Batteries and Fuel Cells: Working, Types and Comparison

A battery and a fuel cell are both devices that convert chemical energy directly into electrical energy through chemical reactions. A battery carries its chemicals sealed inside it and runs until they are used up, while a fuel cell keeps producing electricity as long as fuel, usually hydrogen, and oxygen are supplied to it from outside.

This matters because almost every portable and clean-energy technology depends on one of them. Mobile phones, laptops, electric vehicles and solar home systems run on batteries, while fuel cells power buses, backup generators and spacecraft, offering a way to make electricity without smoke.

Key Concepts

• Primary cell: a single-use battery that cannot be recharged, such as a zinc-carbon torch cell.
• Secondary cell: a rechargeable battery, such as lead-acid or lithium-ion, whose chemical reaction can be reversed by passing current through it.
• Anode and cathode: the negative and positive electrodes where oxidation and reduction take place.
• Electrolyte: the conducting medium, liquid or solid, through which ions move between electrodes.
• EMF (electromotive force): the open-circuit voltage of the cell, measured in volts (V); capacity is the total charge it can deliver, in ampere-hours (Ah).
• Internal resistance (r): opposition inside the cell itself, measured in ohms (Ω), which drops the terminal voltage when current flows.
• Fuel cell: an electrochemical cell, for example the hydrogen-oxygen PEM cell, that produces water as its main by-product.

Formulas

• Terminal voltage: V = E − Ir, where E is EMF in volts, I is current in amperes and r is internal resistance in ohms.
• Electrical power: P = VI, with P in watts (W).
• Stored energy: Energy = V × Q, where Q is capacity in Ah, giving energy in watt-hours (Wh).
• Efficiency: fuel cells convert about 50–60% of fuel energy to electricity, roughly double a petrol engine's 25–30%.

These formulas assume the EMF stays constant during discharge and ignore heating losses. Real batteries lose capacity at high currents and in cold weather, and fuel cell efficiency falls when pushed to maximum power.

Worked Example

A 12 V, 100 Ah lead-acid battery powers a 60 W lamp. How long can it run?

1. Stored energy = V × Q = 12 V × 100 Ah = 1200 Wh.
2. Power drawn by the lamp = 60 W.
3. Running time = energy ÷ power = 1200 ÷ 60 = 20 hours.

Common Misconceptions

• "Batteries store electricity." They do not. A battery stores chemical energy and generates electricity on demand through a chemical reaction. That is why a dead battery feels exactly as heavy as a charged one.
• "Fuel cells need recharging like batteries." They do not. A fuel cell is refuelled, like a petrol tank: keep feeding hydrogen and it keeps making electricity, with water dripping out as the exhaust.
• "A bigger battery always gives more power." Not necessarily. Power depends on both voltage and how much current the design can safely deliver. A small 12 V battery can out-perform a large 1.5 V cell in a high-power device.

Real-World Applications

• Electric vehicles: lithium-ion packs store 200–300 Wh per kilogram, giving cars a 300–500 km range.
• Grid storage: giant battery banks store daytime solar power for use at night.
• Spaceflight: the Apollo missions used alkaline hydrogen-oxygen fuel cells, which also supplied the crew's drinking water.
• Backup power: hospitals and data centres keep batteries and fuel cells ready for instant switchover during blackouts.

Batteries and fuel cells are partners rather than rivals: batteries store energy compactly for portable use, while fuel cells generate it continuously wherever fuel can be supplied.