Electrical systems, battery, charging and starting
Electrical systems, battery, charging and starting tests circuit theory, battery construction and testing, alternator operation and starter diagnosis.
Battery
A 12 V lead–acid battery has six cells of about 2.1 V. Plates are lead and lead dioxide in dilute sulphuric acid; specific gravity of a fully charged cell is about 1.265. Cold cranking amps (CCA) rates starting ability at −18 °C. Top up only with distilled water, keep terminals clean and tight, and never reverse polarity.
Charging and starting
The alternator produces three-phase AC, rectified by diodes to DC, with the regulator holding about 13.8–14.4 V. The starter is a series-wound DC motor with a solenoid that engages the pinion and switches the heavy current. A click with no crank points to a low battery, bad earth, faulty solenoid or seized engine - test, don't guess.
Worked example. A 55 W headlamp on a 12 V supply draws I = P ÷ V = 55 ÷ 12 = 4.58 A. Its resistance is R = V ÷ I = 12 ÷ 4.58 = 2.62 Ω. A voltage-drop test is better than an ohmmeter for cables: cranking should show less than about 0.5 V total drop on the earth side.
Traps
Disconnecting the battery with the engine running, which can destroy the alternator diodes. Reverse-charging a battery. Testing specific gravity immediately after topping up. Assuming 12 V means a healthy battery - a resting reading below 12.4 V is discharged.