Tow Truck University® · Battery Diagnostics

Reading the Voltmeter

Sim S-VOLT-01 · 5-8 min · 6 scenarios

Setup A battery test lives or dies by how you hold the meter. Before you read a single voltage, two things must be right: the dial setting and where the probes touch.

Your meter is set to DC voltage (V―) and the probes are on a resting battery. Walk through the setup below, then start the first scenario.
DIGITAL MULTIMETER DC V — . — V ACV DCV 200 DCA OHM DIODE ACA ACV COM V/mA 10A NEG (−) POS (+) Set here
1
Dial on DC Volts (V—) — not AC, not amps
2
Red probe to battery positive (+) terminal
3
Black probe to battery negative (−) terminal
Scenario 1 of 6
DIGITAL MULTIMETER DC V –.- V ACV DCV 200 DCA OHM DIODE ACA ACV COM V / mA NEG POS
—
correct on first try (out of 6 scenarios)

Key Thresholds — Keep These

Resting voltage: charge and retest if below 12.2 V
~75% state of charge (minimum before testing) 12.45 V
Healthy charging system output range 13.8 – 14.7 V
Undercharging (alternator concern) below 13.5 V
Overcharging (damage risk) above 14.8 V
Normal key-off parasitic drain 25 – 50 mA
Excessive parasitic drain (investigate) above 50 mA
Tow Truck University® · Beta · June 2026 · Sim S-VOLT-01