Gear
A second battery, a way to fill it, and fusing that will not burn the vehicle down. The rest is arithmetic.
Electrical work is the part of a build you cannot see when it is finished, and the part most likely to strand you or set the vehicle alight. It is also the part where the rules are written down by people who take fires seriously, and following them is not difficult.
This guide covers the house battery, how it gets charged, how to work out how big it needs to be, and the fusing and wire sizing that makes the whole thing safe. It names no brands except as the authors of the manuals it quotes.
The starter battery has one job: turning the engine over tomorrow morning. Anything that runs while the engine is off, a fridge, lights, a fan, chargers, belongs on a second battery, usually called the house battery, and the two must be separated when the engine is not running.
That separation is what a DC to DC charger does, and it does something else besides. Modern vehicles have what Victron calls "smart ECU controlled alternators", which vary their output voltage and shut down when they are not needed. A charger with engine-running detection "prevents the converter from discharging the starter battery when the alternator does not supply power", and on those vehicles controlled charging is, in the maker's words, indispensable.
With a lithium house battery there is a second reason. Charging it directly from the alternator "can overload the alternator due to the low impedance of the lithium battery". The controlled charger limits what the alternator is asked to give.
Lead-acid batteries, including AGM, are heavy, cheap, tolerant of neglect and unhappy about being deeply discharged. Lithium iron phosphate batteries are lighter for the same usable energy, take a charge faster, and will give you most of their capacity rather than half of it. They cost more and they have rules.
The rules matter. Victron's own manual puts the charging range of its lithium batteries at 5 to 50 degrees Celsius, and says charging outside it "may cause severe damage to the battery or reduce battery life expectancy". Below freezing, a lithium battery without a heater or a charger that knows to stop is being damaged every time you drive.
Two more from the same manual, which apply to any battery in a vehicle. The terminals "are always live, therefore, do not place metallic items or tools on top of the battery", so use insulated tools. And a battery that is not properly mounted, "if involved in a vehicle accident, can become a projectile".
Do not skipA battery is a block of stored energy with no off switch. Work on it with insulated tools, keep metal away from the terminals, prevent sparks nearby because a charging battery can emit explosive gas, and mount it so it cannot move in a crash.
The alternator is the fastest source you have and it works in any weather, which makes the DC to DC charger the first thing most builds need. Fit it close to the battery, but never directly above it, to keep it away from gassing.
Solar is the source that works while the vehicle is parked, which is exactly when you are using power. It is quiet and needs no thought, and it also needs a fuse between the panels and the charge controller.
Shore power matters only if you stay where there is any. If you fit it, the whole alternating current side changes character: the use of a residual current device is, in Victron's words, compulsory in all AC installations, and every touchable metal part of the vehicle must be bonded together to create a local earth. That is work for someone who does it for a living.
The size of the battery and the panels is not a matter of taste. It is arithmetic, and it takes ten minutes.
List everything that runs off the house battery. For each, take the current it draws in amps, multiply by the hours a day it actually runs, and you have amp hours a day. If the label gives watts instead, divide watts by the system voltage to get amps. Add them up. That total, multiplied by the number of days you want to go without a decent drive or a sunny day, is the usable capacity you need. For a lead-acid bank, double it, because you should not take more than about half out of it.
The figures to use are the ones on your own equipment, not averages from an article. A fridge is the exception that ruins estimates: its draw depends on the outside temperature, how full it is and how often it is opened, so measure it on a hot day rather than trusting the specification.
Then check the answer against reality: what the battery weighs and where it will sit, and whether the panels will fit where the sun actually reaches them.
A fuse does not protect your equipment. It protects the wire. Victron puts it plainly: a fuse "protects wires in a circuit from excessively high currents, which can cause overheating or fire", and "each consumer that connects to a battery needs to be fused", in the positive cable, individually, "no matter how big or small the power rating of the equipment is".
Where the fuse goes is as important as whether it exists. The US Coast Guard's rules for boats, which face the same vibration and damp as a vehicle, require a manually reset trip-free circuit breaker or fuse at the source of power for each circuit, within seven inches of it, or within forty inches if the conductor is inside a sheath or enclosure for the whole of that distance. A fuse a foot down the cable leaves a foot of unprotected wire against the chassis.
Sizing follows the cable, not the appliance. With one consumer, the fuse matches whichever is lower, the appliance rating or the cable rating; with several, it matches the cable. The Coast Guard rule for circuits under 50 volts is 150 percent of the amperage rating for the conductor size being protected.
Lithium adds one more. A lithium battery can deliver enormous short-circuit current, so at least one fuse in the direct current line must have an interrupt rating equal to or greater than the maximum prospective short circuit current of the bank. A fuse that cannot break the fault is not a fuse.
Do not skipEvery positive conductor from the battery gets its own fuse, at the battery end, sized for the wire. This is the single rule that separates a build that is safe from one that is waiting.
Current through a cable does two things: it drops voltage along the length, and it heats the cable. Undersized wire therefore both starves the thing at the end and cooks the insulation. Victron's guidance is to aim for a voltage drop no greater than 2.5 percent, and warns that in extreme cases cable heating can cause a fire.
The practical rules are short and worth memorising: keep cables as short as you can; use enough thickness for the length and the current; use quality lugs crimped with the right tool; make connections tight to the torque figure in the manual, but not tighter; keep contacts clean.
Conductors must be insulated stranded copper, and anywhere a cable passes through a bulkhead or a rigid surface it must be protected from abrasion. Every continuously energised terminal needs a boot, cap or cover so a dropped spanner cannot short it. And do not pull cables drum tight: vibration then works on the crimps and the battery posts until they loosen.
There is no federal building code for a homebuilt camper. The nearest thing is the set of standards the RV Industry Association requires of its members, who pledge in writing to build to them and accept unannounced plant inspections: NFPA 1192 for fuel systems, fire and life safety, plumbing and vehicle requirements; the ANSI and RVIA standard for direct current systems up to 60 volts; and the National Electrical Code for the alternating current side.
If you are paying someone to build your interior, those are the words to use. Ask which standards they build to, and ask for a wiring diagram at handover. A system nobody can trace is a system nobody can repair on a Sunday in the middle of nowhere.
Van and RV service centres, overland builders and van conversion shops do this work. The directory lists them by state.
Whoever does it, insist on three things: every positive conductor fused at its source, a labelled diagram of what you have, and a walk round the finished system explaining what to switch off when something smells hot.
On any modern vehicle with a variable-output smart alternator, and on any lithium house battery, a DC to DC charger is the right answer. It separates the batteries when the engine is off, and it protects the alternator from a lithium battery's low impedance.
Size the fuse to the wire, not the appliance. With one consumer it is whichever is lower, the appliance or the cable rating; with several, the cable. Boat rules for circuits under 50 volts use 150 percent of the conductor's rated amperage.
At the battery end of every positive conductor. Boat rules put it at the source of power, or within seven inches, or within forty inches if the cable runs inside a sheath for that whole distance.
Not without protection. Victron gives 5 to 50 degrees Celsius as the charging range for its lithium batteries and says charging outside it can cause severe damage. Use a battery with a heater or a charger that stops when it is cold.
Add up the amps each device draws multiplied by the hours it runs each day, then multiply by the days you want without charging. Double it for lead-acid, which should not be taken below half. Measure your fridge rather than guessing it.
Reviewed September 19, 2026. Conditions, closures and rules change; check with the managing agency before you travel.