| VEHICLE STATE | VOLTMETER READING | INDICATION |
| Engine not running | Engine off voltage is at least 12V | So far, so good. Note voltage reading. |
| Engine not running | Engine off voltage is less than 12V | Battery run down. Charge before continuing tests. Cars with electronic ignition may not start, even if engine cranks. |
| During Cranking | Voltage should be above 9.5V | If not, battery may not have enough capacity (too small or weakened by sulfanation or "old age.") |
| Increase engine to 1500rpm | Voltage exceeds engine-off voltage by more than 2V | Probable over-charging; look for bad regulator or regulator ground, or wiring problem. |
| Increase engine to 1500rpm | Voltage exceeds engine-off voltage by less than 2V | Proceed to Load Test |
| Load Test: 3000rpm with all lights, heater & radio on | Voltage exceeds engine-off voltage by less than 1/2 Volt | Charging system may be faulty |
| Load Test: 3000rpm with all lights, heater & radio on | Voltage exceeds engine-off voltage by more than 1/2 Volt | Charging system ok |
| Any rpm | Voltage regularly exceeds 14.5V. (May also blow lamp bulbs frequently, and/or tend to make battery run dry.) | Probable over-charging; look for bad regulator or regulator ground, or wiring problem. |
The voltmeter will let you know if there is a problem with your charging system or battery. It is easier to use (and especially to install) than an ammeter. Learn your car's baseline voltages, so you can recognize when something may be going wrong.
Here are a few possible voltmeter indications, assuming the car has been running enough to charge the battery. All accessories should be off: