12 V system 3D wiring model
Travel trailer battery, power center, branches
A travel trailer's 12 V house system. The battery on the tongue feeds the coach through the disconnect to the power center's + bus, and the current comes home through the negative bus bar, the bond wire, the frame and the battery's − cable. Plugged in, the converter makes 13.6 V and feeds the same bus through its reverse-polarity fuses; towing, the truck charges the battery on 7-way pin 4. The circuit is solved for real: every cable and connection has resistance, so a bad one reads fine at rest and eats volts under load, and a clamp reads every cable.
Parts in this model
- Disconnect: Key switch on the A-frame in the battery + cable. Off, the coach has no battery at all; on shore power the converter still runs the 12 V side.
- Lights fuse: 15 A blade fuse in the power center. The red LED beside it lights when it's blown and the circuit is on.
- Light switch: Wall switch for the ceiling lights, on the red feed.
- Water pump
- Furnace
The sequence
- Parked on the battery. Unplugged, unhooked, nothing switched on. Nothing draws current, so every + test point reads what the battery reads, 12.6 V, and every − point reads 0 to the battery − post.
- Lights on. The light switch sends the red feed up to six LED fixtures, about 1.6 A. The current runs out the + cable through the disconnect to the power center and out the lights fuse, then home on the white to the bus bar, down the bond wire, along the frame and up the − cable to the battery.
- Faucet open: the pump runs. Pump switch on and a faucet open: the pump's pressure switch closes and it pulls about 5 A. With 6.7 A out of the battery, every cable and connection drops a little: the long battery cable to the power center about 0.05 V, each good connection a hundredth or two.
- Furnace on too: the full load. The furnace heats, about 7.8 A on its own. Now the battery carries 14.4 A, its posts sag to 12.43 V and the + bus to the bus bar reads 12.24 V: 0.10 V lost in the long battery cable, 0.05 V on the whole return. This is the load that finds a bad connection: the volts it eats show here and nowhere at rest.
- Plugged in. Shore power: 120 V through the converter's breaker, and the converter holds the + bus at 13.48 V to the bus bar with everything on. It carries every load, about 21 A, and pushes about 5.4 A back into the battery through the RP fuses, the POS+ lug, the cable and the disconnect.
- Towing. Unplugged and hooked to the truck with the engine running: the truck's 7-way pin 4 charges the battery through the 30 A breaker, about 5.3 A, and pin 1 brings it home on the frame. Pin 4 reads 13.49 V at the junction box, the battery posts 13.40 V.
Faults you can find with the meter
- Corroded battery + terminal
- Sulfated battery
- Corroded frame bond at the power center
- Disconnect switch failed open
- Lights white wire loose at the bus bar
- Furnace feed chafed through on a crossmember
- Pump fuse blown
- Pump motor open
- Converter not putting out
- Reverse-polarity fuses blown
- Converter breaker tripped
- Charge line breaker open
Numbers
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- :
- :
- :
- :
- :
- :
Keep going on 12V and batteries
Learn it
- Volts, amps and ohms, the simple way
- Voltage drop: the test that finds bad grounds
- Practice: find the bad connection
- Batteries: lead-acid vs lithium
- Fuses, breakers and grounds
Prove it
- Back lights are dim
- Battery dies even on shore power
- Lithium won't charge on cold mornings
- Coach is dead, battery is fine
- Batteries dead after storage