Automotive Electrical System and Battery
This domain covers the 12 V system: battery construction and maintenance, starting motor, charging, ignition and lighting circuits.
Battery
A lead-acid battery has six cells in series, each about 2.1 V, giving about 12.6 V when fully charged. Positive plates are lead dioxide, negative plates are sponge lead, and the electrolyte is dilute sulphuric acid with a specific gravity of about 1.260 to 1.280 when fully charged. Capacity is rated in ampere-hours, and cold cranking current shows its starting ability. Maintenance: keep terminals clean and lightly coated with petroleum jelly, top up with distilled water only to just above the plates, never add acid, and keep vents clear. Charging releases hydrogen, so no sparks or flames.
Starting and charging
A starter motor is a series-wound DC motor giving high torque at low speed, engaged by a solenoid and Bendix or pre-engaged drive. The alternator generates AC which is rectified by diodes and regulated by a built-in regulator; unlike the old DC generator it charges even at idle and needs no polarising. Check the fan belt tension and the charging light before condemning the alternator.
Ignition and accessories
A conventional coil ignition uses a contact breaker; electronic systems use a sensor and module. Spark plug gaps are typically 0.6 to 0.8 mm. Fuses protect circuits and must be replaced with the same rating; lighting, horn, wiper and instruments are fed through the harness with relays for heavy loads.
Worked example
A hydrometer reads 1.250 in five cells and 1.150 in one, after correction for temperature. Reasoning: a single low cell means that cell is failing internally (sulphation or a short), and the battery cannot be recovered by charging. The battery should be replaced; topping up with acid or overcharging will not restore the low cell.
Traps that cost marks
Topping up with acid instead of distilled water; fitting a fuse of higher rating; assuming the alternator is faulty when the belt is slipping; forgetting the hydrometer reading needs temperature correction.