Because the starter battery isn’t very big, most conversion vans use a separate 12 volt power system to run lights, a fridge, fans, chargers, power for a diesel heater and so on.

If you want to run 120 volt appliances like an induction cooktop, microwave, hot water heater or similar, then you’ll also need an inverter to turn 12v DC power into 120v AC power.

You’ll need batteries to store this power, and a way to recharge the batteries like solar, your engine alternator, or a generator.

Luckily, people have been installing this type of stuff in boats for years. Unluckily as soon as you add the word “marine” to the description of a component, its cost seems to double compared to the regular household version. We ended up spending quite a bit of time and money on our electrical setup but we wanted to do it right.

If you’re new to all this stuff, read our intro to 12 Volt systems. After you’ve looked at that, the rest of this area of the site might make more sense.

The 12 volt system that comes with the Sprinter van

The van has a 12 volt system for the factory components. It relies on the van being driven regularly so that the small starter battery can be recharged. Running more than a few accessories for more than a short while will drain the starter battery so the van can’t even start.

That’s why you can specify an auxiliary battery when you order the van, or add one yourself. Our van came with one installed, but it can only handle recharging laptops and running a couple of devices. We wouldn’t recommend the factory auxiliary battery as the basis for a conversion. Here’s why.

Adding your own 12 volt system to run more things

The factory auxiliary battery is not good enough for a van you want to camp in. We chose to add a large “house” battery that will power all our 12 volt needs. Our solution is bigger than what most people may choose, but we did our usage calculations and this is what we decided on. Having a large battery means we don’t need a propane tank for cooking or hot water.

Lithium battery installed in van
Lithium battery installed in van

Here’s a guide we wrote to help you work out what size battery you need. There are also different types of batteries. We used a lithium-based one. Before you get all worried about lithium, read about the different types of lithium and lead-acid batteries, and why despite all the horror stories the lithium batteries used in van conversions are actually very safe. We’re convinced that the lifetime cost and convenience of lithium is a better proposition than using lead-acid batteries.

Recharging your 12 volt “house” battery

Charging from the mains

If you’re close to civilization, you can charge the house battery through a charger or inverter by plugging it in to the mains supply at home. Some people use a 30 Amp socket of the kind you find at RV parks, but we just use a regular 15 Amp extension cable.

Under the van, there are large holes in the rocker panel. Perfect for a cable with a hinged cover.

Charging from solar panels

The battery can also be charged using solar panels. We did some research and decided to use 265W panels made by Grape Solar because we could fit 3 on the roof of our 170″ wheelbase van and still have just enough room for a fan behind them. There are several mounting options, but you will probably either use a roof rack to mount them, or make use of the roof channels (if your van has them) to slide in little fixings for the edges of the panel. The panels are pretty rigid and don’t necessarily need much extra support.

Solar panels on the van roof

Your solar panel doesn’t produce a steady voltage or current, so it’s not a good idea to hook it straight to your devices or battery. Our panels, for instance, can produce 38 volts or more. That would fry any regular 12v equipment.

Luckily, solar controllers are designed to take the varying voltage and current and turn it into a steady output.

And not to worry – so long as you set your solar controller parameters correctly, your solar system will just hook straight in to the rest of your electrical set-up.

Crazy as it seems, you can get a tax credit for installing a solar system on your van if you live in it more than 10 days a year. Obviously, consult with your tax adviser to make sure this applies to you and your situation. Here’s IRS Form F5695 (PDF) and its instructions. The solar system credit is extended until 2021 now, but will step down in credit value from 2019 onwards.

Charging from the engine

It’s also possible to charge the battery from the engine. There are lots of ways of doing this, using the stock alternator or a dedicated alternator. The article describes why we chose not to in the end, and its based mainly on Mercedes’ recommendation not to draw the kind of current that you need for house battery charging from the alternator.

Using a generator

Please don’t do this. We use the van to get out into nature, so we hate camping near people who run their generators. It’s also a really inefficient way to charge a lead-acid battery, because the battery can’t take all the power that the generator provides. Even the little Honda EU generators are noisy and extra hassle. Because the Sprinter runs on diesel, you have to carry an extra gas can with you just for the generator. If you carry the can and the generator inside the van, the fumes are terrible, so that means strapping them on to the outside of the vehicle somehow.

Making 120 volts from the 12 volt battery

If you want to use 120 volt household appliances, you’ll need to add an inverter to create 120 volt AC power to run a microwave, induction cooktop, Vitamix, or other mains loads when you aren’t plugged in to shore power.

Be careful though – inverters are very power-hungry. If you want to run anything more than a small microwave, you’ll need a large inverter. And that inverter will need a large battery too. As a rule of thumb, you’ll need at least 400Ah of battery capacity for a 2kW inverter.

It’s really unlikely that you’ll be able to run a 120v AC unit from your battery unless you build a really large battery bank, install a massive inverter, and run a very efficient air conditioner.

Adding electrical components

Our build has a lot of electrical consumers. You may be able to get away with less, but it’s important to add up the power consumption from all of them when you’re calculating your battery needs.

Coving panel with LED strip lights
LED lights behind ceiling coving


We added several types of lights, including spotlights and LED strips. We moved some of the dome lights that came with the van, too.

We bought a microwave and induction cooktop. We use a small electric kettle and a 4 gallon electric hot water tank. The fridge is a high efficiency compressor fridge that runs on 12 volts.

The diesel heater needs electrical power to run its fan and glow plug element. We sometimes use a 12v mattress pad to warm the bed rather than heating the whole van. The ceiling fan and water pump are both electric. We also have an air compressor for inflating bike and van tires.

We have an internet amplifier and a cell phone booster that both need power to run, and several 12 volt outlets at various points around the vehicle.

Obviously, not all of this stuff is on at once. Most of it is hardly used at all during the day. The power draw from some of the items is minimal. However, it all adds up when you’re trying to work out how much capacity to install.

All the other bits you need

Building out a 12 volt system is very different to working with 120v household wiring. Cables tend to be much fatter and they need crimped lugs on their ends. Components are more expensive because they have to be built to withstand DC voltage arcs. Fuses aren’t sexy but they are very important.

Positive and negative bus bars with 4/0 and 2 AWG cables attached

It’s possible to learn all that you need to and to put together a safe, efficient system. But it takes some research and a lot of time spent reading manufacturer’s manuals.

Monitoring the electrical system

It helps to know how full or empty your battery is. In the same way that you wouldn’t drive without a fuel gauge, you shouldn’t use a battery without some form of monitoring. If you have a lithium battery, it should also come with a battery management system.

Victron Color Control GX system monitor
The electrical system controller

Most important of all to us was to make the whole electrical system usable without having to remember a whole bunch of rules or what position certain switches had to be in. We hid all of this complexity behind a really simple interface. All we really have to know is what percentage charge our battery has. If it gets below 40%, it’s time to either find some sun or find a wall outlet to plug in to – just like an oversize laptop!

Electrical system articles

See more Electrical articles