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Auto Electricals study materials
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Auto Electricals: Batteries, Starting Systems, and Charging Systems

Auto Electricals · Work through your exam, one topic at a time.

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Batteries, Starting Systems, and Charging Systems

This domain tests how a vehicle stores, delivers, and replenishes electrical energy. Expect questions on lead-acid battery construction and ratings, state of charge, charging system operation and regulation, cranking circuit voltage drop, and parasitic draw.

Core rules

  • A 12 V lead-acid battery has six cells of about 2.1 V each. Open-circuit voltage approximates state of charge after a rest: about 12.6 V at 100%, 12.4 V at 75%, 12.2 V at 50%, and 12.0 V at 25%.
  • Cold cranking amps (CCA) is current at −18 °C for 30 s to 7.2 V; reserve capacity is minutes at 25 A to 10.5 V. CCA falls as temperature falls.
  • Flooded, AGM, EFB, and gel batteries need different charge profiles. AGM is sensitive to overvoltage; smart charging systems may regulate 12.5–15.5 V depending on temperature and state of charge.
  • The alternator is a three-phase AC generator with a rectifier, field control, and voltage regulator. It needs a sense circuit and a good ground; the charge lamp or LIN message may control it.
  • Cranking circuit voltage drop should generally stay below 0.5 V total under load; each cable and connection should be tested individually.

Worked example

A battery reads 12.5 V at rest, but cranking is slow. During cranking, battery positive to starter positive reads 0.9 V and starter case to battery negative reads 0.4 V. Total drop is 1.3 V, so the starter sees about 11.2 V instead of 12.5 V. The positive side is the larger problem; test from battery positive to the solenoid, then solenoid to starter, to find the cable or terminal causing 0.9 V. Do not condemn the starter until the circuit is proven.

Traps

Testing state of charge immediately after charging reads surface charge and is falsely high; apply a load or let it rest. Measuring voltage drop with the circuit off gives near-zero and proves nothing. Disconnecting the battery while the engine runs can cause load-dump damage. Replacing an AGM with a flooded battery without configuring the charging system can shorten battery life.