Most van electrical problems are not faults. They are people trying to run a system that was never sized for what they are asking of it, and getting confused because the numbers on two different displays do not agree.
This guide covers what is actually in your van, how the parts talk to each other, and how to work out what you need before you spend money.
Two circuits, not one
The single most useful thing to understand is that your van has two entirely separate electrical systems that happen to share a cupboard.
The 12V DC circuit is the one that matters day to day. It runs off the leisure battery and powers your lights, water pump, fridge control board, heater fan, USB sockets and anything you plug into a cigarette socket. It works whether or not you are plugged in anywhere.
The 230V AC circuit is ordinary mains. It only exists when you are connected to a campsite electric hookup, or when you are running an inverter. It powers the sockets on the wall, and it powers the battery charger that tops the leisure battery back up.
These two circuits meet at exactly one point: the mains charger. When you plug into a hookup, mains comes in, the charger converts it to roughly 14V DC, and it pushes current into the leisure battery. Everything else about them is separate.
Why the numbers never match
This trips up nearly everyone. You arrive at a site with a 6A hookup, plug in, and your control panel says the battery is charging at 12A. That looks impossible. It is not, because those two figures are measured in different places at different voltages.
The 6A limit is the mains draw at 230V. That gives you roughly 1,380W of budget for the whole van. The 12A your panel is showing is DC current going into the battery at around 13.5V. That is about 160W of actual power.
Power is what you have to budget, not amps. Amps only mean something when you say what voltage they are at.
| Load | Real draw |
|---|---|
| Leisure battery charger | ~200W |
| Fridge on mains | ~100W |
| Laptop and phone charging | ~90W |
| Starlink or 5G router | ~40W |
| E-bike battery charger | ~105W |
| Total | ~535W |
That is comfortably inside 1,380W. The thing that will trip the bollard is a kettle at 2 to 3kW, which on its own exceeds the whole budget. On a low-amperage hookup, run the kettle alone or run it off gas.
The leisure battery
The leisure battery is a deep cycle battery, designed to be discharged slowly over hours and recharged. That is a completely different job from the starter battery under the bonnet, which delivers a huge burst for two seconds and then gets immediately refilled.
Never run habitation loads off the starter battery. That is what the split charge relay or battery to battery charger exists to prevent.
Battery chemistry, honestly
Lead acid and AGM. Cheap, heavy, widely available, and you can only really use about half of the rated capacity before you start damaging them. A 100Ah AGM gives you roughly 50Ah of usable energy, or around 600Wh. They also charge slowly as they approach full, so the last 20 per cent takes disproportionately long.
LiFePO4. Three to four times the price, roughly a third of the weight for the same usable capacity, and you can use 80 to 90 per cent of the rated figure. A 100Ah LiFePO4 gives you around 1,100Wh usable, so it does the work of two AGMs. It also accepts charge fast right up to nearly full, which matters enormously if your only charging window is a two hour drive.
The honest summary: if you spend most nights on hookup, AGM is fine and the money is better spent elsewhere. If you spend most nights off grid, LiFePO4 is not a luxury, it is the thing that makes the rest of the system work.
Two caveats on lithium. It needs a charger with a lithium profile, so check your existing mains charger and solar controller support it before you swap. And most cells will not accept charge below freezing without a battery management system that handles low temperature cut-off.
Sizing it
Work backwards from a power budget rather than picking a number that sounds good.
| Item | Watts | Hours/day | Wh/day |
|---|---|---|---|
| LED lighting | 15 | 4 | 60 |
| Compressor fridge | 45 cycling at ~40% | 24 | 430 |
| Water pump | 60 | 0.2 | 12 |
| Heater fan | 25 | 4 | 100 |
| Phones and laptop | 70 | 3 | 210 |
| Router or Starlink | 35 | 8 | 280 |
| Total | ~1,090Wh |
The fridge dominates. It always does. It is the single item running around the clock, and it is the reason a van that seems fine for a weekend dies on day three.
Now apply a target. Two days at 1,090Wh means 2,180Wh, which is roughly 200Ah of lithium or 400Ah of lead acid. That is the honest answer, and it is usually bigger than people expect.
Charging: three ways in
Mains hookup
Simplest and fastest. A mains charger typically delivers 15 to 30A DC. On a low-amperage bollard the charger itself is often the biggest single load in the van.
Solar
A roof panel feeds a charge controller, which feeds the battery. The controller matters more than people expect.
PWM controllers are cheap and crude. They essentially connect the panel to the battery and pulse the connection, which drags the panel down to battery voltage and throws away the difference. Many factory-fitted controllers are PWM.
MPPT controllers track the panel’s maximum power point and convert the excess voltage into extra current. In real conditions that is typically 20 to 30 per cent more harvest from the same panel, and the gap widens in cold weather and low light, which is to say British weather.
If your van came with a basic PWM unit, replacing it with an MPPT is one of the highest value upgrades available, because it costs less than adding panels and you get the benefit on every panel you already have.
Size the controller to the panel array, not the battery. A controller labelled 100/20 accepts up to 100V from the panels and delivers up to 20A to the battery, which suits roughly 250 to 290W of panel on a 12V system.
Realistic UK yield from 300W of roof panel, flat mounted:
- Midsummer, clear: 1,200 to 1,500Wh a day
- Spring and autumn, mixed: 400 to 700Wh a day
- Midwinter, overcast: 100 to 250Wh a day
Solar does not solve winter. It is a summer and shoulder season tool, and anyone quoting you an annual average is hiding the shape of the curve.
Alternator
Driving charges the leisure battery, but how well depends on what is fitted.
A split charge relay simply connects the two batteries once the engine is running. Cheap, and adequate for lead acid on older vehicles. It struggles with lithium, because lithium will happily pull more current than the wiring or the alternator was designed to give.
A battery to battery charger sits between the two batteries and actively regulates the charge, delivering a proper multi stage profile at a fixed current. Essential for lithium, and necessary on most vans built after around 2015, because smart alternators drop their output voltage once the starter battery is full, which leaves a plain relay delivering almost nothing.
A 30A B2B gives you roughly 400Wh per hour of driving. Two hours on the road is most of a day’s usage back.
Distribution and protection
A main fuse at the battery. Within 30cm of the positive terminal, sized above your maximum expected total draw. This exists so that a chafed cable becomes a blown fuse instead of a van fire. It is not optional.
A fuse board for the individual circuits, each fused to protect the cable, not the appliance. The fuse defends the wire.
Cable sized for current and length. Voltage drop is the thing that gets underestimated. At 12V, a drop that would be trivial on mains is a large fraction of your supply, which is why long runs of undersized cable produce dim lights and a pump that sounds tired. Size for under 3 per cent drop over the round trip length.
A battery monitor with a shunt. Voltage alone is a poor indicator of state of charge, especially with lithium, which sits at almost the same voltage from 90 per cent down to 20 per cent. A shunt counts the current in and out and tells you the actual figure. If you are going to spend money on one instrument, spend it here.
Inverters
An inverter turns 12V DC into 230V AC so you can run mains appliances off the battery. Useful, but expensive in energy terms.
Get a pure sine wave unit rather than modified sine, because modified sine upsets anything with a motor or a sensitive power supply. And note the conversion is only around 85 to 90 per cent efficient, with the inverter drawing power just being switched on.
The arithmetic that stops people: a 2kW kettle for three minutes is 100Wh, which sounds fine, but it is drawing over 170A from a 12V battery while it does it. That needs very heavy cable, a large fuse, and a battery that can supply it. Most lead acid banks cannot. Gas remains the sensible way to boil water in a van.
Putting it together
A system that works off grid for several days in the shoulder season looks roughly like this:
- 200Ah LiFePO4
- 300 to 400W of roof solar into an MPPT controller
- 30A battery to battery charger from the alternator
- 25A mains charger for hookup nights
- Shunt based battery monitor
- Properly fused distribution with correctly sized cable
That covers around 1,000Wh a day of usage with enough headroom for two or three grey days.
A hookup focused system is much simpler and much cheaper: 100Ah AGM, a decent mains charger, a modest solar panel to hold things up between trips, and no inverter.
Neither is wrong. The mistake is buying the second one and expecting it to behave like the first.
