Updated on May 25th, 2025
If you want lights, running water, or fans in your van a 12-volt van electrical system is a must. Our 12-volt system includes our roof vent fan, reading lights, overhead lights, and a small water pump for the sink. We recommend taking the time to really plan your system and draw up a wiring diagram before you start.
A quick safety note: if you have never done any electrical work before please proceed with caution. While a 12-volt van electrical system is a great first electrical project, it is still dealing with electricity. This is not a project to be a bull in a china shop with, there is a potential of injury or fire. Ask a friend to help, or we would be happy to answer any questions!
Disclaimer: In the interest of transparency, please be aware that this post contains affiliate links and any purchases made through such links may result in a small commission for us at no cost to you. We only recommend products and services that we use, love, and trust.
We put together a post that covers all the basics of van electrical systems when we built our second DIY van, you can check it out here!
Materials
- Reading Lights
- Disk Lights
- Fuse Box
- Isolator
- 150 Amp Fuses
- Switches
- Fan
- Inverter
- Battery Switch
- Water Pump
- Circuit Breaker
- Heat Shrink
- Ring Terminals
- Battery Capacity Monitor
- Spade Connectors
- Battery Side Post Extensions
- Terminal Blocks
- Ammeter and Current Shunt
- Rosin Core Solder
Tools:
Planning your 12 Volt Van Electrical System
Batteries
Battery Types
Three common types of batteries typically used in vans are, Flooded Lead Acid (FLA), Absorbed Glass Mat (AGM), and more recently Lithium Iron Phosphate (LifePo4). The type you choose would depend on your budget, how long you plan on owning the van, and how you use it.
Flooded Lead Acid Batteries (FLA) have a liquid electrolyte that is vented to the atmosphere. FLA batteries are the least expensive, but require the electrolyte to be topped up occasionally with distilled water and are prone to spilling. When charging these batteries also produce hydrogen gas that must be vented outside. You can only repeatedly discharged FLA batteries to 60% without damage.
Absorbed Glass Mat (AGM) batteries have an electrolight stored in a sponge-like mat that is totally sealed to prevent leaks or the need to top up the electrolyte. These batteries have a lower internal resistance than FLA batteries which means it can charge and discharge faster. You can mount AGM batteries at any angle and they resist vibration damage because the electrolyte is stored in a matt. You can discharge AGM batteries to 50% without damage.
Lithium Iron Phosphate Batteries (LifePo4) are pretty new to the market. These batteries are smaller, lighter and have a much longer life than FLA and AGM. Additionally, they have an extremely low internal resistance. This means it can be recharged quickly and cycled down to 20% without harm! What’s the downside then? LifePo4 batteries are very expensive upfront but cost less in the long run due to a longer life. They require a sophisticated battery management system (BMS) and special chargers. LifePo4 batteries require BMS systems to prevent damage to the battery cells if the temperature or voltage gets too high or low. Most LifePo4 used in vans are self-contained units which have the BMS built into the battery itself. The other option is to build it yourself from loose cells and external BMS, unless you really know what you are doing, I would steer clear of this option.
Since we are doing a budget, weekend warrior / road trip / adventure van, we went with the middle of the road option AGM.
System Capacity
You also need to figure out how big your battery bank needs to be. This means calculating how much power each one of your loads consumes and how long you use them for. Multiply the 12v amperage draw by the number of hours you will use them to come up with amp-hours (AH). For items running off a 120v inverter, multiply its amperage by 10 and then by the number of hours to get amp hours at 12 volts. We will go into greater depth about inverters in our 120-volt post.
This is how our loads broke down:
- Fridge 2 Amps X 25 hrs = 50AH
- Roof Fan 1.5 Amps X 8 hrs = 12 AH
- Lights 1.5 Amps X 2 hrs = 3AH
- Laptop .5 Amps X 10 X 3 hrs = 15AH
- Phone .1 Amps X 10 X 6 = 6 AH
Our total daily power consumption is calculated at 86 AH. Since the AGM batteries we selected can only be discharged to 50% we would need a total of 172 AH assuming that we charge the batteries every day by either solar or from the van alternator.
Charging
When setting up your 12-volt system in a van you have two main ways to charge batteries, with the engine alternator or solar. Which one or both you go with will depend on how you use your van. How long will you stay in one spot? Is it sunny all the time? If you plan on being pretty stationary look into solar or shore power. If you are like us and plan on traveling a lot you can get away with charging from the van alternator.
Our original plan was to go with a system to charge batteries only off of the van alternator, but at the last moment, we got a good deal on solar panels so we decided to install a 200W solar system. We will cover that in our solar post.
To charge the batteries off of the van alternator we used a battery isolator. These are sometimes also called a combiner or automatic charging relay (ACR). This device senses when the engine is turned on and electrically connects the engine battery to the van or “house” battery so they can be charged together as one big battery. This device also senses when the engine is turned off and disconnects the engine and van battery. This is important because you don’t want to inadvertently run down your engine battery, and be stranded without being able to start! This isolator is a good solution for us because our van alternator output voltage (13.9-14.2v) works well with AGM / FLA batteries. Lithium batteries and/or newer vans with smart alternators with varying output voltages will require a different solution such as a battery to battery charger.
Putting the 12-Volt Van Electrical System Together
Now that you have all major components for your van 12-volt system you have to put them together.
Placing Components
Location is important. You need to keep in mind the weight of the batteries and the weight of the other parts of your van. Batteries can weigh a lot and you don’t want to have all of your heavy items on one side. It’s also important to consider where you place your components because you will want to keep your wire lengths as short as possible, especially with the larger gauge wires. Long wires can be expensive and keeping them short reduces voltage drop.
We chose to put our batteries under the bed at the back of the van and on the opposite side of the cabinets and fuel tank. Mounting the batteries at the rear did mean we had to run a long wire for our isolator but it was worth it for weight distribution reasons. We mounted out invertor, isolator and fuse block inside the bed frame under the mattress.
Wire
Vans are not like houses. They move, they flex, and bounce down the road. For that reason, you should only use stranded wire in your van 12-volt system. Solid core wire such as Romex is not designed for the vibration seen in a van and are subject to work hardening and cracking. A good source of smaller guage stranded wire is heavy-duty extension cords. They have a built-in protective jacket. Larger stranded wires can be found online, marine stores or even welding supply stores, but will likely cost you more.
Wire Sizing
The van wire needs to be sized appropriately for the current (amps) it will carry. The voltage has no effect on the ampacity of the wire (only on the insulation required). There are many online tools to calculate how big the wire needs to be I like the one by Blue Sea Systems. When using these tools, don’t forget distance is a round trip!
Typically up to 10% voltage drop is ok for most circuits. Fridges, chargers or combiner wiring which should be limited to no more than 3%. Voltage drop is the amount the voltage will reduce within a circuit due to the resistance in the wire. A 12-volt circuit with a 10% voltage drop will see 10.9 volts at the other end, which is just fine for lights, and small motors such as fans. Battery chargers and combiners which rely on an accurate voltage to function will perform poorly if the voltage drop is too high. Fridges and larger electric motors will run more efficiently with a lower voltage drop.
There is no downside to oversizing wire beyond cost and weight. Undersized wires, on the other hand, could be either cause issues with whatever you are powering or in extreme cases cause a fire.
In our van we used 4 gauge wire for our inverter feeds, 6 gauge for the isolator and to feed the fuse block. All of our branch circuits are 16 gauge.
Wire Connections
At the end of each wire most likely you will need some sort of connector to attach the wire to your device. The type of connector you will use will depend on the device. Most likely you will need a spade terminal, ring terminal, or butt connector if it has wire leads.
For smaller connection 10 gauge and smaller in dry locations, I recommend using an insulated terminal along with a decent set of crimpers. If you need help with this check out our wire crimping video. In wet locations, such as under the hood or underneath your van, use either heat shrink connectors or if you can solder the connections and protect with adhesive lined heat shrink. If you need help with this check out our soldering video.
For larger connections, I used an uninsulated terminal that is either soldered, crimped or a combination of both. Afterwards, I like to cover the connection in a piece of adhesive lined heat shrink. This provides both a waterproof connection as well as a decent strain relief as well as looks pretty spiffy.
Circuit Protection
All circuits need some sort of overcurrent protection to protect the device you are powering and the wiring itself. In vans, fuses and circuit breakers are common. Fuses are the least expensive but require replacement after an overcurrent event (hopefully not very often!). Circuit breakers have a button or lever to allow them to be reset. Avoid the temptation to use household circuit breakers, they are not rated for DC use.
In a typical installation, you would have a centralized 12-volt fuse block or circuit breaker panel that feeds all of your smaller loads or branch circuits such as lights, fans, 12-volt outlets, fridges, etc. Larger loads such as inverters and fuse block or panel feeds should be fused Individually. Inline fuses or circuit breakers are very common for inline use.
You should size the Fuses or circuit breakers to protect the wiring or the load whichever is less. Hint, it should always be the load if the wire is sized correctly! Often, the manufacturer of the fan, light, fuse block, etc. will provide a recommended fuse size. If that is not available, many online calculators are available to help you size your fuse or circuit breaker properly. Again, I like the Blue Sea Systems.
In our installation, we have a 150A fuse that feeds our inverter and battery isolator. Off of that 150 amp fuse, we have a 50A circuit breaker that feeds the fuse block. At the fuse block, we have a either a 3, 5 or 10 amp branch fuse to protect each individual circuit.
Monitoring Your 12 Volt Electrical System
Now that you have your system put together (or spec’d out) you need some method of monitoring the state of the battery(s).
There are many options, from a simple ten dollar voltmeter all the way up to several hundred dollar Bluetooth enabled battery monitors. These pricer options give you amp-hours, voltage, amps, time to empty as well as historical data for weeks or months. At the minimum, you should have at least a voltmeter that you can use to indicate the state of charge.
Again, we went with a middle of the road solution, composed of a battery percentage gauge and combination volt/ammeter. The battery percentage gauge required only connection to positive and negative. We simply hooked to our fuse block via a switch which allows us to turn it off at night since it’s quite bright.
The volt/ammeter also required a battery connection which we connected again to the fuse block through the same switch. In addition to the battery connection, the volt/ammeter requires a current shunt. You will mount the shunt in the main negative cable going to the batteries.
A current shunt allows you to remotely measure amperage without having to run huge battery cables up to your gauge. It works by installing a resistor of known value and measuring the voltage drop across it. As the current goes up so does the voltage drop proportionally and the gauge calculates the amperage from that. Our gauge is very inexpensive and was unable to measure current in and out of the battery. In other terms, if could not handle a negative value from the shunt. To work around this limitation I installed a double throw double pole switch allowed me to reverse the voltage drop signal from the shunt. Flip the switch one direction to read current into the battery (charging) and the other to read current out of the battery (discharge).
As always, we are happy to answer any questions you may have about installing a 12-volt van electrical system. Comment below or email us at info@abroadreachtravel.com!
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