Electrics are the part of a self-build that puts most people off, but a basic 12V campervan system is built from a handful of parts that each do one simple job. This guide explains what those parts are, how they fit together, and the few numbers you need to understand before you buy anything. It’s aimed at complete beginners planning a UK conversion, and it points you to the manufacturer and industry guidance worth reading before you pick up a crimping tool.
If you’re still at the planning stage, our step-by-step guide to converting a van shows where electrics fit into the overall build order.
The three systems in a campervan
Most conversions end up with up to three separate electrical systems. Keeping them clear in your head makes everything else easier.
| System | What it powers | Who usually fits it |
|---|---|---|
| Vehicle 12V | Engine, lights, dashboard, starter battery | The van manufacturer (leave it alone, apart from a charging connection) |
| Leisure 12V | Lights, water pump, fridge, USB sockets, heater controls, fans | Many self-builders fit this themselves |
| 230V mains (hook-up) | Sockets and a mains charger when plugged in at a campsite | A competent electrician should install or at least inspect and test it |
The leisure 12V system is the heart of a campervan build, and it’s what the rest of this guide focuses on.
Volts, amps, watts and amp-hours in plain English
You only need four terms to read a product label or a wiring guide.
- Volts (V) are the electrical “pressure”. A campervan leisure system is nominally 12V.
- Amps (A) are the rate of flow of current. Cable and fuse sizes are chosen by amps.
- Watts (W) are power. Watts = volts × amps, so a 60W appliance on a 12V system draws about 5A.
- Amp-hours (Ah) describe how much a battery stores. In theory a 100Ah battery could supply 5A for 20 hours.
A simple worked example
Here’s how you would estimate daily use. The appliance figures below are example numbers only; take the real current draw from each product’s label or manual.
| Appliance (example figures) | Current draw | Hours per day | Amp-hours per day |
|---|---|---|---|
| LED lights | 1A | 4 | 4Ah |
| Water pump | 5A | 0.25 | 1.25Ah |
| Phone and laptop charging | 3A | 2 | 6Ah |
| Compressor fridge (average) | 2A | 24 | 48Ah |
| Total | about 59Ah |
That daily total is what you size your battery and charging around.
Leisure batteries: AGM vs lithium
The leisure battery stores energy for when the engine is off and you’re not plugged in. It’s separate from the starter battery, so running the lights all evening can’t leave you unable to start the van.
The two common choices in self-builds are AGM (a sealed lead-acid type) and lithium iron phosphate (LiFePO4). The key difference for a beginner is how much of the label capacity you can actually use. Victron Energy says the generally accepted economic and practical depth of discharge is 50% for AGM batteries, while 80% is used for LiFePO4 (Victron Energy: Lithium-ion vs AGM).
So in the example above, roughly 59Ah a day would need about 118Ah of AGM capacity, or about 74Ah of lithium capacity, before you add any margin for cloudy days or extra nights.
| AGM | Lithium (LiFePO4) | |
|---|---|---|
| Usable capacity (planning figure) | About 50% | About 80% |
| Weight | Heavier | The Camping and Caravanning Club says typically about half the weight of an equivalent lead-acid battery |
| Price | Lower | Higher up front |
| Cold-weather charging | Possible with the right charger settings | Cannot be charged below 0°C |
| Charging kit | Most chargers have an AGM setting | Needs chargers with a lithium profile |
The Camping and Caravanning Club’s leisure battery guide notes that LiFePO4 batteries have their own on-board charge management, can’t be charged below zero degrees Celsius, and that quality varies, so it’s worth buying from a reputable retailer. It also says an AGM battery’s charge voltage must not exceed 14.4V, though you should check this with the manufacturer.
Weight matters in a van, too: every kilo of battery comes off your payload, which we covered in the conversion guide.
How the leisure battery gets charged
There are three common ways to top up a leisure battery, and many vans use all three.
1. From the engine while you drive
Older vans often used a split charge relay (or voltage sensitive relay, VSR), which simply connects the starter and leisure batteries when it detects the engine is charging. Nohma explains that Euro 5 and Euro 6 vans commonly have a “smart” alternator controlled by the engine ECU, which drops its voltage before the starter battery is full, so a voltage sensitive relay won’t properly charge a leisure battery on these vehicles. Some smart alternators also produce voltage peaks from regenerative braking that can damage a leisure battery (Nohma: split charge relays).
The usual answer on a modern van is a DC-DC charger (also called a battery-to-battery or B2B charger). It takes power from the starter battery while the engine runs and delivers a proper multi-stage charge to the leisure battery, with a setting for the battery type.
2. From solar panels
Roof-mounted solar panels charge the battery through a solar charge controller, which manages the voltage going into the battery. Solar output varies a lot with the season and weather, so treat it as a top-up rather than something to rely on in a UK winter.
3. From mains hook-up
When you’re plugged in at a campsite, a mains battery charger (often built into a combined unit) charges the leisure battery from 230V. Mains electrics in a motor caravan are covered by Section 721 of BS 7671, the UK wiring regulations, which sets requirements for the inlet and a main isolator inside the van (IET Wiring Matters). Unless you’re qualified, have a competent electrician install or inspect this side.
The parts of a basic 12V system
A simple leisure system usually includes the following, wired roughly in this order from the battery outwards.
- Leisure battery, securely mounted so it can’t move in a crash.
- Main fuse close to the battery positive terminal, protecting the main cable.
- Battery isolator switch, so you can switch everything off for maintenance or storage.
- Battery monitor (shunt), which measures current in and out and shows the state of charge more reliably than a voltage reading.
- Charging devices: DC-DC charger, solar charge controller and mains charger, each fused.
- Fuse box (distribution board), with a separate fuse for each circuit.
- Switches and loads: lights, pump, fridge, USB and 12V sockets, fans.
- Optional inverter, if you want to run small 230V appliances from the battery.
Cable sizing and voltage drop
At 12V, thin or long cables waste a surprising amount of energy as heat, and appliances can misbehave when the voltage at their end falls too low. Victron’s Wiring Unlimited guide recommends aiming for a voltage drop below 2.5% and offers several ways to pick a cable size: the product manual, the free Victron Toolkit app, a rule of thumb, or its recommended battery cable table (Victron: DC wiring).
Here’s an extract from that table, which shows the maximum current for each cable size at a voltage drop of 0.259V. The length is the total of the positive and negative cables added together.
| Cable cross-section | Max current, total length up to 5m | Max current, total length up to 10m |
|---|---|---|
| 1.5mm² | 4.5A | 2.3A |
| 2.5mm² | 7.5A | 3.8A |
| 4mm² | 12A | 6A |
| 6mm² | 18A | 9A |
| 10mm² | 30A | 15A |
| 16mm² | 48A | 24A |
| 25mm² | 75A | 38A |
| 35mm² | 105A | 53A |
Source: Victron Energy, Wiring Unlimited. Always check the specific product manual too: Victron notes that its manuals state the cable and fuse size each product needs.
Beginner tip: doubling the length of a run halves the current a given cable can carry at the same voltage drop, so keep the battery, charger and fuse box close together and keep heavy-current runs short.
Fuses: the part you must not skip
A fuse protects the cable from overheating if something shorts out. Victron’s guidance is to choose a DC-rated fuse that matches the circuit and the equipment on it, considering current rating, voltage rating, interrupt rating and speed (Victron: Wiring Unlimited).
In practice for a beginner:
- Fit a main fuse as close to the battery positive as you can.
- Fuse each charging device as its manual specifies.
- Give every circuit its own fuse in the fuse box.
- Never fit a bigger fuse because one keeps blowing; find the fault.
Keeping it safe: a beginner’s checklist
- ☐ Plan your loads and daily amp-hours before buying a battery
- ☐ Check whether your van has a smart alternator before choosing a relay or DC-DC charger
- ☐ Choose battery type and charging kit that match each other
- ☐ Size every cable from the manual or a voltage-drop table
- ☐ Fuse the main cable near the battery and every circuit separately
- ☐ Fit an isolator switch and secure the battery firmly
- ☐ Use proper crimp terminals and a ratchet crimper, and protect cables where they pass through metal
- ☐ Keep cables away from gas pipework and out of the gas locker where possible
- ☐ Have the 230V hook-up installed or tested by a competent electrician
- ☐ Tell your insurer about the electrical work as part of the conversion
On that last point, our guide to self-build campervan insurance explains what insurers usually want to know, and our conversion kits buyer’s guide covers the main UK furniture kit suppliers.
FAQ
Can I wire my campervan’s 12V system myself?
Many self-builders do. The 12V leisure side is low voltage, but it can still cause fires if cables are undersized or unfused, so follow the manufacturers’ manuals closely. The 230V hook-up side falls under BS 7671 Section 721 and should be installed or tested by a competent electrician.
Do I need a DC-DC charger or a split charge relay?
If your van has a smart alternator, which is common on Euro 5 and Euro 6 vans, a basic voltage sensitive relay won’t charge the leisure battery properly, so a DC-DC charger is the usual choice. A DC-DC charger also lets you set the right charging profile for AGM or lithium batteries.
How big a leisure battery do I need?
Add up your daily amp-hours from each appliance’s real current draw, then divide by the usable fraction: roughly 0.5 for AGM or 0.8 for lithium, using Victron’s planning figures. Add a margin for days when you can’t recharge.
Next steps
Start with a list of everything you want to power and how long you’ll use it each day. That one sheet of paper decides your battery size, charging set-up and cable sizes. In the next post in this series, we’ll look at drawing up a wiring diagram for a self-build campervan.