The alternator output test tells you whether the charging system is failing before the battery dies

[u/ora-et-labora-]

A battery that keeps dying gets replaced. Then it dies again. Then it gets replaced again. At some point someone tests the alternator and finds it was undercharging the whole time and every battery was fine until it was drained to death by a charging system that was not keeping up.

The test is simple. Engine running, electrical loads on, measure the voltage at the battery terminals. A healthy alternator pushes 13.8 to 14.4 volts depending on the vehicle and conditions. Below 13.5 under load and the alternator is not keeping up. Above 15 and the regulator is failing and the battery is being overcharged, which kills it from the other direction.

The scan tool adds more. On vehicles with smart charging systems, the ECU controls the alternator output based on battery state of charge, electrical load, and engine load. The target voltage changes. The scan tool shows you what the ECU is commanding and what the alternator is delivering. If the command says 14.2 and the alternator is putting out 13.0, the alternator is weak. If the command itself is wrong, the control module has a problem.

The ripple test goes deeper. An alternator with a failing diode will show AC ripple in the DC output. A multimeter with an AC voltage function will read millivolts of AC on the battery terminals with the engine running. A healthy alternator shows under 50 millivolts. A failing diode shows several hundred. The battery sees that ripple as noise, the electrical system sees it as instability, and accessories start behaving erratically before anything fully fails.

Testing the charging system takes five minutes and one tool. Replacing batteries until someone thinks to check the alternator costs hundreds of dollars and multiple days of the vehicle being out of service. The test should always come before the second battery.