In order to minimize eventual residual surface charges, consider to leave the radio or headlights on for a couple of minutes prior to the measurement procedure.
1. Set your multimeter to DCV and select a range at least above 15V.
2. Connect the multimeter leads (black is ground) to the battery. The optimal reading is around 12.6V.
3. Now start the car with the multimeter connected to the battery. Voltage should not drop lower than 10V (it’s ok to drop to 11V).
4. After the sudden decrease on the reading, the value shall increase again and then stabilize around 14V, because the alternator will start charging the battery.
– If it goes above 14.7V, the alternator is overcharging the battery, which can damage it.
– If it goes under 14.2V, the alternator is still charging the battery (14.2V > 12.6V), but it might not be enough if you are consuming extra power with your headlights, radio, etc.
5. Now let’s add some load on the by turning on everything you can (headlights, internal lights, fan, radio, etc). The reading should not go below 12.7V (or, better yet, 13V).
– cleaning your battery terminals
– checking the back of the alternator for disconnected cables
– checking if the alternator is connected to the battery
– checking if the alternator belt tight enough