12v dc

Battery state (lead-acid, rested)

Rest the battery at least 4 hours with no charging and no loads, then measure at the terminals. Typical readings: 12.7 V = full, 12.4 V = about 75 percent, 12.2 V = about 50 percent, 12.0 V = about 25 percent, 11.8 V or less = discharged.

Lithium (LiFePO4): voltage stays almost flat around 13.2 to 13.4 V for most of the charge. Voltage is NOT a good gauge. Use the battery monitor or the BMS.

Charging voltages (typical)

Converter or charger float: about 13.2 to 13.8 V. Absorption: about 14.2 to 14.8 V. Alternator: about 13.8 to 14.4 V.

Lithium batteries need a charger set for lithium. Check the battery and charger labels.

Load test

A battery that reads fine at rest can collapse under load. Run a real load (lights, pump, fan) and watch the voltage. Dropping below about 11 V quickly means a weak or discharged battery. Also check terminals for corrosion and tightness.

Voltage drop test

Find bad wires and connections without unplugging anything. Run the load. Put the meter across the suspect wire or connection, red on the source side, black on the load side. The reading is the voltage lost there.

Target: about 3 percent or less (about 0.4 V on 12 V). Anything over about 0.5 V across one connection or one short run is suspect. Test the ground side too: black lead on battery negative, red lead on the load ground.

Parasitic draw

Everything off, shore power unplugged

Wait 20 minutes so modules go to sleep

Meter on DC amps in series with the battery negative

Note the draw. Pull fuses one at a time to find the circuit

Many coaches sit around 20 to 100 mA (LP and CO detectors, fridge board, stereo memory, monitor panel). A lot more, for days, flattens the battery.

Fuses and wire (copper, short runs)

Max amps, 60 C wire rating: 14 AWG = 15 A, 12 AWG = 20 A, 10 AWG = 30 A, 8 AWG = 40 A, 6 AWG = 55 A, 4 AWG = 70 A, 2 AWG = 95 A, 1/0 AWG = 125 A.

Long runs need thicker wire to keep voltage drop low. The fuse protects the WIRE, not the load. Never put in a bigger fuse to stop it blowing; find out why it blows.

Converter / charger

With shore power on, measure at the battery terminals: 13.2 to 14.8 V is normal. No charge: check 120V input, converter fuses, the DC distribution fuses, and the connection to the battery. Listen for the converter fan.

Common 12V faults

Bad ground or corroded battery cable. Loose terminal at the converter or fuse block. Warm or discolored connections (high resistance). Tripped thermal breaker. Reversed polarity from a previous repair.

Why 12V matters for codes

Furnaces, tankless heaters, fridge boards, slides and leveling controllers all throw odd codes on low voltage. Measure at the appliance WHILE it runs before you replace any part.

A shorted battery can deliver hundreds of amps. Remove rings and watches. Fuse close to the battery. Lead-acid batteries give off hydrogen: no sparks, ventilate.