12 Volt electrical systems for vans – the basics

Here’s a basic overview of how a 12 Volt system works in vehicles. If everything that goes on under the hood of your car is a mystery to you, this article might help out.

It also covers the most important pieces of adding a house battery and charging system to your van conversion, and the things you need to consider when you wire up your fridge, fan, and electrical outlets.

Disclaimer: I’m not a professional electrician. I have no qualifications in this space. Although I learnt the physics underlying this stuff back in school, there’s a big difference between theory and practice. I made a complex 12v electrical system work in our van without anything bursting into flames, but that doesn’t make me an expert. Consult other sources of information before you start your own build.

We cover lots of topics in this post. It’s worth reading all the way through, but you can jump to the piece you want using this index, too.

12V DC in a vehicle
12V DC in a van conversion
Calculating what size battery you need
   How battery size is measured
   Stated battery size is different to usable battery capacity
   Match battery size to your expected usage
Charging the 12V house battery
   Different charging options
   Three stage charging
   Charging voltages
   Knowing how full your batteries are
Linking the battery to the things it powers
   12 Volt cables are different to household wiring
   Choosing the right cable thickness
   Connecting cables to your devices
   Protecting the cables with fuses
   Simplifying wiring with bus bars and distribution panels
Safety first
Using 12V to make 120V
   AC versus DC - and why you care
   Inverters often charge the battery too
   Inverters are power-hungry
What next?

12V DC in a vehicle

Most vehicles on the road today use a 12 Volt electrical system. They use a battery for starting the car’s engine. They use an alternator that runs from the engine to provide the 12 Volts that recharges the battery and to run all the electrical components (lights, ignition, stereo, engine management system, etc.) while the engine is on.

Alternator, battery, and light bulb

What we call a “12 Volt” battery might actually provide anything from 11 Volts to 13.5 Volts depending upon what type of battery it is and how charged or discharged it is.

To recharge the battery, the alternator provides a voltage slightly higher than the battery’s regular resting voltage. This “pushes” more charge into the battery. Because the alternator is attached to the vehicle’s electrical system in just the same way as the battery is, this means that when the engine is running, the voltage in the whole system could be 14 Volts or higher.

When the engine is off, the battery has to keep providing power to things like the car’s alarm system, parts of the engine management system, the courtesy lights that come on when the doors open, the stereo system (so it doesn’t lose its programmed radio stations), and anything you’ve plugged in to the 12v sockets like your iPhone charger.

This can run the battery down over time. It’s why some of the 12v sockets switch off when the engine isn’t running. It’s also why your stereo may turn itself off a while after the engine stops. Obviously, things that take more power will run the battery down faster.

Car manufacturers don’t want to add weight or cost to their vehicles. They tend to specify the smallest battery that will do the job. The battery has to be able to start the car after it’s been sitting unused for a while, but that’s about it. Once the car is started, the alternator takes over to provide most of the power that the car’s electrical system needs.

The type of battery used in cars is designed to be kept fully charged. It’s good at delivering a lot of power for a short period of time, so the starter motor can turn the engine over, but it’s not designed to be left in a partially charged state for any length of time. In fact, doing so can damage it.

12V DC in a van conversion

Now let’s look at what’s different about the electrical stuff in your van conversion.

You’ll probably want some lights, some charging sockets for your electrical devices, and a way of powering a vent fan, fridge, water pump, the electrical control unit for a diesel heater, and so on.

These things run all the time, whether the engine is on or off. As a result they need a different type of battery than the starter battery. Rather than one that’s designed to give a lot of power for a short period of time, they need one that’s designed to give a little bit of power over a long period of time.

The type of battery used as the “house” battery in van conversions needs to be capable of being discharged more, and sometimes left in a partially charged state. This is known as a deep cycle battery. Typically this type of battery costs more than a starter battery because it has to be built differently in order to stand up to the harder way it’s being used.

Often, people put their house batteries inside the van. That also means using a different type of battery construction than starter batteries. Lots of the time, starter batteries are wet cell lead acid batteries. That means they are made of lead plates which are submerged in sulphuric acid. When they are recharged, that sulphuric acid heats up. It gives off hydrogen. That hydrogen vents out of the battery. Hydrogen is quite explosive. It’s perfectly fine under the hood of your car where your starter battery lives because it just wafts away. It’s not such a good idea inside the living space. That’s why it’s more common to see sealed lead acid (SLA) or absorbed glass mat (AGM) batteries used as house batteries. They don’t vent under normal conditions, so they’re safer to use inside.

Calculating what size battery you need

How battery size is measured

Batteries come in different sizes. The larger the battery, the more capacity it tends to have. The capacity is measured in Amp-hours. For instance a 100 Amp-hour battery could provide 10 Amps of current for 10 hours, or (theoretically) 100 Amps of current for 1 hour.

I say theoretically, because weird things happen if you try to suck the power out of a lead battery too fast. The guy who first documented this was called Peukert, so now it’s known as the Peukert effect. Basically, if you put a load on a battery that makes it discharge too fast, it doesn’t give you as much energy as it should. The rest gets lost to heat and inefficiencies in the chemical reaction going on inside the battery.

Why should you care about that? Well, if your house battery is too small for how you use it, the power will be sucked out of it too fast, and so you won’t even get the rated Amp-hour capacity from it.

Stated battery size is different from usable battery capacity

Another important consideration is that lead-acid batteries should not be used all the way until they are empty. In fact, most recommendations say to only use them half way before you recharge them. Using more than 50% of their charge can reduce their ability to recharge. The further they are discharged, the more damaged they are likely to get.

Lithium batteries don’t suffer nearly as much from the Peukert effect and they can be discharged by 70% to 80%, so you can specify a smaller capacity lithium battery for the same type of use. They are also physically smaller and lighter than a comparative lead-acid battery, they have a longer life, and the kind used in conversions isn’t likely to catch fire like hover boards and Boeing Dreamliners. The big issue with lithium batteries is that they cost more to buy up front than lead-acid batteries. You can learn more about the differences between battery types here.

Match battery size to your expected usage

Each of the things you want to run from your battery consumes power. It’s normally written on the device either in terms of Amps or Watts. You can convert between Watts and Amps. Once you know how many Amps all of your devices use, and how long each of them is used for every day, you will know how many Amp-hours of power you need each day.

Multiply this by the number of days’ usage you need between recharges. That’s how much battery capacity you need to run your devices on the road. Then, you double this number (because your lead-acid battery can only be discharged to 50%), and that gives you an estimate of what capacity of battery you need.

I’ve written a step-by-step guide to doing this calculation. Check it out and see what your power needs are.

This whole calculation thing might seem like hassle, but making a mistake in your battery purchase is expensive when you find you need to upgrade because your batteries keep running down too far. Better to do the thinking now and get it right first time.

Charging the 12V house battery

So far, we’ve talked about how much power you’ll want to get out of your battery. Now it’s time to think about how to put energy back in to it.

House battery charging options

Different charging options

The different ways you can charge your house battery are:

  • Using the van’s alternator (the same one that charges the starter battery and powers the van’s electrical system).
  • Using a second, dedicated alternator just for the house battery.
  • Using a generator mounted to the van.
  • Using solar panels mounted to the van.
  • Using “shore” power – mains electricity from an outlet.

There are pros and cons to each approach.

Using the van’s alternator is a relatively cheap option, because the alternator already exists, and the components you need to hook in to it are pretty cheap. But Mercedes has some warnings about hooking stuff into the van’s system, and most sensibly sized house batteries fall outside the limits they set.

Using a second, dedicated alternator is a great option if you can afford it. The alternator can charge your house batteries fast. But the whole installation can cost a couple of thousand dollars.

Lots of commercially available RVs include a generator that’s powered either from the vehicle’s fuel tank or from the propane tank. They tend to be noisy, not too efficient, and not too reliable. They are also expensive to install. However, they do provide on-demand recharging wherever you are.

It takes a large area of solar panels to successfully recharge a house battery. We’ve covered almost our whole van roof in panels and that works as our primary charging source. The cost of buying that many panels, the roof mounting brackets, and the solar charge controller can again be a couple of thousand dollars. That’s probably why most people use solar as a secondary charging source, with just enough capacity to keep essential items like the fridge running.

Most vans end up also installing a shore power connection. This is basically a way of getting a 120 Volt supply from a regular household outlet (or the 30 Amp outlets at campgrounds) into the inside of the van. From there, you can run the 120 Volt power to a battery charger or an inverter. This is often a cheaper option, but it does require you to park up in places where you can reach an outlet. Lots of people build conversion vans to escape from civilization, and there aren’t many outlets in the wilderness.

Each of these charging sources should get run through some kind of charge controlling device so that they deliver the right voltage and current to the battery to make sure it gets charged properly.  The solar panels use a solar charge controller. The inverter and battery charger have the controls built in. The dedicated alternator kits come with a controller. Using the van’s alternator or a generator does not provide this level of control unless you add a DC-to-DC charge controller.

Obviously, you can install more than one charging option. In fact, you can use more than one charging source at the same time without much trouble, so long as they each have a way of controlling how much charge they provide.

Three stage charging

Lead-acid batteries don’t like to be charged. The fuller they get, the more they resist having more power pushed in to them. For that reason, good chargers change the way they charge the battery depending on how full or empty it is. They have a bulk charging mode for when the battery is quite empty and isn’t resisting being charged. They have an absorb charging mode for when the battery is getting fuller. Then they have a float mode for keeping the battery topped off once it’s full.

Chargers with this three-stage system cost more than basic ones, but they are able to keep your battery in a better condition so it will last longer. The comparative cost of the charger versus the battery means it’s worth your time buying a better charger so you don’t have to keep replacing the expensive batteries so often.

Charging voltages

Different battery types take different voltages to charge. Wet cell lead-acid batteries charge at a different voltage than Gel or AGM lead-acid batteries. Different manufacturers specify different charge voltages even for batteries that use the same chemistry.

Lithium batteries use a different voltage to lead-acid. In fact, there are even different types of lithium battery chemistries, and each of those charge at different voltages.

This is important because if you charge the battery at the wrong voltage, it either won’t charge fully, or you can damage it. That means your charging sources need to be matched to your battery. Normally, that means buying programmable chargers so you can enter the exact voltage your batteries need. Chargers with a programmable function cost a bit more money, but it will be cheaper in the long run because it will keep your batteries in better condition so they’ll last longer.

Knowing how full your batteries are

Like the gas gauge in your car tells you how far you can drive, a battery monitor lets you know how much power you can use, and when you need to recharge.

The battery’s voltage is one way of measuring how much power it has left. The voltage drops slightly as the battery gets used more. Cheap battery gauges display the voltage and let you work out what that means in terms of charge left. Unfortunately, it’s not very accurate.

A more accurate way is to use a battery monitor that actually measures how much power goes in to and comes out of your battery. This type of monitor has a special device called a shunt that hooks on to the battery. From this device, the monitor can tell you your state of charge, in other words what percentage of the battery is left to be used. Because it directly measures how many Amps go in and out of the battery, it is much more accurate.

Linking the battery to the things it powers

There’s one interesting thing about wiring a van, and that is that the whole van is made of metal, which is a good conductor of electricity. Normally that’s a problem – you don’t want your cables to short out against the van body. However it’s also a benefit.

Vehicles almost always use what’s called a negative ground system. They use the vehicle chassis as the negative cable. That means you can run a short piece of wire from the battery negative terminal to the chassis, and then run another short piece of wire to each of your devices from a part of the chassis close to them. You still have to run the positive cable all the way from the battery (or distribution panel) to each device, but you end up needing a lot less cable overall.

You may decide not do to this for your house battery system. Although it works fine when it works, it’s not always easy to make good, electrically conductive connections to the vehicle chassis. Those connection points might well be hidden away inside your walls and ceiling after you finish the conversion process. If one comes loose, the device attached to it will stop working. Then you have to spend time troubleshooting. That might mean ripping half of your van apart.

We ran both positive and negative cables to each device and outlet. Our house battery is not attached to the van chassis. Theoretically you could pull our entire electrical system out of the van and it would still work just fine. There are some (slight) safety advantages to doing things this way, and it means you don’t have to search for grounding points on the chassis, or grind the paint off to make sure you have a good electrical contact.

12 Volt cables are different to household wiring

The type of cable used in 120 Volt AC household wiring systems is normally 12 gauge solid copper. It has two insulated conductors for power (live and neutral) and a bare earth wire. You can join two pieces of cable using wire nuts that twist on to the copper wire.

Household wires and wire nuts

It’s not much use in a van conversion. Yes you can use it, but there are several reasons why you should not use it.

  • The solid copper wire is not flexible. It can get a lot of vibration in a van, which can cause it to snap at its connection points. That’s especially true if you use screw-on wire nuts to hold it together, because they score the copper cable, making it thinner and creating a stress point.
  • The cable normally comes with three conductors, whereas you need two or just one if you are using negative ground wiring.
  • The wire doesn’t bend well around the tight corners you’ll find in a van.
  • It’s harder to crimp the right kind of cable ends on to solid copper wire.
  • 12 Volt DC supplies often need (much) thicker cable than 12 gauge.

Instead, when you’re wiring a van, you probably want to use multi-strand cable.

4/0 welding cable and lugs

Multi-strand cable is still made of copper. However, it uses lots of thin wires twisted together instead of one thick one. This makes the cable more flexible. The multiple thin wires also work better with the crimp-on connectors that you use in a van.

Choosing the right cable thickness

Because they work at a lower voltage, 12 Volt devices sometimes need more current than 120 Volt devices. You can think of current as the “flow”of electricity. In order for the larger current to flow properly, you need thicker cables.

If your cables are too thin, they will create electrical resistance. They are resisting the flow of the current. Resistance is wasted power. It gets wasted in the form of heat, which can be dangerous.

Think about how a toaster works. It runs current through a grid of very thin wires. That makes the wires get hot. Using the wrong gauge of cable in the van turns it into a toaster. There are much more efficient and less dangerous ways to heat the inside of your van.

It’s pretty easy to use an online cable thickness calculator. To tell you the correct cable to use, it needs to know what voltage you are running, how many amps might be flowing through the cable, and how long the cable is. Then, it will tell you what gauge of wire will meet your needs.

Connecting cables to your devices

To connect the multi-strand cables to the battery and to your electronic components and devices, you’ll need crimp-on cable ends.

crimp-on cable ends

These ends are made in different sizes for different cables. For most of the cables you use, you’ll want one of the three most common sizes, which are color coded red, blue and yellow. Red is for cables up to 18 AWG. Blue is for 16 to 14 AWG, and yellow is for 12 and 10 AWG. Yes, wire gauge numbers get smaller as the wire gets thicker. You can crimp them on with a regular cable crimper.

Your batteries, inverter, solar charge controller, and some of the other devices in your system will need much thicker cables. 8 AWG, 2 AWG, 0 AWG or even (in our case) 4/0 AWG, which is four sizes larger than 0 AWG!

These take a different type of lug, which needs to be very well attached to the cable end. A bad connection causes more resistance, which (you guessed it) leads to dangerous heat build-up.

You can read all about the crimping process here.

Most of the time, it’s worth also covering the crimp connection with some heat shrink tubing. This helps to insulate the connection, adds a little strength and strain resistance, and (if you use the right kind of heat shrink with glue inside it) waterproofs the connection so it can’t corrode.

Protecting the cables with fuses

Fuses are a great way of protecting your van. We’ve talked about how too much current in a cable can make it heat up. Fuses are designed specifically to heat up and melt before the current gets strong enough to melt the cable that the fuse is attached to.

That’s good news for a couple of reasons. First, fuses are normally all in the same place, in a fuse box, so it’s easy to find the burnt-out one and replace it. Secondly, fuses are a lot cheaper to replace than burnt-out cables would be. Thirdly, burnt-out cables tend to also burn your van down. Fuses are a lot cheaper to replace than a whole van.

You’ll notice the description above talks about the fuse protecting the cable, not the devices you put at the other end of the cable. Those devices also need their own fuses, but when we’re wiring a van we’re most concerned about making sure the cables themselves don’t get damaged by excess current.

Simplifying wiring with bus bars and distribution panels

You could run wires from each of your devices straight back to your battery and connect them all to the battery terminal. But that’s not sensible.

We just covered the need for fuses. It would be hard to put fuses on each wire if they are all individually connected to the battery.

Your battery probably won’t be in a place that is easy to get to on a regular basis. It’s better to run a single fat wire from your battery to a fuse box or distribution panel in a convenient location, and then run the wires from that panel to each device.

Blue Sea 6-fuse block

Distribution panels are just fancy fuse boxes. Rather than fuses, they tend to use breakers. Breakers sense when the current is too high and rather than melting a fuse, they just switch the circuit off. That makes them easier to reset. But of course, you need to work out why the breaker tripped in the first place before you just switch the circuit back on.

The distribution panels - DC, AC, shore power and Victron BMV700

In some electrical system designs, it also makes sense to use bus bars. These are strong pieces of metal with attachment points on them. The metal is insulated from everything around it. Bus bars are designed to let you run multiple cables to one location without dangerously clamping them all to a single terminal.

Clamping to a single terminal is dangerous because the cable ends might not form a solid electrical connection with each other. Then they’ll have more resistance to the current flowing through them, so they’ll get hot, and you know the rest of that story by now.

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

Bus bars remove that problem by giving each individual cable a home. It also makes the wiring job easier because you don’t have to bend all your cables into weird positions to get them to join together.

Safety first

You aren’t likely to get much of a shock from touching the two terminals of a battery with your hands. A 12V DC system doesn’t have the “ooomph” to hurt you that way.

But the battery is storing a lot of power. If you drop a wrench over the terminals, or if you’re wearing a metal watch strap or a metal bracelet or necklace and it touches across the terminals, it will suddenly have a lot of current running through it.

That will make it heat up. Lots. People get bad burns from jewelry that shorted out batteries. Discharging the battery that fast also makes it heat up. The heat boils the acid inside the battery and creates a lot of hydrogen gas. Lead-acid batteries can explode in that situation.

You can get a shock from other components connected to the battery. Some of them convert the voltage to one which can hurt you. You can also get a fatal shock from the 120 Volt components in your van.

The best advice is to always switch off and disconnect the power source before you work on the electrical system, and to always treat wires as if they are connected. Make sure your battery terminals and bus bars are always covered so that things can’t short out against them. Always install fuses in the system so that the fuse blows before the wire or the battery get too hot.

Using 12V to make 120V

Some of the electrical devices you want to use in your van might be household appliances that run on 120 Volts. For that, you need something that can turn your 12 Volt DC power into 120V AC power. That device is an inverter.

AC versus DC – and why you care

It’s time to talk about the terms DC and AC. DC stands for Direct Current. The current flows one way in the circuit. AC means Alternating Current. The current flows backward and forward (alternates) 60 times every second (50 times in the UK). There are advantages to each approach. For various historical and practical reasons, vehicles standardized on DC, whereas houses standardized on AC.

Once you start using house-style devices in a vehicle, you need a thing that converts from your battery’s 12 Volts DC to the 120 Volts AC that the device needs.

Different inverters do this conversion in different ways. Some just switch the current back and forth. If you plotted how this looks on a graph, it is a square wave.

A square wave signal

The current provided by your energy company to your household outlets has a smooth transition between going forwards and going backwards. It looks more like this.

Sine wave signal

This is a sine wave. Some devices – especially microwaves – expect the current to move smoothly between going forwards and going backwards. If it doesn’t, they don’t work very well. They may hum loudly and work on reduced power, or they may not work at all.

As you can imagine, sine wave inverters cost more to make than square wave ones, because it’s harder to turn the DC power into a smooth AC current. There is also a middle ground, where some inverters create a modified sine wave.

Modified sine wave signal

Modified sine wave inverters cost less than full sine wave ones. They work fine for some devices, but not so well for others.

So, when you’re choosing an inverter for your conversion, you need to think about whether you can get away with a cheap square wave one, whether you need to step up to a modified sine wave one, or if you need to lay down the cash for a pure sine wave model.

Inverters often charge the battery too

Lots of inverters also double as battery chargers. Because they can convert from 12 Volts DC to 120 Volts AC, it’s pretty easy for them to convert back from 120 Volts AC to 12 Volts DC. So, plugging power from your house in to the inverter lets it recharge the battery.

As we already mentioned, you need to be able to tell the inverter what type of battery it is charging. The better quality inverters from Magnum, Outback, or Victron have that capacity. Sometimes though you need to buy additional remote controls or programming devices to change the settings.

Inverters are power-hungry

When an inverter is creating enough power to run your microwave, it’s sucking a lot of juice from your battery. For that reason, your battery needs to be sized properly so that it doesn’t get hurt by the demands the inverter places on it.

As a rough recommendation, you’ll need at least 400 Amp-hours of battery capacity in order to run a 2kW inverter safely.

What next?

OK. We’re at more than 4,000 words now. That’s plenty for an intro article. We’ve looked at how 12 Volt systems work in cars, how you might want to install your 12 Volt house battery, what size it needs to be, how to charge it, and how to connect other devices to it, including an inverter to make 120 Volt AC power.

Once you understand the basics, lots of the things they talk about in manuals for batteries, chargers, solar systems, inverters, and other devices will make a lot more sense.

You need to read those manuals because it’s up to you to set your whole system up so that all the components can work together safely. The manuals will tell you what size of fuse to use, what type and size of cables are recommended, what charging voltages and discharging currents are acceptable, and so on.

Obviously there’s a lot more information to learn about than we can cover in this article. Check out the links in the text on this page, the other information in the electrical section of this site, the information available on the Sprinter forum, and any other sources you can lay your hands on. One book you might want to read is “Managing 12 Volts – how to upgrade, operate and troubleshoot 12 volt systems” by Harold Barre.

9 Replies to “12 Volt electrical systems for vans – the basics”

  1. Did you make the copper connectors shown at the ends of your bus bars (with heat shrink on them) or are they something you buy?

    1. Terri, we made those connectors from copper bar stock. If you’re planning on doing the same thing, you can calculate the required cross sectional area using one of the online cable calculators – just be sure to specify that it’s solid copper rather than stranded. Stranded can be a slightly smaller cross section for the same load because it is better at dissipating heat.

      Also, it’s important to keep the areas that will be in contact with your buss bars and other components as flat and smooth as possible. You want to maximize the contact area in order to reduce resistance.

      1. Thanks, good to know.

        I just came across your website from a link from the sprinter forum. What a great site you’ve created! (Not to mention the van itself.) I’m very impressed. Thanks for sharing so much information and presenting it so well.

  2. Hi Diesel fumes, question for you. The Blue Sea DC panel ships with 15 amp breakers. When connecting a line that has a maximum 5 amp draw for example, is it necessary to replace that breaker with matching 5 amp breaker, fuse the line or leave as is?

    1. Theoretically, the fuse in the breaker panel is protecting the wire, not the items attached to the wire. Obviously though it can protect both.

      You normally size the fuse for the gauge of wire you used, and then ensure that anything you put on that circuit is also individually fused. However, if you have dedicated circuits for certain items (roof vent fan, water pump) then you can replace the breaker with one that’s the correct size for your consumer item. We did that on several circuits.

      On the Blue Sea panel, it’s really easy to swap the breakers out.

      1. Thanks for the clarification. I was second guessing after spotting one of your panel photos having all 15 amp breakers. I’m going to upgrade several breakers then since they are all dedicated runs. I have

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