Technical
Electrics in your camper: how much battery, how much solar, and which cable
Almost every electrical plan starts back to front: buy a battery, then see whether it is enough. The order that works is the other way round — add up what you use per day, decide how many days you want without sun, and only then pick the battery. This article walks through that sum and gives the cable sizes to match.
Updated on 15 min read
Step one: add up your daily use
Work in amp-hours at twelve volts. For each appliance: current in amps times the hours it runs per day. A fridge is not on for twenty-four hours but cycles on and off; so reckon with what it uses per day, not with what it draws while running.
| Load | Per day | Note |
|---|---|---|
| 50-litre compressor fridge | 25 – 50 Ah | Doubles in a hot summer; by far the biggest item |
| LED lighting, 4 hours | 2 – 5 Ah | Negligible, even though it does not feel that way |
| Water pump | 1 – 3 Ah | Brief but with a high inrush current |
| Diesel heater, fan | 5 – 20 Ah | Strongly dependent on how cold it is outside |
| Charging phones and a laptop | 5 – 15 Ah | Add another ten per cent loss through the inverter |
| Kettle or coffee machine on 230 V | 15 – 30 Ah | A few minutes at a time, but at enormous current |
Two people with a fridge, lights, a pump and some charging usually land between forty and eighty amp-hours a day in summer. That number is your starting point; everything below follows from it.
Step two: the battery
The capacity on the label is not the same as what you can use. A lead battery — AGM or gel — lasts longest if you do not take it below half; so reckon on half the printed figure. A LiFePO4 lithium battery can be used to eighty or ninety per cent, lasts two to four times as long and weighs half as much. It costs more to buy and less per amp-hour used, and the latter is the sum that counts.
Rule of thumb: take your daily use, multiply by the days you want without sun, then divide by 0.8 for lithium or 0.5 for lead. Sixty amp-hours a day, two days of buffer, lithium: 60 × 2 ÷ 0.8 = 150 Ah. The same in lead: 240 Ah, and that is over fifty kilos.
Step three: three ways to charge
- 01
From the engine, with a DC-DC charger
This is the most reliable source, because it also works in November. A modern van with a smart alternator will not properly charge a second battery on its own: the voltage is regulated for the starter battery and drops once that is full. A DC-DC charger between the two solves it, and with lithium it is mandatory anyway, because otherwise the lithium draws so much current that the alternator fails.
- 02
Solar panels with an MPPT controller
Two panels of 150 to 200 Wp deliver roughly 40 to 70 amp-hours a day in a Dutch summer. In December that is a tenth of it, and that is not a disappointment but physics. Take an MPPT controller rather than a cheap PWM one: it makes a quarter of the difference in winter, exactly when you need it.
- 03
Shore power, with a mains charger
On a campsite, the fastest way back to full. Choose a charger matched to your battery type — lithium has a different charge profile from lead — and set it so it does not keep charging once the battery is full.
Cable sizing: the table
At twelve volts, voltage drop is the whole story. The same cable that is ample at 230 volts loses so much voltage at 12 that your pump will not start and the cable warms up. Always count the run there and back: a load three metres away has six metres of cable. Below are common cases, generously sized.
| Run | Current | Cross-section |
|---|---|---|
| LED lighting, up to 5 m | 2 A | 1.5 mm² |
| Water pump or fridge, up to 4 m | 6 – 10 A | 2.5 mm² |
| Solar panel to MPPT, up to 6 m | 10 – 15 A | 4 – 6 mm² |
| DC-DC charger from the starter battery, 4 – 6 m | 30 A | 16 mm² |
| Battery to busbar, up to 1.5 m | 100 A | 25 – 35 mm² |
| 1000 W inverter, as short as possible | 85 – 100 A | 35 mm² |
Every positive cable needs a fuse, as close to the battery as possible — within thirty centimetres. A fuse protects your cable, not your appliance: a 100 Ah battery can deliver hundreds of amps into a short, and the fuse catches exactly that. The good news is that this is also the cheapest part of the whole installation: a few euros per circuit, and it is settled for good.
And then 230 volts
Everything above is twelve volts, and with common sense and a good fuse you can install that yourself. At 230 volts, take a different approach: that is work with a standard of its own, and it is worth having the shore-power installation — inlet, residual-current device, breaker, bonding to the bodywork — fitted or at least inspected by someone qualified. It costs an afternoon, you get a certificate, and that certificate is exactly what your insurer wants to see.
The order of installation
- All wiring goes into the wall before you insulate. A cable you forget is one you drill through a finished wall for later.
- Run one spare cable to every location. It costs three euros now and a day's work later.
- Put the battery, the fuses and the busbar together in one accessible place, low and between the axles.
- Use crimped terminals rather than screw connectors. Vibration turns every screwed clamp into a loose connection eventually, and a loose connection gets hot.
- Draw your installation on paper while you build it. In three years you will not remember which brown wire goes where.
See the layouts with a technical bay
What you need for this
We do not (yet) sell these parts separately. The list is here because this is what should be in your basket before you start.
- 100 or 150 Ah LiFePO4 battery with a monitor
- 30 A DC-DC charger for vans with a smart alternator
- MPPT charge controller matched to the panel output
- 150 – 200 Wp solar panel with roof mounts and a gland
- 25 and 35 mm² battery cable with crimped lugs
- Fuse holder with a main fuse, within 30 cm of the battery
- Busbar with numbered circuits
Frequently asked questions
- How much battery capacity do I need in my camper?
- First add up your daily use — two people with a fridge, lights and a pump sit at forty to eighty amp-hours in summer. Multiply by the days you want without sun and divide by 0.8 for lithium, 0.5 for lead. Sixty amp-hours a day with two days of buffer comes to 150 Ah lithium or 240 Ah lead.
- What cable size do I need for 12 volts?
- It depends on the current and on the length there and back. Lighting needs 1.5 mm², a pump or fridge 2.5 mm², a 30 A DC-DC charger over four to six metres 16 mm², and the battery-to-busbar link 25 to 35 mm². At twelve volts, voltage drop dominates, so size up rather than down.
- Do I need a DC-DC charger?
- With a modern van and a smart alternator, almost always; with a lithium battery, without exception. A smart alternator regulates voltage for the starter battery and backs off once it is full, so your second battery never fills. With lithium the charger is also needed to stop the battery drawing so much current that the alternator fails.
- How much does a solar panel yield in winter?
- Roughly a tenth of the summer yield. Two panels delivering forty to seventy amp-hours a day together in July will not exceed five to ten in December. Anyone travelling in winter charges from the engine or on shore power in practice, and uses the panels as a supplement.
- Where should the fuse go?
- In every positive cable, and as close to the battery as possible — within thirty centimetres. The fuse protects the cable, not the appliance: in a short circuit a 100 Ah battery delivers hundreds of amps, and an unprotected cable glows red in seconds.