Caravan & Motorhome Electrics – Sample Chapter
Overview of an RV’s electrical needs
There are two main approaches to RV electrical power. The first, mostly in the USA, is to stay only in ‘trailer parks’ where ample and reliable 120-240 volt (60 Hz) energy is available, and to rely on alternator power to run a fridge while driving. In Australia, however, increasing numbers of people camp wherever they can. Many stay in travel trailers only occasionally. Some never at all.
A few purists still do without electricity except for dry battery powered torches and possibly a radio, but the majority seek at least the basic electrical facilities they have at home. A few would like them all. There are various approaches to the above, and even the latter can be provided - at a cost.
The early days
Figure 1.3. Dynamo (from 1930s Alvis Speed 25). Pic: Car & Classic Ltd.
Early systems relied on energy held within pre_charged batteries used only for lighting. They used (kerosene-powered) fridges that were only marginally adequate. This enabled a few days stay on-site, until the batteries became discharged.
Direct current dynamos (Figure 1.3) of often 6.0 volt for running the car’s electric lighting, etc., were available by 1911, but sometimes only as an ‘after-sale’ option. Until 1918 or so, many car makers resisted attempts to use them at all.
The dynamo assisted recharging and powered the on-road lighting, but its limited output (and hence charging ability) required several hours driving each day to generate even the modest electrical power of early travel trailers and motorhomes.
Figure 2.3. This 1920s Angela travel trailer needed no electrical power! Pic: Dennis Publishing.
Magnetos (that provided ignition as the engine turned over) were used until the mid-1920s. Coil ignition was introduced by Delco’s founder, Charles Kettering (initially for Cadillac) in 1910. This remained much the same until the major introduction of the alternator (by Chrysler) in 1960. At much the same time, the output was increased - to 12 volts.
The alternator’s output was rigidly controlled by an associated regulator that caused the alternator to maintain a constant (typically) 14.2-14.4 volts. Its most vital task was to ensure the starter battery had sufficient charge to provide sufficient current for the starter motor.
Today, starting big 4WD engine requires 500-600 amps but only for two to three seconds. This depletes the starter battery by only 2%-3% of that battery’s capacity (typically less than two amp hours). The alternator replaces that in only two to three minutes (Chapter 12).
While the vehicle is stationary, that battery also energises interior lights, electric door locking, immobiliser warning light, and the electric clock but these, collectively, draw little more than the battery loses in self-discharge.
The alternator’s major role nowadays is to provide power for all of a vehicle’s ever-increasing on-road electrical loads.
An alternator’s generally surplus output enabled RVs to charge battery banks capable of running reasonable auxiliary needs. This, however, began to change in the late 1990s, when computer engine management systems were introduced.
Alternator-derived energy was increasingly needed for additional functions as well, e.g. electrical power steering, suspension stability systems, etc.
Using the alternator for charging auxiliary RV batteries became increasingly controversial. It was deemed to adversely affect the operation of those systems. It also raised warranty issues.
From 2001 or so, some alternators were temperature controlled. They ran at too low voltage to adequately charge batteries. This trend continues: most post-2014 vehicles have variable voltage output alternators that often drop below 12.3 volts and only increase when the (starter) battery requires charging to a now 80%.
It is possible that some RVs may eventually have alternator output being substantially or totally confined to the original vehicle use, but it is currently (late-2019) too soon for anything but speculation. If/when that happens, fuel cells (Chapter 21) are likely to be an adequate replacement.
While lighting and TVs have become increasingly efficient, microwave ovens, and pre-2014 colour TVs, etc., still gobble power, resulting in many bigger RVs drawing more electrical energy than previously.
For most vehicles, dc-dc charging (Chapter 13) isolates the RV’s ‘house’ electrical systems from the alternator and provides fast, deep and safe auxiliary charging. Chapter 15 covers post-2013 vehicles.
Figure 3.3. Many fluorescent and incandescent light fittings can be retrofitted to LED. The pic on the right shows a fitting converted to LEDs. Pic: ledsunlimited.co.nz.
Around 1980 or so battery technology, that had been stagnant for close to a century, began to improve. Despite this, most practical RV batteries still store only 50% or so more energy (re size and weight) than in the 19th century. Lithium batteries (Chapter 9) can store up to four times as much but in mid 2019 were still three or so times the price of most other batteries.
Solar module prices fell 75% from 2009 to 2011, and continue to fall. Given space for the modules and associated battery capacity, most electrical appliances that people seek to use in a travel trailer or motorhome can now be feasibly driven from solar.
As this book shows (and explains how to do), an RV electrical system that is designed and scaled appropriately will enable an RV to remain indefinitely away from mains-power and to do so reliably. It is necessary to think ahead, however because vehicle electrical technology is changing fast. It already requires a new approach in several areas.
Warm white compact fluorescents are practical and economical. They have internal electronics that run at high frequency, and by so doing, avoid the slight flickering of full length fluorescent tubes.
Ultra-efficient LED globes are replacing other forms of RV lighting: they are the only ones to consider for new installations. They can usually be retrofitted (Fig 3.3).
RV electrical appliances
Until recently it was often more efficient to use 12 volt dc appliances rather than an inverter’s 230 volts (an inverter converts a low dc voltage to grid voltage ac). Inverter efficiency and performance, however, is now a typical 93%-96%. A few are even higher.
Using 230 volts ac all but eliminates voltage drop in RVs, and enables a wide choice of good and affordable products. These vary in efficiency but the ‘Energy Star’ ratings system enables easy assessment of the main previous heavy gobblers - particularly fridges, and items (such as cooling fans) driven by induction motors.
Away from mains-supplied power or power from a generator, electrical usage is best limited to lights, radios, DVD players, laptop computers and communications equipment, iPads, TVs, fans, blenders, water pumps, fridges, etc.
That which cannot realistically be run from small scale solar and/or alternator power, is anything that, as its intended function, generates heat over time. As there’s more energy stored in 9.0 kg (20 lb) of LP gas than can be held in 1000 kg (2200 lb) of lead-acid batteries or 250 kg (550 lb) of lithium batteries, LP gas (or diesel) is still best used for ovens, grills and water heating.
Microwave ovens are fine in large RVs but borderline in smaller ones with limited available energy. An ‘800 watt’ microwave oven produces 800 watts in ‘effective heat’ but draws about 1200 watts, or about 1330 watts if run via an inverter. It needs an AGM battery of at least 150 Ah. Ten minutes usage may be half a day’s energy draw in a small RV. It is possible to accommodate this but it makes more sense to use the microwave oven only where 230 volts mains-power is available. If not, that under $299 unit may cost $1000 more in solar and battery capacity.
Figure 4.3. A CPAP in use. Pic: ResMed (USA).
These continuous positive airway pressure devices are used by people who suffer from snoring, sleep apnoea and other forms of sleep-disordered breathing. They are largish volume, low pressure air pumps, some of which incorporate humidifiers. Some also heat the supplied air.
Until 2002 or so these units drew from 130-300 watts. This was really too high for realistic 12/24 volt power excepting in large coaches with equally large battery banks. Their efficiency since then has improved greatly.
By and large CPAP energy draw is related to the treatment pressure: typically from 590 Pa (6 cm H²O) to 1960 Pa (20 cm H²O). The more efficient (non-heated) units draw (at 12 volts) 0.65 to 1.4 amps for the above, but a few older ones draw twice that. Those with heated humidifiers draw several times more.
Some have inbuilt inverters but most are 230 volt ac units that require a high quality pure sine wave inverter if run from 12/24 volts.
CPAP machines may have a poor power factor (Chapter 1 and 2). If so, these require an inverter that has an output of 50% or so higher wattage than that of the machine.
This is a very specialised area. Anyone with (or suspected) sleep apnoea syndrome, sleep disorder, snoring or similar condition should consult a doctor or other specialised professional in this area. Advice on specific CPAP machines should be obtained directly from the major vendors in this field.
An electric refrigerator typically accounts for 50 to 70% of an RV’s electrical consumption. The larger RV fridges need 250 to 350 watts of solar capacity to drive them and, at minimum, a 200 Ah battery to run them at night (particularly in tropical areas), plus a fuel cell or generator for times of little sun. Solar is now cheap, and thus practicable for RV use if there is space for the solar module area required.
Three-way (12 volt/mains-voltage/gas) refrigerators running on gas are less convenient and while more costly, the saving on electricity approximates their higher initial cost. They use about 0.45 kg (1 lb) of gas per day. Their 12.5 to 25 amp draw from 12 volts precludes running on battery power except while driving and for short roadside stops. They need 230 volts, or to be run on gas whilst on remote sites.
Some users are prejudiced against three-way fridges as early models were ineffective in extreme heat. The later ST and T-rated units work fine in Australia if installed correctly.
The 230 volts ac water pumps draw too much energy for camper van and motorhome use. Even those supplying only one or two taps draw about 500 watts, and twice that while starting. RV 12/24 volt pumps typically draw about 60 watts (about 5 amps and 2.5 amps respectively). Some made recently are quieter and provide smooth constant flow - yet cost no more to buy. refers - and also shows manufacturers’ typically claimed pump draw.
Excepting in large RVs with ample space for solar modules and substantial battery capacity, it is not as yet feasible to run an air conditioner for more than a few hours during the night from battery-stored solar. This situation may well change because the very best small (domestic) reverse-cycle air conditioners are becoming increasingly more efficient.
As of 2019 the Daiken US7 (2.5 kW unit rated reverse cycle unit) draws only 420 watts on its cooling cycle. If run via an efficient inverter, this corresponds to about 33 amps at 12 volts. This is still marginal for solar but a few costly USA units are now claimed to use even less.
Most production RVs are now fitted with a so-called converter that, when 230 volts is connected to the vehicle, provides an unregulated 13.65 volts directly to power the RVs 12 volt needs.
These units present major problems for those intending to stay away from mains power for more than one overnight stay. This issue is covered on Chapter 11.
There are known to be major electrical compliance issues with some US imports. A loophole in the regulations enables the original buyer (only) to use them in Australia by adding a 230 to 110 volt transformer. Many owners assume (wrongly) that this confers electrical compliance. It does not. To be 100% compliant they may not have any form of 110 volt anything - from wiring to all appliances.
Before being offered for sale, such units must be brought totally into full electrical (and gas) compliance. Take this seriously. Selling a non-compliant RV is a criminal offence in some states of Australia and a civil offense in others. This issue is covered in depth in Chapter 40.