Multimeter-based test sequence to diagnose alternator, battery, and ground problems. Voltage drop testing finds bad grounds and connections the straightforward voltage test misses.
Standard charging system test sequence:
1) BATTERY RESTING VOLTAGE (engine off, 4+ hours sitting): 12.6V healthy, 12.4V partial, 12.0V or less = problem or discharged.
2) CRANK VOLTAGE (probe battery while cranking): >9.6V healthy. <9V = weak battery or starter draw issue.
3) IDLE CHARGING VOLTAGE (engine running, no loads, 800 RPM): 13.8-14.4V healthy. >14.7V = overcharging (regulator). <13.5V = undercharging.
4) LOADED CHARGING VOLTAGE (engine ~1500 RPM, headlights+blower+rear defrost on): should stay 13.5-14.2V. Drop below 13V = alternator weak or wire/ground problem.
5) VOLTAGE DROP TESTS (the key step most people skip): with the engine running and loads on, measure between (a) battery+ post and alternator B+ stud — should be <0.3V; (b) battery- post and alternator case — should be <0.2V; (c) battery- and engine block — should be <0.1V. Any drop bigger = bad connection or undersize wire. This is exactly why Big-3 is recommended.
6) AC RIPPLE on alternator (multimeter on AC volts, engine running): should be <0.5V AC. Higher = bad diode in alternator stator.
A $35 carbon-pile load tester or 'battery tester' from Harbor Freight can also stress-test the battery. Most autopart stores will free-test charging systems.
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.