Lead-Acid Battery
160-year-old chemistry that still starts your car
The 12-volt battery is six 2.1-volt cells in series, each a stack of lead plates bathed in sulfuric acid. Its one heroic job is delivering a few hundred amps to the starter motor for a couple of seconds; after that it smooths the electrical system while the alternator does the work.
What it does
Stores chemical energy to start the engine and stabilizes the car’s 12 V electrical system.
In a corner of the engine bay (some cars hide it in the trunk).
How it works
- Each cell pairs lead dioxide plates (+) with sponge-lead plates (−) in dilute sulfuric acid.
- Discharging converts both plates toward lead sulfate, releasing electrons — current flows.
- Charging (from the alternator) reverses the reaction, restoring the plates and the acid.
- Six cells × 2.1 V gives the familiar ~12.6 V at full charge.
- Cranking demands 200–600 A — that’s why the plates are huge and the cables are thick.
Key numbers
| Voltage | 12.6 V full / ~14.4 V charging |
| Capacity | 45 – 80 Ah typical |
| Cold cranking amps | 400 – 800 A |
| Cells | 6 × 2.1 V in series |
| Life | 3 – 6 years |
Warning signs of trouble
- Slow, laboring crank — especially on cold mornings.
- Resting voltage below ~12.4 V after sitting overnight.
- Swollen case or rotten-egg smell — overcharging or internal fault.
- White-green fuzz on terminals — corrosion robbing current.
Maintenance & care
- Keep terminals clean and tight; coat with dielectric grease.
- Avoid deep discharges — they sulfate the plates permanently.
- On infrequently driven cars, use a trickle maintainer.
- Test capacity yearly once the battery is 3+ years old.
Frequently asked questions
What does the lead-acid battery do?
Stores chemical energy to start the engine and stabilizes the car’s 12 V electrical system.
How does a lead-acid battery work?
The 12-volt battery is six 2.1-volt cells in series, each a stack of lead plates bathed in sulfuric acid. Its one heroic job is delivering a few hundred amps to the starter motor for a couple of seconds; after that it smooths the electrical system while the alternator does the work.
What are the signs of a failing lead-acid battery?
Slow, laboring crank — especially on cold mornings. Resting voltage below ~12.4 V after sitting overnight. Swollen case or rotten-egg smell — overcharging or internal fault. White-green fuzz on terminals — corrosion robbing current.
How do you maintain a lead-acid battery?
Keep terminals clean and tight; coat with dielectric grease. Avoid deep discharges — they sulfate the plates permanently. On infrequently driven cars, use a trickle maintainer. Test capacity yearly once the battery is 3+ years old.