How a Lead-Acid Battery Works

Cells, plates and electrolyte — and what ageing really does.

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What you will learn

Describe the internal construction of a lead-acid battery
Explain state of charge versus state of health
Recognise the signs of a failing battery

A conventional 12 V battery contains six 2 V cells in series. Each cell has lead and lead-dioxide plates in sulphuric acid electrolyte. Charging converts lead sulphate back to active material; discharging does the reverse.

Over time, deep discharges and vibration break down the plates and build permanent sulphate. The battery may still show an acceptable open-circuit voltage yet collapse under a starting load — which is why a load test matters.

Batteries contain sulphuric acid and produce explosive hydrogen during charging. Wear eye protection, avoid sparks and ventilate the area.

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Key points

Six cells in series create the nominal 12 V
State of charge is not the same as capacity
Sulphation permanently reduces capacity

All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.