly. Oxidation and reduction are always a pair."
Toma supplemented. "If one loses electrons, the other receives them. Balance."
"Then why does it get tired... depleted?"
"When all the zinc becomes Zn²⁺, it can't emit more electrons," Rei answered.
"Out of materials."
"Yes. When the chemical reaction stops, the current stops too."
Kana thought. "What about rechargeable batteries?"
"Like lithium-ion batteries," Toma showed interest.
Rei explained. "The reverse reaction is possible. When you apply current, it goes back."
"Lithium ions go back and forth."
"But it doesn't completely return. It gradually degrades."
Kana asked. "Why zinc? Won't other metals work?"
"Ionization tendency," Rei wrote a list.
"Li, K, Ca, Na, Mg, Al, Zn, Fe, Ni, Sn, Pb, (H), Cu, Hg, Ag, Pt, Au"
"The farther left, the easier to emit electrons."
Toma pointed. "Zinc is at a moderate position. Not too strong, not too weak."
"If too strong?"
"Like lithium. Reacts violently with water. Dangerous."
"If too weak?"
"Like copper. Hard to emit electrons. Low voltage."
Kana was satisfied. "Balance is important."
"Battery voltage is determined by the potential difference between two metals," Rei supplemented.
"Potential difference?"
"The difference in power to emit electrons. The larger it is, the higher the voltage."
Toma brought a measuring device. "Let's measure new and old batteries."
New: 1.5V, Used: 0.8V.
"The voltage dropped!" Kana observed.
"When materials decrease, reaction rate falls. Potential also drops."
"That's why it looks tired."
Rei said quietly. "Batteries don't get tired. They've just run out of what to give."
Kana felt wistful. "Sad."
"But it fulfilled its role," Toma laughed.
"It converted chemical energy to electrical energy."
Rei continued. "Living things are similar. ATP is like a battery."
"ATP?"
"Adenosine triphosphate. The cell's energy currency."
"When tired, recharge?"
"Recreate it from food. Through chemical reactions."
Kana understood. "Life is also a chemical battery."
The three stared at the depleted battery. A small cylinder that carried energy. The reason the battery got tired is simple yet profound.