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12 Volt Systems

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12V SYSTEMS. (Author = Sallytrafic)

The simplest of 12V systems contains a battery, a method of charging the battery and a dc distribution. Indeed in the very simplest system the battery isn't a separate one but is the one that starts the engine, the charging system is the vehicles alternator and the distribution is a number of fused connections directly off the battery terminal. More complicated systems have a separate leisure battery, possibly more than one, and a central control system as well. The control system may be fully integrated with the charging system.

DISTRIBUTION. This feeds all the 12V loads, normally through a fuse box and also probably a 12V socket of which more later. Although the system works at 12V, this is the nominal voltage, when the battery is being charged this voltage rises considerably (often above 15V), so all loads attached must be rated at these higher voltages.

A simple fuse strip and a more complex one

Examples of typical DC loads include the following:

Lights: LED florescent and halogen
Fresh Water pump (and toilet pump if separate)
Fridge: If compressor, (The 12V section of a three way absorption fridge is normally fed from the vehicle battery controlled by the ignition switch)
Video and audio: TV, Freeview boxes DVD players Satellite systems and the like.
Roof vent fans
Electric steps

Many, but not all, distribution systems on UK built vans are relay controlled so that none of these devices can be operated whilst moving.

12V sockets are not standardised and you may see any of the following types. It is important to appreciate that 12V loads are often high current loads and to ensure that the rating of the socket is not exceeded.

Cigar lighter, Euro and two flat pin types

All wiring needs to be flexible (ie not solid core) and rated for the circuit involved, then a fuse that will protect the size of cable chosen (not the size of load) needs to be fitted.


There are separate FAQs about batteries, and for the purposes of this FAQ we will mention that most vans have at least one leisure battery that feeds the loads in the habitation part of the motorhome but this is probably a good time to mention batteries in parallel.

Your battery is a collection, or string, of six 2V cells in series. If you want a bigger battery you get bigger cells up to a practical limit of being able to lift the battery in and out of your motorhome or fit the space. Although not all cells may have reached the same state of charge because its a series circuit all the cells have the same current passing through them and tend to reach fully charged over time. If you want more capacity you may have to consider two batteries in parallel. Now there are two current paths so the current is not the same in each battery and therefore the possibility of a circulating current arises from the more charged to the less charged. This tends to charge the weaker of the two batteries from the stronger. So it is a good thing if both batteries are very similar and at the same state of charge. Buying two identical batteries instead of adding a new battery to an old improves the chances of them being similar. Extending the charge time at the maximum voltage charge voltage will bring the batteries to the same state of charge. This phase of charging is often referred to as 'equalisation' it brings the cells in a battery to the same charge and the cells in any parallel battery (and its cells) to the same charge.

Note that batteries should be connected together by very thick wires or busbars and if not adjacent be protected by high current fuses. Also if possible take the feed from one positive terminal and the negative connection from the other (this is a counsel of perfection designed to ensure both batteries are equally charged and discharged).


These are many and varied and often more than one coexist in your motorhome. They may be integrated with each other or separate and they may often be after market additions.

Mains charger powered from the EHU or a generator. These chargers may be simple constant voltage chargers often limited to 13.8V, or three or four stage chargers these also may have temperature compensation allowing charging voltages above 15V when the temperature is very low.

Split charging system powered from the vehicles alternator. Charging voltage depends on vehicle and may reach 14.4V on newer vehicles but thin cables fuses relays etc reduce this voltage. The simplest of these systems relies on a relay to connect the leisure battery to the vehicle battery when the alternator is producing any output. There are others that don't connect the leisure battery until the vehicle battery voltage has risen to above about 13V. Note that if the batteries are not adjacent it is wise to protect both ends of the cable with a fuse as a short to earth (battery negative) would be catastrophic.

Battery to Battery chargers powered by the vehicle battery. These are able to charge the leisure battery at a high rate perhaps as much as 50A. Note it is unwise to charge a battery at a rate higher than 20% of the Ahr rating. So for a 110Ahr battery this would be around 22A.

Solar and Wind powered systems and their regulators. Covered by other FAQs


Some motorhomes have completely integrated 12V dc systems, top of the range Zig and Schaudt Electroblock are examples. These control where the mains battery charger is switched to (allowing it to charge the vehicle battery) and to, for example, select either the leisure or vehicle battery to power the loads. Some of these are completely automatic and often the control panel is remotely mounted from the control unit.

This leads to the oft repeated question 'does my charger charge the vehicle battery when on EHU' and the best answer is 'perhaps'. A slightly more useful answer is 'in a UK built van unlikely unless a switch exists for it, in a european van especially German built very likely though it still may require a switch setting. There are after market devices that can be fitted designed to allow the leisure battery to trickle charge the vehicle battery, this is a useful addition for vehicles that are stored or not moved for periods over a couple of weeks.

All control panels give some measure of control even if its only to be able to switch off the charger.

Solar and wind regulators allow loads to be directly connected to them and can disconnect them automatically if the battery voltage falls too low. Some integrated control systems do that as well.


As part of some control systems or as a stand alone some motorhomes have a battery monitoring system. Those based on voltage are unlikely to give an accurate reading of charged the battery is. This table gives a rough guide:

To accurately measure the state of battery charge electrically it is necessary to monitor the current in and out of the battery. When they are calibrated with the battery's correct capacity they can be quite accurate but need resetting from time to time. If you say that you have a 110Ah battery fitted but its performance has actually fallen to 90Ah then the gauge will not be accurate.

Some integrated control systems give a battery monitoring facility. If you add loads or another charging source it may be that you bypass the measuring device (normally a current shunt) this will also lead to incorrect monitoring.

The only always reliable methods of determining charge state are measuring SG or using an impedance measurement system neither can be done on a day to day basis.
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