Battery testing from start to finish
The 12.6 V Trap
Predict, then check
Terminal Voltage Under Load
A 2017 Ford Focus presents with a non-start. Resting voltage across the battery posts measures 12.62 V (100% state of charge indicator). When the key is turned to crank, the starter solenoid engages with a loud click, but the starter motor does not turn.
What reading do you expect on your multimeter when the key is held in the crank position?
Think of a 12 V battery as a pressurized water tank. Open-circuit voltage measures static head pressure. A high pressure reading proves water is in the tank, but it reveals nothing about the size of the outlet pipe. When the starter engages, it opens a massive valve demanding over 150 A. If chemical degradation or plate sulfation has restricted internal current flow, electrical pressure collapses instantly. The meter reads static pressure, but the starter requires volume under load.
Good resting reading: 12.60 V to 12.72 V (100% charge). Bad load reading: dropping below 9.60 V during cranking. A healthy battery holds at least 9.60 V at 20°C under full starter load.
Check your understanding
A battery reads 12.62 V key-off, but drops to 6.20 V during starter engagement. What does this combination of readings prove?
Answer the question on this page to carry on