Batteries, Starting Systems and Charging Systems
Battery behaviour
Learn state of charge from open-circuit voltage after removing surface charge (load for a few seconds or leave to rest): roughly 12.6–12.8 V = 100%, 12.4 V ≈ 75%, 12.2 V ≈ 50%, 12.0 V ≈ 25%, below 11.9 V is discharged. Capacity is rated in Ah and cranking ability in CCA - they are different figures and questions swap them deliberately. Flooded, EFB, AGM and gel batteries need different charging profiles and, on many vehicles, registration/reset after replacement so the smart charging system relearns. Parasitic drain is typically in the tens of milliamps; measure it with a clamp or after the modules have gone to sleep.
Starting
A starter circuit is a high-current series path: battery → earth strap → engine block → starter earth, and battery → positive cable → solenoid → motor. Test by voltage drop under cranking load, not by ohms. Typical limits are around 0.2–0.5 V per cable leg; a total cranking drop over about 1 V means a cable or connection fault. A clicking solenoid with no crank can be a flat battery, high resistance cable, poor earth or faulty solenoid contacts.
Charging
Alternators produce AC that a rectifier converts to DC, with the regulator controlling field current. Modern systems are LIN-controlled and use a battery current sensor, so charging voltage may sit anywhere between about 12.5 V (energy recovery) and 15 V. Warning signs are low output at idle, excessive AC ripple, a noisy or glazed belt, or a battery that never reaches full charge.
Traps. Never test an alternator by disconnecting the battery with the engine running - the resulting load dump can destroy electronics. Do not test state of charge straight after charging without removing surface charge.