Understanding The Basics Of Onboard Electrics


The vast majority of us will have some form of on board electrical system(s) in our units, be it 12 Volt, 240 Volt, or both, however, it is apparent that many users still don’t fully understand what systems power what appliances, and we regularly receive enquiries from people concerned that their three pin mains sockets aren’t working when there is no electric hookup (EHU) or from those unable to understand why their fridge won’t work on 12V, if there is no mains supply.


A 240V Mains Socket Will Not Work Without EHU.

Most modern units will have two different electrical systems, a 12V system and a 240V system, however, it is more than likely, both systems will be inter linked to a greater or lesser extent.

What System Powers What Appliances?

Starting with the 12V system, this will, most likely, be powered by an on board leisure battery.

For more information on how these work, see our earlier article; An Introduction To Leisure Batteries.

In most units, the 12V system will run the following;

  • 12V sockets (these come in a variety of formats, depending on the age of the unit)
  • 12V lighting (some lights may be mains powered, but the vast majority are 12V)
  • Water pump (for main / wash room water supply to taps etc)
  • Toilet flush (where applicable)

The 12V Switch Will Only Work Whilst Connected To A Car With The Engine Running

Please note; the 12V supply will NOT power the three way refrigerator, where fitted.

This is because it would drain the battery in a matter of hours, or, even, less.

Many fridges are not, even, connected to the leisure battery, for just this reason.

More modern fridges do tend to be connected to the leisure battery, but not for cooling, only to power the light(s) and electronic ignition.

The 12V setting on a three way fridge is only for use when connected to a towing vehicle, and, then, only when the engine is running (an on board relay will, normally, disconnect power to the fridge, as soon as the car engine is turned off).

The 240V system, on the other hand, is available for much heavier loads, especially those requiring some form of heating element. The 240V system (via the on site EHU system) will power the following;

  • 3 pin 240V sockets (like those you have in the wall at home)
  • Fridge
  • On board heater (where fitted)
  • Hot water system
  • In some units, there may, also, be mains lighting, as well, although this tends to be more the case with caravans, as opposed to folding campers.

How Do The Systems Work, In Practice?

The way the electrical systems work will, in the main, depend on the age of our unit. The very oldest units may well have no electrics, whatsoever, relying on manual foot pumps for water, and gas for cooking and, maybe, a fridge, if you’re lucky.

Slightly newer units will, often, have one system or the other, but not both, although you will, often, see one, or the other, or both, fitted as an after-market installation, by subsequent owners.


A Typical On board Switch, allowing Us To Select The 12V Source, From Car To Leisure Battery

If the on board electrics are 12V only, this is fine, but does present us with one significant challenge; how do we charge the leisure battery, without a mains supply?

Of course, these days, we have various options, such as solar panels etc, but the original answer, usually, came via the grey 12S lead, connected to the vehicle tow bar electrics.

This gives us the option to charge the leisure battery, whilst in transit, but, of course, is not ideal, for longer stays, when the car may need to be hooked up, for a while, in order to top up the leisure battery.

Some models, actually, take this a step further, and use the vehicle battery to power the camper’s on board 12V system, thereby excluding the need for a leisure battery, at all. This is not ideal, as you need the car to be permanently connected, in order for the 12V system to work, but a surprisingly large number of models did, still, use this system.

On units with 240V power, only, there is no such issue with charging the battery, although, of course, there can be a bit of an issue, when visiting sites without electric hookup facilities.

So, of course, the ideal scenario is to have both systems installed, and, for the vast majority of modern units, this is the way they are configured, thereby enabling us to operate reasonably well, wherever we may be situated.

Acer Image

Typical Power Supply Unit, The Sargent PSU 2005

Most, if not all, modern campers will have an onboard power management system. For most of the Pennine / Conway models, in recent years, this has been the very popular PSU2005 unit. These units perform three main functions;

  1. To control and distribute the onboard mains electric supply
  2. To provide 12V power, via an in built transformer, to the 12V system
  3. To charge the leisure battery, where fitted

This unit, also, contains all fuses / trip switches for both 12V and 240V systems.

The PSU provides a bridge between the 12V system and the 240V system. It allows the mains supply to charge the battery, but, also, allows the 12V system to run, even without a battery in place. Whilst this facility is available, and can be used in the short term, it is not recommended for long term use, in most units, as the battery is not only used for storing 12V power, it is, also, used as a buffer, smoothing out the power supply from the transformer, and protecting any in line appliances from damage. Having a dual electrical system really is the best of both worlds, with the facilities needed to charge the battery, whenever we are on EHU, and the backup of a leisure battery when we aren’t.


Typical Pennine Folding Camper Electrical System Configuration

Working Without EHU

We looked at the general topic of working without EHU in our earlier article; Making The Most Of Life Without Electric Hook Up, but, for now, we are just looking at the specifics of what will, and won’t work, when we have no external mains supply.

We have, already, covered, above, those appliances that will work on 12V power, from a leisure battery, but what about those that won’t? The simple answer, to be honest, is that most will have to work on gas, in the absence of a 240V power supply. Typical examples of this include;

  • Fridge
  • Heater
  • Hot water

All of these contain a heating element, rendering them unsuitable to use on 12V, which is why many come with a gas option. If they don’t, then you will be unable to use them without electric hook up. Unfortunately, all new model folding camper models, from Pennine, are provided with electric only heating / hot water, and only UK competitors, Opus, have electric only heating, and no hot water system, at all, so off grid camping, in modern units, can be a little more of a challenge.


A Solar Panel Will Help To Keep A Leisure Battery Topped Up

Of course, if we are staying away without electric hookup, for any period of time, we may well require some form of battery charging facility.

The most popular of these, today, is the solar panel.

An 80W plus model should give you all you need for moderate to average usage over extended periods.

If you do need a little 240V power, whilst away, without EHU, this is achievable with the use of an inverter.

This converts 12V power to 240V, for running the odd domestic appliance.

Of course, we need to be mindful of the power usage of those appliances, and, once again, anything containing a heating element will kill the battery in a matter of hours, or, even, minutes. Consequently, it is best to keep to inverters of no more than 300W, in order to protect battery life.


About Alan Young

MD and owner of the Woodhurst Group, including Praxis Accountancy Limited and Blue Sky Recreation Limited. Also Commercial Director of The Sky visor Group
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