Electrical System, Battery and Starting/Charging
What is tested
Electrical System, Battery and Starting/Charging tests battery construction and testing, starter and solenoid operation, alternator charging, wiring, fuses, lights and safe jump-starting.
Core rules
- A 12 V lead-acid battery has six cells. A healthy, fully charged battery rests near 12.6–12.8 V; about 12.2 V indicates partial charge, and below 11.9 V is discharged.
- Specific gravity roughly tracks state of charge: about 1.265 full and 1.120 discharged. Correct for temperature and never add acid to a discharged battery; add distilled water only.
- The starter draws very high current. Check battery terminals, earth strap, solenoid contacts and voltage drop before condemning the starter.
- The alternator should charge at about 13.8–14.4 V. A warning lamp with the engine running can indicate a failed alternator, belt or exciter circuit.
- Disconnect the battery before working on wiring; observe polarity and never short the terminals with a spanner.
Worked example
A tractor will not crank. The battery reads 12.5 V at rest but falls to 8.5 V while the key is turned. The reasoning: a good battery should hold above about 9.6 V during cranking. A large drop suggests a discharged or failed battery, or a high-resistance connection. Check terminal voltage drop and the earth strap; if the battery is good, test the starter current draw.
Traps
Reversing jump leads, disconnecting the battery with the engine running, and over-tightening terminal clamps. Another error is testing a battery immediately after charging without removing the surface charge. Also, a blown main fuse can mimic a dead battery.