Free 12-volt diagnostic guide
Car battery, starter & alternator test
Use a basic digital multimeter to combine resting, cranking, running and loaded voltage. The result identifies the next test—not a part to buy.
Previous test found
Saved locally on . Complete a new test to compare the readings.
What the guided test measures
StartVitals treats each voltage as evidence from a different operating state. The resting reading mainly screens state of charge. Cranking minimum shows how the 12-volt supply behaves under starter load. Running and loaded voltage screen whether charging support is visible.
That combination is more useful than asking whether the headlights look bright, but it still cannot measure battery capacity, alternator amperage, diode ripple or voltage loss inside individual cables.
Retest after charging
Your latest result is stored only in this browser. When you repeat the test on the same vehicle, StartVitals compares every usable before/after reading and explains whether resting and cranking performance improved while charging stayed stable.
Mechanic-ready report
The printable result includes vehicle context, battery type, age, temperature, symptoms, all measurements, assessments, reasons, recommended action and limitations. It is designed to communicate evidence—not instruct a shop which part to replace.
Why the result avoids “replace this part”
A weak battery and excessive starter load can both pull cranking voltage down. A discharged battery can also make alternator testing misleading. The engine therefore recommends the next confirming test and uses phrases such as “most consistent with,” not certainty.
Do not disconnect a battery cable while the engine runs. Voltage spikes can damage vehicle electronics, and the battery area can release flammable gas.
Read the complete threshold methodology and safety boundaries.