An RV has two electrical systems that share a battery. Once you see them as two separate things with a couple of bridges between them, everything else makes sense: which lights work without hookups, why the microwave does not, what an inverter is for, and why the battery is dead after a weekend. This guide is for someone who has never opened the electrical panel. No prior knowledge assumed.
The two systems
The 12 V DC system runs on the battery. Direct current, low voltage, the same as your car. It powers everything that must work while you are off-grid: interior lights, the water pump, roof vent fans, the fridge controller (and a compressor fridge entirely), the propane detector, the furnace or diesel heater fan, slide and awning motors, the control panel, and USB outlets. It is fed by the battery and protected by blade fuses or small breakers in a 12 V panel.
The 120 V AC system is house power. Alternating current, the same as the outlets at home. It powers the wall outlets, the air conditioner, the microwave, and (on some RVs) the water heater's electric element. It comes from the campground pedestal through the shore cord, or from a generator, or from an inverter. It is protected by household-style breakers in an AC panel.
The bridges:
- A converter (or charger) turns 120 V AC into 12 V DC to run the 12 V side and charge the battery when you are plugged in.
- An inverter turns 12 V DC into 120 V AC so you can run a few AC things from the battery when you are not.
- An inverter/charger does both in one box and switches between them automatically. The Victron MultiPlus is the common RV example.
The battery
The battery is the heart of the 12 V system and the thing beginners struggle with most. Three points cover most of it.
- Capacity is in amp-hours (Ah). A 100 Ah battery can deliver 1 A for 100 hours or 10 A for 10 hours, roughly. Lead-acid batteries should only be discharged halfway, so a 100 Ah lead-acid battery is really 50 Ah of usable energy. Lithium (LiFePO4) can use 80–100% of its rating.
- Voltage is not a fuel gauge. A rested 12 V lead-acid battery reads about 12.7 V full and 12.0–12.1 V at half. Lithium sits near 13.2 V for most of its range. Under load, both read lower. The only reliable gauge is a shunt monitor like the Victron SmartShunt, which counts amps in and out.
- The chemistry sets the charger. Lead-acid, AGM, and lithium want different charging voltages. A charger set for the wrong type either undercharges or damages the battery. The flooded vs AGM vs lithium comparison explains the choice; the Eco-Worthy 280 Ah LiFePO4 ($399.00) is a common first lithium upgrade.
Where the charge comes from
There are four ways to put energy back into the battery, and a well set up RV uses at least two.
- Shore power through the converter or a smart charger like the Victron Blue Smart IP65.
- Solar through a charge controller. The Victron SmartSolar MPPT is the standard. Panels make DC; the controller converts it to the right voltage for the battery.
- The tow vehicle through a DC-DC charger like the Victron Orion XS. The 7-pin trailer plug's charge wire is too thin to do much; a DC-DC charger on a dedicated heavy cable does the job.
- A generator, which is just shore power you carry.
The shore vs generator vs solar comparison goes deeper.
Fuses and breakers: what they protect
A fuse protects the wire, not the appliance. Every wire leaving the battery needs a fuse or breaker rated below what the wire can carry, mounted as close to the battery as practical. The main battery fuse on an RV with an inverter is a large bolt-in type (MEGA, MIDI, or ANL). The 12 V panel then splits power into circuits with small blade fuses. On the AC side, breakers do the same job.
If a fuse blows, find out why before replacing it. If a fuse blows twice, stop and find the fault. Carry a full assortment; the Fuses & Circuit Protection collection has the Victron sizes.
The shore power cord and the pedestal
A 30 A RV uses a three-prong TT-30 plug and gets up to 3,600 W. A 50 A RV uses a four-prong plug and gets two 50 A legs, up to 12,000 W. Adapters let a 30 A RV plug into a 50 A pedestal (fine) or a 15 A household outlet (fine for lights and charging, not for the air conditioner). A 30 A cord, 20 ft, 10 AWG ($89.99) is the standard; anything thinner overheats.
Check the pedestal with a plug-in tester before connecting. Reversed polarity and open grounds are common and can energize the RV's skin.
Ground, chassis, and why it matters
On the 12 V side, the negative of the battery is connected to the trailer frame, and many circuits use the frame as the return path. A corroded frame ground causes dim lights, flickering trailer lights, and weak trailer brakes. On the 120 V side, the RV's ground is separate from its neutral and is bonded at the pedestal or generator, never inside the RV. Do not connect the two systems' grounds yourself unless you understand why.
Five things to check when "nothing works"
- Battery disconnect switch. Many RVs have one; someone turned it off.
- Main fuse or breaker at the battery. A blown main takes down everything on 12 V.
- State of charge. A battery below its cutoff (or a lithium BMS that has disconnected on low voltage or temperature) looks dead. Charge it.
- Shore cord and pedestal breaker. The pedestal breaker trips more often than the RV's.
- GFCI outlet. On the AC side, one tripped GFCI receptacle can kill several downstream outlets.
A sensible first upgrade path
Beginners often buy an inverter first. Do not. Buy in this order: a shunt monitor so you can see what is happening; a proper multi-stage charger for your battery type; solar if you camp without hookups; a DC-DC charger if you tow long distances; a lithium battery when the lead-acid one dies; and only then an inverter sized to what you actually run. The Victron buying hub lays out each step, and the 12 V vs 24 V vs 48 V guide matters only once you get to large inverters.
Shop this
- Victron SmartShunt Battery Monitor — from $99.99
- Victron Blue Smart IP65 Charger, 120V — from $52.50
- 30A Shore Power Cord, 20 ft — $89.99
- 15A GFCI Receptacle — $9.99
- Fuses & Circuit Protection (collection)
- Electrical & Solar (collection)