How Batteries Work

Your phone battery stores about 40 kJ of energy, enough to lift a 1-ton car about 4 meters off the ground. That energy sits in chemical bonds, waiting. When you turn on the screen, a chemical reaction begins that forces electrons through a circuit. The reaction is reversible: plug in a charger and the electrons flow backward, restoring the original chemistry.

A battery has three essential parts. The anode (negative terminal) holds a material that wants to release electrons. The cathode (positive terminal) holds a material that wants to accept them. The electrolyte between them conducts ions but blocks electrons, forcing electrons to travel through the external circuit (your device) to complete the reaction. That electron flow is electric current. The voltage depends on the chemistry: how strongly the cathode material pulls electrons away from the anode material. Lithium-ion cells produce about 3.7 volts; lead-acid cells produce about 2.1 volts.

Charging reverses the reaction by pushing electrons backward with an external voltage source, restoring the anode and cathode to their original chemical states. No battery is perfectly reversible: each cycle loses some energy to heat, and side reactions gradually degrade the electrodes.

Worked Example

A lithium-ion battery pack is rated at 3.7 V per cell, with 100 cells in series.

• Calculate pack voltage. 3.7 x 100 = 370 V.
• The pack stores 50 kWh of energy. Current capacity = Energy / Voltage = 50,000 Wh / 370 V = 135 Ah.

If this pack achieves 90% round-trip efficiency, how much energy is lost per full charge-discharge cycle?

50 kWh x 0.10 = 5 kWh lost as heat per cycle. Over 3,000 cycles (a typical battery lifetime), that is 15,000 kWh of cumulative heat loss.

Batteries convert between chemical and electrical energy. Their voltage, capacity, efficiency, and lifespan all trace back to the chemistry at the electrodes.

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