Motorhome electrics: a complete guide to your motorhome electrical set-up

Basic motorhome electrics explained

The modern motorhome has two electrical systems: mains 230V AC – as in a bricks-and-mortar home – and 12V DC. Both have different jobs, but they work together at certain points, with mains, at times, powering heavy loads such as heater, fridge, hot water boiler and battery charger, and 12V providing power for the systems that control them.

Motorhome electric set-up explained

12V power comes from a battery, referred to as a leisure battery, while the 230V power is normally supplied by plugging into a mains point – generally a bollard on a campsite.

What is a leisure battery?

The heart of the 12V system is the leisure battery. The name is really a misnomer as this is the hardest working piece of kit in any motorhome.

It is charged either by the base vehicle’s battery charging system or when hooked up to the mains.

When driving, the leisure battery is connected to the engine’s alternator, which charges and tops it up. When stationary a special device, called the split-charge relay, ensures the living area electrics do not flatten the starter battery.

On all but the most basic campervans and motorhomes, a mains-powered charger keeps the batteries topped up. Some charging systems play a dual role, also charging the engine battery once the leisure battery is fully charged, while still keeping the two separated.

Traditional lead acid batteries are similar to car batteries, but leisure batteries should have much thicker plates to better withstand more frequent and more intense charge/discharge requirements. Sealed lead acid batteries have a recombination lid, to trap the hydrogen and oxygen gases and condense them back to liquid within the battery.

Lead acid batteries are best protected by not draining them down to more than half of their capacity. Life cycle figures are given for this 50% depth of discharge figure (DOD). So, a 100Ah battery should be considered to be only a 50Ah battery. Cheaper batteries will have a shorter service life and less life cycles.

In AGM (absorbed glass mat) batteries, the electrolyte is soaked up in a form of fibreglass blotting paper and sealed to guarantee no spillage and no maintenance. The plates are of deep-cycle construction (thicker).

Gel batteries are similar to AGM, but the electrolyte is stored in a gel form and can put up with more extreme conditions and temperatures. However, they require a specific charger and charging routine. If gel batteries are charged at too high a rate, you’ll actually lose some of the electrolyte through gassing and drying out of the battery (shorter life).

Lithium batteries: are they right for your motorhome?

These are lighter, charge faster, offer higher efficiency and last far longer – costs are higher, but prices are quickly becoming more affordable. Whereas a few years ago a 100Ah lithium battery would set you back close to £1,000, and the equivalent high-quality traction battery would be around £300, now a LiFePO4 battery can be had for around £400 on average, although there are cheaper and more expensive versions.

Don’t get confused with the lithium ion batteries in devices such as phones and tablets, as well as electric bikes, the ones powering motorhomes use iron, and are called LiFePO4 or lithium ferrous.

Other advantages provided by lithium ferrous batteries are the ability to discharge regularly to 90% without seriously compromising the battery life. If you factor in the general advice not to discharge a lead acid battery below 50%, then a 50Ah lithium ferrous battery can provide the same amount of usable energy as a nominal 100Ah lead acid battery.

However, this technology is fragile. They all need built-in battery management systems (BMS) that monitor the voltage of every cell to switch off the battery if the set voltage limits are reached for any cell.

Another difference is the working temperature range. You can use the battery between -10˚C up to 60˚C, but the temperature when charging is between 0˚C up to 40˚C. Don’t try to charge a lithium ferrous battery if it is very cold, as you can do permanent damage if the BMS does not provide temperature protection.

Motorhome electric hook-up explained

Once on a campsite pitch, a bollard with a mains socket will supply your motorhome with 230V electricity. The cable that connects the bollard to motorhome is known as the hook-up lead and is usually coloured orange and is heavier-duty than a domestic extension lead.

One end connects to the bollard, the other plugs into the motorhome's externally mounted hook-up point. It’s best to plug into your motorhome or campervan first then connect to the mains.

Inside the motorhome or campervan, the mains distribution is similar to that at home, with an RCD and breakers protecting mains circuits. The RCD detects faulty appliances or wiring and disconnects the power if a fault occurs.

Heating and hot water in motorhomes

Most heating systems produce hot air, distributed throughout the interior using a fan and system of ducting to distribute the heat. The most used brand is Truma and its Combi unit, which combines a 10-litre hot water boiler with the blown-air heater function. All are primarily powered by a gas supply, but many (denoted by an ‘E’ in the name) can work on mains electric.

Some systems, such as Alde, provide hot water in the same way (and are fully programmable), but provide heat using concealed wet radiators – the result is near silent heat with even heat distribution.

Truma also has a diesel-fired version of the Combi, and Webasto and Eberspächer also make compact blown-air heaters working off the vehicle’s fuel tank. As long as you have fuel in the tank, you’ll never be short of heat. However, they can draw more power from the leisure battery that the more common gas-fired units.

Remember, a 6kg gas bottle contains as much energy as over 60 fully charged 100Ah batteries, so it can often be better to use gas for anything that requires heat.

A Whale heating system takes about 1.7A when running and 0.01A on standby. The Truma Combi has similar electrical requirements. The Alde wet system is more economical, requiring 0.4A for water heating only and 0.6A for hot water and space heating.

Powering a motorhome's fridge

Motorhome fridges are divided into two types: three-way and compressor-types.

A three-way fridge can be powered in three ways, hence the name. When driving, a 12V element powers it. When hooked up to mains, a mains equivalent is used and, when camping away from the mains, it’s a tiny gas flame. Some fridges switch between the source automatically, or you’ll have to do it manually.

The other fridge, a compressor, is similar to your fridge at home but it runs on 12V, not mains, so just switch it on and forget about it. These consume more precious leisure battery power and the hum of the compressor can disturb light sleepers.

These fridges are also more tilt tolerant and can work at higher outside temperatures. Top-loading compressor fridges are the best of this bunch in terms of efficiency.

Common motorhome electrical problems

The most useful tool for electrical fault-finding is the digital multimeter. These are inexpensive and there are many to choose from. This tool can be set to measure voltage (electrical pressure), current in amps (the electric stuff that flows through the wires) and electrical resistance, which is measured in ohms.

The motorhome manual should detail the use of every fuse and its location. There is frequently a fuse in the leisure battery compartment and these can suffer corrosion. Similarly check the battery connections are clean, bright and tight.

If no mains equipment is working at all and your battery is not being charged, then the RCCB, hook-up lead, breaker at the hook-up point and connections are all suspects.

Faulty hook-up lead

A common fault is associated with the hook-up lead. In most cases the three individual wires at each end are secured by pinch screws. These screws can – and do – come loose, especially as they get shaken about while the motorhome is being driven on the road.

With the multimeter set to the ohms range, check between the L pin at one end and the L socket on the other end for continuity.

The electrical contacts of the socket are a long way down the hole, so a long test prod is required. Then check between N and N and finally between E and E in the same way.

They should all read 'zero' ohms.

It is good practice to check the tightness of screws regularly as preventative maintenance.

Also check that there is no physical damage to the cable sheathing along its length and, if there is, replace it.

Mains charger not working

Some built-in chargers have a separate on/off switch and some have the mains input to them via a kettle-type plug and this is worth a check if the charger isn’t working.

Exceeding mains supply

Whilst British campsites often supply 16A (the maximum amount of electricity that you can use at any one time), it can be as low as 10A, 6A, 4A or even 2A, particularly on the Continent – remember to ask when booking in.

If you use too many appliances at once and overload the supply, the circuit breaker on the mains bollard will switch off and need to be reset. Although you may be able to reset the switch or button yourself, these parts are commonly locked up and only accessible to the site warden, who you will need to ask to come and reset it for you.

A tripped switch

If you have a fault or an overload, one of the circuit breakers (or a safety switch known as an RCD) may have switched to the ‘off’ position. Before attempting to reset it, unplug or switch off all electrical items then reset the switch and reconnect each appliance in turn individually.

The appliance which triggers the circuit breaker may well be the cause of the fault or overload. The mains switch box (fusebox) will also have two or three over-current circuit breakers, located in the back of a cupboard and can sometimes be quite easily knocked and switched off.

Typically, one circuit breaker will cover the battery charger, the mains part of the fridge and various other accessories. Another circuit breaker will provide protection to the mains socket outlets inside the motorhome.

So, if your motorhome’s 230V sockets don’t work when you are hooked-up, but everything else does, then it is likely that the circuit breaker has been knocked off.

Steering away from doing any live mains voltage tests (best left to qualified experts), you can plug a mains-powered piece of equipment into a socket to see if it works – a phone charger or even a toaster. If it doesn't, then borrow an alternative hook-up lead to check whether that is the culprit.

Electric step does not retract

Electric steps have a small plunger-operated switch built into the mechanism, which switches off the motor when the step is fully retracted.

When in their retracted position, the steps are in-line with the tyres and, consequently, dirt and road debris is thrown up at them when travelling. This often results in the switches getting full of water and grime and, so, eventually they fail and, hence, the steps fail to retract.

In many cases a bit of work to access the switch and remove it pays off, as they can frequently be dismantled, cleaned and put back into service, or replaced.

Leisure battery does not charge when driving and fridge stays running on gas

In most cases a pair of relays, the split-charging and the fridge relay, need to be energised to enable the leisure battery to charge when driving and thus to switch the fridge to be powered by electric not gas.

The coils of these relays will connect between the battery negative (chassis) and the D+ terminal on the vehicle alternator. When the engine is running, the alternator will be charging and about 14V will be present at the D+ terminal. You can check this with a multimeter set to volts.

One set of relay contacts will close and connect the leisure battery and starter battery positives together so that the leisure battery is also charged.

The other set of relay contacts will close and provide a 14V feed to the fridge to cause it to switch over to 12V mode (for automatic fridges).

In some cases, these two relays and associated fuses are under the bonnet. The relays have their coil-negative connection connected to the bodywork and frequently this is done by the relay fixing screws. Check for sound connections to the relays and measure the voltages there.

Others incorporate the relays and fuses within the main control box; however, some more complex. Sargent systems are different and diagnosing these and any with battery-to-battery chargers are best left to the experts.

Solar panel not charging

This is where the use of a multimeter set to volts is invaluable. First, find the solar voltage regulator, often a separate unit but sometimes its function is incorporated into the main electrical control box.

Without any 230V hook-up connected and with some sunshine, measure the input voltage to the solar regulator, that is the pair of wires connecting it to the solar panel. Expect to see a reading that is at least the same as the battery volts.

If the previously measured battery voltage is around 14V or a tad above and the control panel in the motorhome indicates zero charging amps, then expect to see even more volts from the panel as the regulator will have switched off because the battery is charged.

This is correct and not a fault. Most modern solar regulators and mains chargers charge the battery up to a peak voltage then reduce it to a maintenance level.

The typical maintenance voltage will be 13.5V to 13.8V. Peak voltages will be around 14.4V for wet lead acid batteries, 14.7V for AGM lead acid batteries and ideally 14.6V for the new lithium ferrous (LiFePO4) batteries.

Solar panel not charging

If your motorhome's starter battery keeps going flat, you need to check if there is any continuous battery discharge when everything is supposed to be turned off.

Again, use the multimeter set to the amps range and move the test leads to the amps position. Then disconnect the starter battery negative lead from the battery.

Attach one test lead to the battery negative terminal and the other to the removed cable. The meter will indicate any residual current flow. Anything more than 0.03A is excessive. Items such as aftermarket alarms and reversing camera systems are frequently the culprit.

Remove fuses one at a time to see which of the circuits is causing the battery drain. The same arrangement also applies to the leisure battery.

Motorhome electrical accessories

There is a host of options and upgrades that will improve power supply and maintain a battery.

A mains battery charger controls the charge into the battery, to improve a battery’s performance. If you spend a lot of time wild camping, consider battery-to-battery chargers. If you are electrically minded, you may be able to fit this device yourself.

If your motorhome has an ‘intelligent alternator’ that converts braking energy into electrical energy to fast charge the starter battery then your motorhome may have a B2B unit already installed.

If you only stay at campsites with hook-up then most standard leisure batteries will satisfy your needs, as they only need to power your systems when you stop for a cuppa en route.

However, if you want to power a microwave or hairdryer without being hooked up, you’ll need an inverter. These inverters generate a ‘mains’ AC voltage and the more-expensive pure sine wave variants also match the shape of the mains voltage. The cheaper quasi-sine wave units may not work with all equipment, so check before buying.

Typically, inverters are used for powering phone and laptop chargers, where a 150W unit should be sufficient. If you’re charging electric bikes then look at a 600W inverter. If you want a microwave, you will need to use an inverter of around 1,500W to power it, which will use 125A so you’ll need beefy cabling to handle this. Even a 600W (output) microwave will still take nearly 90A from a leisure battery via the inverter.

If this is your lifestyle then probably two 100Ah batteries will be required to support such an inverter and microwave combination, even if you’re only using it to warm pasties!

If your camping lifestyle cannot quite be supported by one leisure battery, then adding another matching battery is the best way to double your independence. Make sure that each leisure battery has its own in-line fuse for safety.

Motorhome solar panels explained

Solar panels are ideal if you have the roof space, the budget (decent panels are not cheap) and want your batteries always charged to optimum.

Fit the biggest you can. Make sure you have the appropriate MPPT (maximum power point tracking) solar regulator to maximise the panel’s output and match the battery technology and, if required, a regulator that can charge both leisure and starter batteries.

Modern electrics in motorhomes

‘Now you can control your home from your phone’ – we’ve all seen the TV ads for home automation products such as Hive, which use a smartphone app to control domestic equipment. Increasingly, the same is true of motorhomes. Initially, apps were introduced to control one or two functions, with Truma’s iNet system, for example, allowing the setting of its heating and air-conditioning systems.

Now things are moving up a gear, with products that let you control and view the status of many of the life-support systems on board and functioning in conjunction with the motorhome’s built-in control panel.