The Electrical Systems
Part One

During my time as a member, I've noted the many letters to The Wanderer seeking advice on motorhome electrics. This four-part series will hopefully shed some light on the subject - and I trust may also assist those many motorhome owners whose systems are operating unsatisfactorily. The material is basically an abridged version of the relevant chapters from my forthcoming book on campervan, motorhome (and caravan) mechanical and electrical systems that I hope to have in print in about nine months.It is abundantly clear that a substantial proportion of campervan and motorhome electrical systems do not work to their owner's satisfaction. Attempts at rectification (larger alternator, bigger batteries) are commonly less than successful.
To understand why it is first essential to appreciate that motorhome electrical systems are functionally quite different from those in cars. Campervan systems are different too, but to a lesser degree.The difference is this. Cars and trucks require electrical energy primarily whilst they are moving. Campervans/motorhomes require electrical energy primarily whilst they are at rest. Whilst the above might seem blindingly obvious, the fundamentally different approach that is required in the respective electrical systems is rarely understood by motorhome owners. Worse (and as equipment suppliers in this field will confirm) - these different requirements are rarely understood either by auto-electricians. Campervans/motorhome owners do not require a deep understanding of the technicalities of vehicle electrics, but rather an appreciation that their vehicles' and car/trucks' electrical systems have substantially different functions - and must be put together accordingly.

We'll start by having a brief look at the electrical systems of cars and trucks. These vehicles have one main battery (or, as with larger trucks, two batteries coupled together to function as one). The major role of this battery is to start the engine. The battery also powers an interior light, energises electric door locks, keeps the electric clock running (and enables the alternator to start charging when the engine first turns over), but negligible power is required for these tasks.

Once the engine has started, every single electrical need (ignition, fuel pump, lights, radio, air conditioner etc) is thenceforth supplied by the alternator.

To start the engine, the starter battery supplies a very high current, but as it only does so for a few seconds the energy typically drawn is that required to power a sidelight globe for less than an hour. The alternator replaces this energy in a few minutes, but only to the level where the battery can reliably restart the engine (70%-75% of full charge). The charge rate is then drastically reduced, to a level that is so low it will take ten to twelve hours for the battery to achieve 85% of full charge this is done so that the battery will not be overcharged in taxis, long distance transport vehicles etc where charging may continue virtually non-stop.

This reduced charge rate is just acceptable for a campervan that is driven several hours each day, but it is next to useless for a motorhome where, (unless supplemented by other sources of energy) the batteries must supply sufficient electrical energy to supply all needs when the vehicle is stationary perhaps for several days.The 'obvious' cures of (a) fitting a larger alternator, and (b) fitting larger batteries, make less difference than one might reasonably expect. Regardless of battery or alternator capacity, a standard car/truck voltage regulator 'perceives' your alternator/ battery as a starting system and charges it accordingly, i.e. reasonably quickly to 70% - 75% charge. Unbelievably slowly thereafter.

At first sight this might not seem too bad: after all, you might reasonably say, 70% - 75% battery charge is close to three-quarters full. But it IS that bad - a battery cannot be safely discharged below 50 or so per cent, and if you do so its life will be drastically shortened, typically to a score or so of such cycles.

In practice then, usable battery capacity starts at 50% charge. A 100 ampere/hour battery thus has a theoretical capacity of 50% - but only if l00% charged. Which it will never be. If the battery is only 70%-75% charged (as is typical of most systems) the usable capacity of our 100 ampere/hour battery is 20 - 25 ampere/hours.

It's actually worse than that because, like people, batteries degenerate with age - losing at least 10% of total capacity each year.

Realistically, using good newish batteries, in a well thought-out motorhome system, a nominal 100 ampere/hour battery can be reliably charged to a bit over 90% capacity, thus providing about 45 or so ampere/hours.

In many instances, particularly where the owner has accurately calculated the battery requirements, the batteries already installed may well be adequate - they have only appeared to be undersized because they were not being adequately charged.

In other instances, larger batteries may be required. But whilst larger capacity batteries are capable of storing more energy, that energy must first be produced - and as we have seen, the standard charging system is fundamentally incapable of doing so in a realistic length of engine-running time.

To charge your existing or larger batteries adequately you will almost certainly need a larger output alternator than that fitted as standard, but fitting a larger alternator is not in itself enough.

However it is also necessary to replace the existing voltage regulator, which is increasingly commonly housed within the alternator, by a so-called 'smart regulator' or other system that enables the alternator to charge the batteries at a faster rate. This is the vital key to a successful electrical system, and will be covered in detail later in this series of articles.

Fixing the Problems

Once the fundamentals are understood, fixing the various problems is relatively easy. Here's what must be done:
If physically feasible, an excellent approach is to install a second, electrically separate, alternator, regulator and batteries to supply the house needs, retaining the original system for starting etc. This is generally possible with coach and truck based motorhomes, but there is not usually sufficient space around the engine in cars and campervans.

If you do use this approach, you still need to apply the general principles outlined in this series of articles.If the existing single alternator system is preferred, the essential requirements are (1) and (2) below.

1. To install a system that parallels the starter and house batteries (for charging) when the engine is running, but separates them automatically the moment the engine stops.

2.To install a high output alternator and modified, or 'smart' regulator that will allow the alternator to recharge the batteries quickly and fully.

It may also be necessary to install house batteries large enough to supply two/three days needs - preferably so-called 'deep-cycle' batteries that will withstand frequent discharging to 45%-50% of full capacity (my second article discusses batteries in more detail and shows how to estimate needs and assess required battery capacity).

Optionally, we may also add solar panels, battery chargers or a generator to assist battery charging whilst stationary. This approach, particularly the use of solar panels will be covered in the third article in this series.Also optionally, it helps to have a system that measures and displays the batteries' state of charge. Note here though that this cannot be assessed from battery voltage - unless the batteries have 'rested' for about 24 hours.

Component Changes

The existing starter battery can be retained, or upgraded if there is doubt about its adequacy. Starter batteries can be used to supply the house needs, but will degrade quickly. Deep-cycle batteries are a better although more expensive choice for prolonged life and long-term reliability. Good deep cycle batteries cost about $1.70 per nominal ampere/hour.

A high output alternator is required - preferably 120 amp minimum (with a suitable regulator it is safe to charge batteries up to 25% of their nominal ampere/hour capacity). A high output alternator will cost $150 - $300.

The alternator regulator must be modified or replaced by a so-called 'smart regulator' (about $400).Other car, truck, caravan or boat components, such as switching solenoids, light globes, fans etc are readily usable. These components are available in a wide range of quality and prices, with the better quality items stocked mainly by boat equipment vendors.

Quantifying Your Needs

Having established what is realistic to use, and what is not, the next step is to calculate one's daily electrical consumption and assess whether what you'd like is electrically realistic.

Using Table I as an example, make up your own Table and calculate the average daily current consumption as follows:

1. Take the current drawn by each device and enter it against each item in column. If there's more than one of each item (as with lights) enter the total current drawn by them all.

2. For each entry in column 1, write down (in column 2) the maximum number of hours that each device will be in use each day.

3. Multiply each entry in column I by each entry in column 2.Enter this in column 3.

4. Total the entries in column 3. This figure is the consumption in amp/hours each day.

This table is for 12 volt systems. For 24 volt systems, halve the current consumption. A refrigerator cycles on and off continuously. The figure used above is typical for a small (40 litre) well-ventilated chest-type unit set to 4 degrees Centigrade, on a typical summer day. This when figure may vary by 50% or more from model to model .

Deep cycle batteries cannot be discharged continuously - repeatedly below 45%-50% of full capacity without progressive damage. Thus a nominally I 00 ampere/hour battery has an effective capacity (new) of 40-45 ampere/hours if discharged over a period of not less than 20 hours or so, i.e. it's good for 5 amps.
The examples in Table I are typical for a campervan, caravan or small/ medium sized motorhome (and are those of my OKA). If your totals are more than 30% or so higher than shown, you may need to rethink your requirements, or provide supplementary means of charging (see below).

The next step is to calculate the battery capacity required to store this energy. This depends on the intervals between recharging (by whatever means) - and on supplementary sources of energy such as solar panels.

Calculating Battery Capacity

As noted in the first part of this series, a battery can only realistically be charged to 90% of full capacity (and even that requires a optimal charging system) and even the so-called deep cycle batteries cannot be discharged repeatedly below 45% - 50% of full capacity without progressive damage.Thus a nominally 100 ampere/hour battery has an effective capacity (when new) of 40-45 ampere/hours if discharged over a period of not less than 20 hours or so, i.e. it’s good for 5 amp for about 20 hours, but less if discharged at a higher rate (typically 10amps for about 9 hours, 20 amps for about 4 hours etc. Taking our daily consumption (from Table 1) of 60 ampere/hours, we thus need battery capacity nominally (i.e. what the maker claims) rated at 140 ampere/hours or so for every 24 hours between charging. (it’s advisable to have a bit more)

TABLE 1

This will allow for battery aging - 150/160 ampere/ hours for every 24 hours between charging adds a comfortable but not excessive 'ageing' margin.If you'd like to have 48 hours between charges, you'll need about 300-320 ampere/hours/. For 72 hours, you'll need about 450-480 ampere/ hours.

You must now decide whether this is feasible - in terms of, cost weight and space.
As a rough guide, good deep-cycle batteries cost about $175, and weigh about 40-50 kg, per nominal 100 ampere/hours. Batteries themselves are not physically large but they, unless of the totally sealed variety, must be located in a well-ventilated area. Allow about 0.02 cubic metres per nominal 100 amp/hours.

Starter batteries can be used for 'house' needs, but they are less than enthused about being discharged to 50% at all,-and will suffer virtually certain failure if you do so more than a score or so times. Starter batteries are however a lot cheaper and a bit lighter than deep-cycle batteries. They are worth considering if you are doing a six-month trip and can afford to write off their cost afterwards. But for long term continuous use deep-cycle batteries are really the only choice.

The so-called suspended gel batteries are a viable alternative. They are more costly than conventional lead

acid deep cycle batteries, but because they can be discharged more deeply without damage, a 70 amp/hour gel battery adequately replaces a 1 00 amp/ hour lead acid battery.They require no ventilation and can be housed at any angle (including upside down!). Before specifying however, check out your proposed system with the manufacturers, as these batteries are deeply upset if charged at higher than 14.4 volts.

Reducing Battery Capacity

The need for battery capacity can be reduced by:

1. Reducing the demand.
2. Reducing periods between recharging.
3. Supplementary charging. Reducing the demand is obvious, but if you intend to use only electric lighting and refrigeration, your absolutely minimum daily consumption will still be 30 - 40 ampere/hours.

Reducing times between charging is also obvious (but not necessarily convenient). The remaining alternative is supplementary charging. Here, solar panels are by far my preferred choice. (The two by 77 watt panels on my OKA provide virtually all of our required daily energy requirement).

Here's a few solar panel facts.

Firstly, disregard solar panel makers' claims for wattage output - the actually achieved maximum wattage of all solar panels, of which I am aware, is about 80% of their manufacturers', quoted figures. The discrepancy is due to an industry-standard method of measurement that is so electrically romantic one wonders that the promotional literature is not written in iambic pentameter.

Reality is about 80% of the advertised output. To convert to amps - divide that figure by twelve (for 12 volt systems),

For most of Australia you can safely assume you'll average the resultant amount for four to five hours a day in the summer; and three to four hours a day in the winter. Continued overcast conditions will reduce this by about half. In desert, central and northern parts of Australia, daily output may be IO- 15% higher.My OKNs two 77 watt panels generate a bit over 60 watts per panel about 10 amps - at full output. I thus have up to 10 amps available for three to five hours each day, i.e. 30 - 50 ampere hours, or about 35 - 60 ampere/ hours in central and northern latitudes.

The above figures are for the panels mounted more or less flat on the roof on my vehicle. Except perhaps for Tasmania, it's not worth optimising solar angles. The gain is well under 10% even in mid-winter - and you'll only achieve that if you remember to move them several times a day.

Some users like to have the panels completely portable so that the vehicle can be placed in the shade and the panels in the full sun. This is fine in theory, but (a) it's not clever to park under trees, (b) you'll get sick' and tired of positioning the panels every time you stop - and they are also eminently steal able. Better by far to have them permanently mounted and permanently connected.

The nominal (claimed) wattage output of solar panels is 90 - 100 watts/ square metre, they weigh about 15 kg per 100 watts and cost about $1000 per 100 watts. You'll also need a solar regulator ($80 - $180).So - if, from Table I you've worked out that you need 60 ampere/hours per day, than from the above data you can see that, adding one 77 watt panel producing (in winter) 15-20 ampere/hours will reduce total energy to be stored by that amount - i.e. to 40 ampere/ hours. Two panels will produce 30-40 ampere/hours, thus reducing required stored energy to only 20-30 ampere hours.

This of course results in huge savings in required battery capacity - or much longer periods between charging.

Adding solar panels provides a further major benefit: batteries last very much longer if kept close to fully charged. Solar panels achieve just that. Alternative methods of supplementary charging are to run the vehicle's engine and alternator whilst stationary, or to use a motor/generator.

Both alternatives are noisy. The former is practicable given a high-output alternator and smart regulator. A well set-up combination will charge the batteries at 60 - 75 amps and, given such a regulator, batteries can safely be charged at up to 25% of their nominal amp/hour capacity.

The latter is theoretically practicable but as most small generators have very limited 12volt output, it is necessary to use the 240 volt output via a high capacity battery charger.

A further and not overly-complex alternative (for the mechanically Sept) is to make your own generator using a suitable (preferably 4-stroke engine) and a high output alternator and smart regulator.

An absolute must for any campervan/ motorhome electrical system is to ensure that the 'house' batteries are automatically isolated from the starter battery when the engine is not running, i.e. reserving the starter battery for that purpose alone. A good and theoretically simple solution is to have two quite separate systems: i.e. a second, engine-driven alternator and regulator connected directly to the house batteries, retaining the original vehicle electrical system as is. This is an excellent and virtually foolproof approach, but installing such a system is not a job for the inexperienced, nor is it mechanically practicable unless there is ample room around the engine to mount a second alternator.

A more common approach is to connect or isolate the starter battery by changeover switches, switching diodes, or solenoids. Changeover switches and diodes are rarely specified nowadays, but I've included them as many older vehicles use these methods - and they can be a cause of unsuspected woes.

Battery Changeover Switches

Still commonly used on boat systems, a battery changeover switch enables both starter and house batteries to be connected to the alternator for charging; either or both batteries to be paralleled for winching or starting; and either or both batteries to be used for supplying the house loads - although in practice it is more than prudent to reserve the starter battery exclusively for that purpose.

With this system the changeover switch must be manually operated every time you stop and start the engine for any length of time - and sooner or later you will forget to do it. Usually sooner.

There is a further and potentially major drawback. An alternator totally relies on the connected battery to provide its voltage reference. If the battery is disconnected, even momentarily, the alternator goes into voltaic lunar orbit, burning out its rectifying diodes and often the regulator as well.

Most battery changeover switches safeguard against this by connecting the second battery before disconnecting the first, or momentarily disabling the alternator's field winding (which cuts alternator output). But a dirty or loose switch contact represents instant death to the alternator (as can a dirty battery connection).

Diode Isolators

Diode isolators allow the alternator to charge all batteries in parallel, and automatically separates them when current is required to be drawn. This method is reasonably effective (but see below) with older vehicles in which the voltage regulator is located externally from the alternator, but it can cause havoc with modern vehicles which have complex electronic management systems. This latter problem can be overcome, but still leaves a further major failing of diode isolation: that it delivers the greatest charge to the flattest battery. However it's the starting battery that needs charging priority - and it is not helpful for it to have to compete for power with a battery supplying, say, a blender and the TV!

Solenoid Switching

This is the most common way of isolating the starter battery. It is simple and relatively trouble-free.

A solenoid is a switch that closes when electric current flows through an associated coil. A lead from the ignition system applies power to the solenoid coil when the ignition is switched on thereby actuating the solenoid, and causing its contacts to parallel the starting battery and the house batteries across the charging circuit. When the ignition is turned off the solenoid deactivates, thus electrically separating the batteries. The system can also be used with diesel-engine vehicles by wiring the solenoid across any electrical device that is activated when the engine is running, and is de-activated when the engine is turned off, e.g. an oil pressure sensor.The above system works reasonably well, but has a few less obvious problems.The biggest such problem is that interconnected but unequally charged batteries have Marxist tendencies they attempt to equalise their charges, i.e. the more charged battery discharges into the lesser charged battery until charges are equal.

This is not necessarily helpful. If the house battery is flat, the starter battery will begin to charge the house battery (via the now-closed solenoid contacts) the moment the engine is started. If the engine were to stall at any time during the next two or three minutes, both batteries may now be too discharged to restart the engine.

To overcome this, we need to give the starter battery priority, by preventing the solenoid from energising until the engine is well and truly running, or preferably only energising when the starter battery is at a voltage high enough to ensure restarting.

A partial solution is to energise the solenoid via the oil pressure sensor here the engine must have started and be running above a certain speed before the solenoid cuts in. But if the engine stalls shortly after starting, a flat house battery could still have drained the starting battery before either had a chance of being charged by the alternator.

A safer way (and that used by the author) is to use a device that continuously monitors starter battery voltage and energises the solenoid only when that voltage exceeds (say) 13.2 volts. If at any time, and for any reason, this battery drops below 13.2 volts, the full alternator/regulator output is directed to this battery -thus ensuring that it is available for starting.

Paralleling Batteries for Winching

Apart from a manually operated changeover switch, none of the above solutions, on their own,necessarily allow batteries to be paralleled for winching. This can be done by incorporating a dash-mounted switch (and associated pilot light) that energises the solenoid directly thus paralleling the batteries whether the engine is running or not. If you intend to do this, you'll need to use a solenoid capable of handling at least 200 amps - preferably more, or, as I've done in my OKA, using a second heavy duty paralleled solenoid just for this purpose.

(A simpler way of course is to use jumper leads, but do be aware that even the so-called 'heavy duty' jumper leads sold by most suppliers are generally useless for starting a large (3 litre plus) diesel when cold.) Cheap solenoids have brass contacts - they are fine for a time, but eventually the contacts tarnish, leading to burnt contact surfaces, and large voltage drops. Better-quality solenoids have silver alloy or pure silver contacts. These are less prone (but still not totally immune) to corrosion.

Follow solenoid fitting instructions carefully - some solenoids must be fitted apparently upside down to enable gravity to assist the closing action!

Ensuring Adequate Charging

As emphasised in the first two parts of this series, only in the rarest of circumstances will a battery ever achieve 100% charge. To do so requires specialised charging equipment and entails charging for very long periods of time. In practice, most standard vehicle systems will, at best, bring a battery to 70 - 75% of full charge, thus leaving a mere 20 - 25% of nominal battery capacity available for use. A realistic target to work towards is 90% -95% of full charge (thus leaving 40 - 45% capacity available). To achieve this is not difficult but it can only be done by using an adequate capacity alternator and a regulator (or battery control system) that is made specifically for the purpose. A fully charged battery has a potential of about 12.8 volts. This falls to about 12.2 volts when the battery is about 50% discharged - and to about 11.8 volts when 70% discharged. Today's alternator regulators and battery chargers are set to limit charging voltage to somewhere between 14.2 and 14.4 volts (earlier regulators were set at 13.8-13.9 volts) and the difference between the charging voltage and the battery voltage determines the charging rate, i.e. as battery voltage rises, charging rate automatically falls. Charging batteries from a standard alternator and regulator is cheap and simple but is not effective unless the vehicle is driven for many hours. Typically, an alternator and regulator will charge a starting battery at 30 - 50 amps for a few minutes, but this rate quickly drops, typically to four or five amps and falling. With the standard car system almost invariably supplied with a motor home, a 100 ampere/hour battery that is 50% discharged will require ten hours or more driving time before it is even 80% to 85% charged. It is unlikely ever to be charged beyond 85% unless the vehicle is driven 12 or more hours each day.

As a result, vehicles that are not in more or less constant use are likely to have batteries that are around 70% charged. This is no use for house batteries that must operate from about 50% to 90% of full charge.

I cannot emphasise too strongly that house batteries in motorhome electrical systems have little chance of being adequately charged by a standard vehicle alternator and regulator. Installing a larger capacity alternator will not help because the regulator will maintain the previous charging regime.If a vehicle charging system is to cope with house batteries, it must be modified accordingly. The almost universal failure to understand this is the cause of most recreational vehicle electrical problems.Such modification may, or may not involve upgrading the alternator, but it will involve installing a 'smart' regulator or battery management system designed for the purpose.

Part Three: The Alternator

The second part of this series showed how to determine the amount of electrical energy you need to generate each day - probably 50 - 80 ampere/hours for a modest system.We need now to determine whether the standard alternator can cope with this extra load, or whether (as is probable) we need to specify and fit an alternator of higher capacity.

Standard car or truck alternators typically generate 50 - 60 amps for short periods and about 60% of that continuously, i.e. about 30 amps. Of that 30 amps, 20-25 amps will be required for the vehicle's running needs. This leaves only 5 - 10 amps safely available for house battery charging, without risking overloading the alternator.

A good rule is to fit an alternator rated at about a quarter or third of the total amp/hour capacity of the batteries that it will be charging. A 120 amp alternator is usually adequate. Alternators with outputs greater than 120 amps will require larger width pulleys and drive belts to take the extra load..High output alternators are readily obtainable from auto electricians. They are usually direct replacements for the original (which are worth keeping as spares).A 120 amp alternator is quite safe to use indefinitely at 60% output and for short periods at 80% capacity, so with the vehicle's running needs as before we now have at least 50 amps available for battery charging - more than enough for all but the larger coach-based motor homes with all mod cons.

A different and alternative but more costly approach to the above is to install a 'KKK' specified alternator. These are designed to produce their rated output continuously (if anyone knows of an Australian supplier I'd appreciate a call - but they are readily obtainable in the USA).

Regulators

We have now determined the energy requirement we need, the battery capacity required, and whether we need to upgrade the alternator. (But once again I reiterate that upgrading the alternator alone will not result in substantially increased charging).

An alternator's output is controlled by varying the amount of current through its field winding (a series of connected coils surrounding the revolving stator inside the alternator).

The voltage regulator, which is located inside the alternator or externally, controls the alternator field winding current - and hence the alternator output voltage.

To operate satisfactorily with recreational vehicles, the regulator must be modified or overridden in such a way that the alternator is allowed to charge the batteries at a higher current for a longer period of time.

If you have a good practical knowledge of electricity, manually overriding the regulator is quite easy. All that is required is to supply a controlled de voltage to the alternator, field via switched resistors or a high current rheostat. For fast charging, the manual control is set so that the voltage across the battery is about 14.4. Once the batteries are around 80% charged, the voltage is reduced to around 13.9 volts, or the manual control switched off.

This approach has been used for decades by cruising yachties but it does require a thorough understanding of the charging process, and considerable self-discipline to monitor what's happening. There is a very real risk of cooking the alternator or batteries - or both (although the former can be safeguarded against by arranging for the maximum manually set charging current to be less than the alternator's safe continuous maximum output).Whilst I recommend against it, if you are prepared to risk this technique, a good practical guide to all this is included in 'The 12 Volt Doctor's Handbook' by Edgar J. Beyn (published by Weems & Plath, ISBN; 1-878797-00X) obtainable from boat equipment suppliers. Commercial devices that work in similar ways are available from marine suppliers; generally similar versions incorporating overcharging limiting are also commercially available.

Smart Regulators

In the author's opinion, a better but more costly solution is to fit a so-called ‘smart regulator'. A smart regulator charges batteries at a high but safe level until the batteries are about 90% charged, and then cuts the rate to avoid overcharging. These regulators reduce charging time to well under half.

Some, such as the Hella regulator in our OKA, use a cycling technique alternating high and low charge rates. In our OKA the high rate is up to 100 amps - a bit alarming when experienced for the first time!

(A few motor home owners have told me of bad experiences with Hella regulators. These problems seem to concern an earlier type that had inadequate cooling for Australian conditions.)These types of regulator are extensively used on cruising and 'round-the-world' racing boats. They cost $300 upwards and are readily available from boating suppliers.

The best known are made by the (US) Ample Technologies, and Cruising Equipment Company, and the (British) Adverc BM, TWC, and Hella companies.The devices cost from $350 upwards. They are reasonably simple to install, but it's best to have someone experienced in this area to do it for you unless you are sure you know what you are doing. Boat electricians (who typically have the know-how) are often strangely reluctant to work on anything except boats, but telephoning around will usually locate a willing soul.

Monitoring Batteries

It is extremely useful to know the batteries' state of charge: apart from the convenience of knowing how many hours lighting etc are left, batteries are damaged by over-discharging (i.e. below 50%) and some electrical equipment (particularly motors) may be damaged if run at low voltage. Unfortunately battery voltage provides only a very rough indication of the state of charge; any more than very minor rates of charge and discharge initially and primarily affect the surface layer of the battery plates. The voltage across a fully charged battery is about 12.8 volts, but so, for a minute or two, is that of an almost flat battery that has been briefly charged at a high charge rate. A 50% discharged battery will have a rested voltage of about 12.2 volts (as may a far more charged battery if measured directly after applying a heavy load).

It takes time for a charge or discharge to 'soak through'. Hence, unless a battery has had a chance to rest for 12-24 hours, a voltage measurement reflects only the surface conditions.

An accurate expanded scale (i.e. reading from 11 - 15 volts) enables one to monitor charging voltage but unless it's of high quality it's next to useless remember that the range from fully charged to 50% charged is only 6%.

Apart from the very messy and tedious process of taking hydrometer readings of the battery acid, the only accurate way of monitoring battery charge is via specialised, and currently expensive, battery charge monitors.

These devices measure both the charge current going into the batteries, and the current drawn out of the batteries, and (allowing also for charging inefficiencies) indicate the present battery charge. There's so much to this subject that, editor permitting (which, having been one myself, seems improbable in the extreme), this series of articles could continue for at least 12 more issues.So rather than pushing my luck I've finished by covering various matters, including the use and abuse of 240 volt inverters, and mains power; and have included a few hopefully useful tips on wiring. I've also included a selection of questions asked by members.
In the latter regard, 1 am happy to answer short queries over the phone, but please, I am a working writer/engineer hence cannot provide unlimited free services. My apologies to the innumerable readers who've left messages (in one case, abusive) asking me to call interstate at my expense. You're welcome to my help. But you pay for the call!My comments that few auto electricians understand motorhome electrics applies primarily to the battery charging system. On that I stand totally unrepentant. The remainder of a motorhome's electrics are generally similar to car and truck practice and I did not intend to imply a general lack of competence in these areas.

How Big an Inverter?

Appliance and inverter makers think in watts. To determine the size inverter you need, check the wattage rating of each appliance. If bought in Australia there will be a label or stamp denoting volts and watts, e.g. 1 00 watts - or I 00 W. You may encounter an appliance that just gives volts and amps - if so multiply amps by 240 to obtain watts.

Total the wattage rating of all the appliances you are likely to have on simultaneously. That's the wattage rating of the inverter that you'll need. Good inverters will withstand short-term overloads so you don't need to panic if you've forgotten to allow for the coffee grinder whilst watching TV!. Overload ratings will be specified in the makers' literature.

The current drawn by an inverter from a 12 volt battery is approximately the wattage drawn by each appliance (when it's in use) divided by I 0- I 1 for an inefficient inverter, and divided by 12 for a more efficient unit. Thus a I 00 watt blender will draw somewhere between about 8 and 1 0 amps; an 800 watt microwave - between 65 and 80 amps.