Electricity for Camping

Friday, Sep 21, 1990 at 20:49


Revised March 2013

On an extended trip it's good to have some of the comforts of home. Electricity is obviously one of them and a good system can make all the difference to how comfortably we live while on the road. A reliable source of electricity allows us to have good camp lighting, a cold beer, battery charging for the laptop, camera, phone, ipod, gps, torches, run shower, hf and uhf radios ......and the list goes on. But all this gear costs energy to run and we've met people on the track and on the EO forum with little understanding of the power drawn by the gear they "need".

While for example, 5 watts will provide good camp lighting using LEDs, heating gadgets like a hair dryer or microwave will demand 100 times that or more. While it's theoretically possible to run these "essentials" from an inverter driven by vehicle electrics, it is generally not practicable to do so.

We'll discuss amps and amphours and watts and such stuff later, but for now please accept that we are measuring our electricity consumption in amphours (Ah).

So just how much electric power do we actually use in camp each day? A fairly typical electricity budget might look something like this:

Fridge (small compressor type running about 1/3 of the time): 3.6 Amps x 7 hrs = 25 Amphours
Camp lights : 1.7 Amps x 3 hours = 5 Amphours
Laptop: 4Amps x 1 hour = 4 Amphours
Charging cameras, torches etc: 1 Amp x 1 hour = 1 Amphour
Radios etc = 1.5 Amphours

TOTAL: 36.5 Amphours

The critical numbers here are the TOTAL amphours and the fact that, even though we are talking about a small fridge, the fridge accounts for 2/3 of that total. Some fridges use less than this, many use more, so this is a reasonable figure for our discussion here.

How do we know how much power our gadgets require?

The only reliable way is to measure the current they draw. Most power packs for laptops and entertainment gear will be labelled with the maximum power the pack can supply, but not the average drawn by the appliance, which is what we really need to know. Talking of measurement, a multimeter (available for less than $20) is a very useful tool when working on these electrical systems.

So, where's that electricity going to come from?

It can originate from the vehicle's alternator, from a battery charger running off 240V mains supply, from solar panels or from a petrol or wind driven generator. In all cases though we will need a battery to store the electricity produced for times when those sources aren't producing. We will put electricity into the battery, then draw it out as we use our appliances. We'll discuss later just how much storage capacity we need.

The vehicle's own (cranking) battery has only one critically important function - to start the vehicle's engine. Once the engine is running, the alternator driven by the engine will meet all the vehicle's needs, and can create a surplus which can be used for our domestic needs. While it's reasonable to take a little from the vehicle's battery for general camping use, a dedicated auxiliary battery is really called for. An auxiliary battery will avoid the problem of running down the cranking battery perhaps to a point where it can't start the engine. Moreover a battery designed for this sort of service, a deep cycle battery, will perform the domestic chores better than a cranking battery can.

Before we start talking about batteries though, lets talk briefly about watts and volts and amps, the units by which aspects of electricity are measured.

The FLOW of electric current is measured in units called amperes or amps for short (abbreviated as A). Voltage (V) refers to the force that drives that flow of electric current. A watt is a unit of electrical power, obtained by multiplying the voltage by the amps (the force by the flow rate.) Our vehicle electric systems operate at a nominal 12 volts. Since that voltage is set, the amount of electrical power (watts) flowing is directly proportional to the electric current, the amps. So, in vehicle electrics we often take an intellectual shortcut and think of amps as relating to power. We refer to the total flow over time as so many amphours (abbreviated Ah). If 1 amp (A) flows for 10 hours, that's 10 Ah. So is 5 amps for 2 hours or 10 amps for 1 hour. The amount of electrical energy is the same in each case.


All of the batteries we are concerned with here use lead-acid chemistry, which pretty much defines their voltages. Trace amounts of other elements, notably calcium, may be included to produce particular characteristics and these will change the basic voltages a little. A standard 12V lead-acid battery consists of 6 cells, each with a voltage of close to 2.1V. These cells are connected in series inside the battery to provide about 12.6V

Battery capacity figures can be a mystery.

Cranking batteries (used for starting a vehicle) usually have a CCA (cold cranking amps) rating. They are intended to deliver high currents (maybe 100 to 200 A) for a few seconds. For a big 4WD we'd choose the highest CCA rated battery that will fit in the space available. While a CCA rating of 400 might be ok in the sedan, we'd look for double that, and maybe use two batteries, to start a 4 litre diesel on a cold morning.

For the auxiliary ("house") battery we choose a deep cycle one. They are intended to supply smaller currents (fewer amps), but for longer periods. (eg maybe an amp for lights for a few hours.) These deep cycle batteries are usually rated in amphours (Ah), relating to the number of amps, multiplied by the number of hours we can expect from the battery when fully charged. So a 100 Ah deep cycle battery might deliver 5 amps for 20 hours, or 20 amps for 5 hours, and that's roughly what happens. The rated capacity is measured in Ah at a particular rate of discharge (usually the 20 hour rate), and varies a bit with the rate of discharge.

It is important to be aware that fully discharging any battery will seriously shorten its life, especially if it's left discharged for very long. Generally it is recommended that in spite of the name "deep cycle", a battery should not be discharged more than about half, 2/3 at the most if it is to have a good life span. So while our 100 Ah battery might deliver 5 amps for 20 hours to be totally exhausted, if we value it, we shouldn't discharge it at this rate for more than 10 or 12 hours.

There are various types of deep cycle batteries. A comprehensive discussion of the pros and cons of these different types, wet, gel, flat and spiral wound AGM, may be found on the Battery Value sitehere. Their Deep Cycle FAQ also provides valuable information. We will be concerned mainly with AGM batteries, though gel types are also commonly used by travellers. A constraint to bear in mind is that gel and AGM batteries are not suitable for mounting in the engine bay due to the high temperatures there.

Historically, the electrodes in batteries were immersed in baths of concentrated sulphuric acid, dangerously corrosive stuff. Most cranking batteries and the cheapest deep cycle batteries still use this so-called wet cell arrangement. Now though, gel and absorbent glass mat (AGM) batteries are generally available and are much more friendly to handle. These are based on much the same chemistry and use the same acid, but the acid is contained in a gel or absorbent mat. With this construction, the battery may be fully sealed and even used upside down without losing acid. (Although the acid is captive, these batteries are best mounted right way up since the internal structure is designed for maximum strength when mounted this way.) Because the acid is captive, gel and AGM batteries may be safely carried inside a vehicle. (Wet cell batteries should not be carried inside a vehicle as not only is the acid a hazard if you accidentally go upside down, but they give off small amounts of explosive hydrogen when charging.)

Now, let's go back to our energy budget that called for about 36 amphours each day, most of it just to run the fridge. A 100 Ah battery is typical of what we'll need for our auxiliary battery. In 2 days at 36 amphours per day we will use about 72Ah or 2/3 of the capacity of our 100 Ah battery, and that's about as far as we would wish to discharge it.

Usage Considerations

The fridge is energy hungry, so managing the fridge is important - add the beer in the morning when the beer is cool, not late in the afternoon when it is as hot as it can be. Don't run the fridge too cold, and avoid opening it frequently. Larger fridges are becoming more common and some travellers carry two fridges, which adds considerably to the daily energy demand. Note that we are talking here of small compressor style fridges, not the 3 way type which would be impossibly demanding (about 300 Ah per day!) when running from our battery. Also note that if we run the fridge as a freezer, the power requirement gets very much higher.

If we are going to run a fridge, it is going to be by far the biggest energy user, so saving a little on more efficient lighting doesn't count a lot in the daily energy budget. Remember though that many small 12V devices if used for a long time will have a cumulative impact on our energy budget. Personally, we don't stint on lighting. LEDs and fluorescents are certainly more efficient than halogen lighting, but we use dichroic halogen lamps, which produce white light of excellent quality. A single 20W halogen, or 5W LED, provides good camp lighting; a 10W halogen (or 2W LED) makes a good reading light.

Something to be aware of: Any heating device uses lots of energy. Jugs, hair dryers, microwaves, toasters, electric frypans etc are far too power hungry for a system based on one or two 100 Ah batteries.

How do we charge an auxiliary battery?

First, a few preliminaries and general considerations:

Any battery in the auxiliary system MUST be fitted with a fuse or circuit breaker close to the battery to protect the wiring and minimise the risk of fire. A 30A fuse or 50A circuit breaker is usually suitable. I favour a fuse, since it is inexpensive, operates more rapidly in the event of a fault, and the battery can be conveniently disconnected from the system by simply removing the fuse. Self resetting circuit breakers have the unfortunate habit of re-connecting at an inopportune moment!

Regardless of what charging system is used, it is essential that heavy wiring and good connectors such as Anderson plugs be used to minimise resistive losses. Just how heavy gauge the wire needs to be varies with each setup, but 6 B&S is often a good choice. (Details are in Appendix 2.) The charger or controller should be mounted close to the battery so as to minimise the effect of wiring losses on the charger's operation.

It is possible to use two or more batteries connected in parallel (ie positives connected together, and negatives connected together) to produce effectively one large capacity battery. Because of the different threshold voltages and other differences between different types of batteries, only batteries with similar age, history, chemistry and structure should be used this way.

Now to business

In a perfect world batteries would be charged by a three (or more) stage charger. In the first stage (bulk) these deliver a constant current (amps) while the voltage slowly rises to a predetermined maximum. After this the voltage is held constant during the second stage (absorption) and the charging current will slowly fall as the battery approaches maximum charge. When the current has dropped to a low value, around an amp for our deep cycle batteries, the charger switches to the third stage (float), the charging voltage is reduced a little to provide a very slow trickle of current.

Different structures and chemistries lead to differences in these threshold voltages and currents. For present purposes though, the constant current phase should use a current no higher than 1/5 of the battery's Ah rating (e.g 20A for a 100Ah battery). The constant voltage phase may be around 14.4V for wet and most AGM, batteries, usually lower for gel and a little higher (14.7V, sometimes up to 15V) for some AGM types. The particular battery will carry the manufacturer's recommendations, which should be observed. This applies particularly to AGM, gel and calcium doped batteries.

In the real world, the auxiliary battery may be charged by the alternator that delivers a voltage that varies with alternator temperature and is intended to suit the cranking battery. The voltage will be a bit low for charging the auxiliary battery. (Note that while the alternator is running, it will be powering the fridge and all the other gear too. This loading can cause a voltage drop that further limits the voltage available for charging the battery. Heavy cabling is essential to minimize this drop.)

The most critical limitation is that the charging voltage supplied by the alternator is generally too low to fully charge the auxiliary battery. Three stage dc-dc chargers are available to accept the alternator voltage and increase it to that required by the "house" battery. (Something not to be overlooked - a dc-dc charger is intended to deliver the rated current to the battery in spite of its source voltage being too low. Larger ones can make excessive demands on the alternator. Before fitting one, check that the alternator can deliver!)

Charging from the vehicle

In the simplest case, we can connect the auxiliary battery to the cranking battery while the engine is running and the alternator will provide charging current to both. This connection can be done manually, but it's far better to use a proper controller (a Voltage Sensitive Relay (VSR) such asthis one.) which makes the connection only after the cranking battery has been recharged, and will reliably disconnect the batteries from each other when the engine stops. Disconnecting is important, since otherwise the "household" electricity will be drawn partly from the cranking battery. It is essential too that the batteries be disconnected from each other when starting the engine so that the high starting currents don't damage the "house" battery and wiring.

Alternators fitted by the vehicle manufacturer are often rated at 55 amps. It's worth keeping this in mind as we start adding extra loads such as fridges, extra batteries, and especially if we include a big dc-dc charger. Higher capacity alternators (80, 120A) are also available and may be required.

These simple systems have limitations, as the charging voltage from the alternator is tailored to the needs of the cranking battery rather than the deep cycle battery. The available voltage is a little low for charging an auxiliary battery and drops further with increasing temperature in the engine bay. This low voltage problem becomes worse if the auxiliary battery is mounted some distance from the cranking battery, perhaps in the back of the vehicle or in a trailer, as more voltage is lost due to resistance in the wiring. In addition, the chemistry of most deep cycle batteries will be slightly different from that of the cranking battery and call for higher charging voltages (at least 14.4 and some over 15V instead of 14 -14.4V for the cranking battery). It is not possible to FULLY charge these deep cycle batteries directly from a vehicle alternator.

Dc-dc chargers, referred to above, may be used to increase the available voltage and provide multistage charging to the deep cycle battery. They provide a constant current stage, followed by a constant voltage stage, before switching to the float stage, and are usually rated according to their constant current output, 20A, 30A etc. They effectively draw extra input current and trade it for higher output voltage, so draw considerable current from the alternator. Alternator capacity needs to be considered before fitting a dc-dc charger.

Another option to increase the charging voltage from the alternator is to modify the vehicle's voltage regulator circuitry, but this requires electrical expertise. A simpler approach that avoids tampering with the alternator itself is to include a small voltage drop in the voltage sensing line. Neither approach is recommended for the novice and both may be highly confusing to the computers in recent model vehicles!

Charging from the 240V mains

Different types of batteries, Gel, AGM, wet, calcium..., require different charging regimes. Good chargers are not cheap, but cheap chargers are not good and can cost battery life. The better chargers (multistage chargers) provide constant current charging, followed by a constant voltage phase and once charging is virtually complete, a slow trickle maintenance current, as described above. (Generally, the charging current should not exceed 20% of the battery's amphour rating - the maximum rate for most 100 Ah batteries is 20 amps.) It is the voltage at which the change from constant current to constant voltage occurs that differs between battery types; around 14V for Gel, 14.4V for AGM, 14.4 to 14.7V for wet and over 15V for most calcium batteries.

240V Generators

If you are running a fridge and you find the perfect camp spot you can stay there for a day or two relying on the power stored in your 100 Ah auxiliary battery. But if you are stopping for more than a couple of days some other way of charging the auxiliary battery will be needed. The options are solar panels or a generator.

Too often, you get what you pay for with the small cheap generators. A respectable generator may cost over $1000, and even then its inbuilt battery charger will usually not be satisfactory as most lack the regulation necessary for a good battery life span. You will then need a suitable 240V charger (preferably a multistage one) to run from the generator.

A generator may have other applications - you might carry a power hungry hairdryer or microwave, which would embarrass a battery based system. Don't forget though that in many campgrounds and national parks, generators are not welcome, and the noise may be a selfish intrusion on other's peace and quiet.

Another generator option that's worth considering, especially if you only camp occasionally: There is already a generator under the bonnet of your vehicle! Some travellers run their engine on fast idle for a while (say an hour) each day when stationery just to put some charge in the auxilliary battery. This costs fuel, but doesn't require any expenditure on extra gear, so may be a cost effective way to go, especially if the auxiliary battery has the benefit of a dc-dc charger to make best use of the alternator

Solar panels

Solar panels are quiet and increasingly affordable , though big and can be awkward to carry. Provided the sun shines, solar panels from about 100W upwards can probably meet the needs of an energy frugal campsite. Given good sunlight, 150W of solar capacity should handle a less frugal camp. The technology in this area is changing rapidly.

There are some important considerations when using solar panels. The maximum power output from a solar panel intended for 12V battery charging occurs when the panel operates at about 17-20V. This is a considerably higher voltage than a battery will tolerate, so a controller is essential to manage battery charging.

There are basically two types of controllers. In the past, simple Pulse Width Modulated (PWM) controllers were used to pass the current available from the panels directly to the battery. With this simple system the current drawn by the battery puts such a load on the panels that the panel voltage is pulled down to match the battery's requirements. Consequently the panel voltage is 25-30% below its optimum power generating point. As the battery approaches fully charged it draws less current, allowing the panel voltage to rise. When the voltage reaches the battery's maximum acceptable charging voltage the PWM controller disconnects the panel from the battery. These simple controllers are often included free with the solar panel.

Maximum Power Point Tracking (MPPT) controllers are now available at reasonable prices. These convert the panel voltage to that required by the battery, allowing the panel to operate at its optimum voltage, and effectively trading its excess voltage for additional charging current. Since the charging process is optimised by these controllers, they deliver to the battery about 15% or more solar power than do the simple PWM controllers. Also, they offer the advantages of multistage charging. Most will also monitor the discharging of the battery and disconnect it in the event of a fault, or to protect it from excessive discharge.

There is a big range of MPPT controllers, and some of the cheapest ones appear to rely more on "witchcraft" and marketing than on technology. They are simply not MPPT controllers. Others are excessively sophisticated and expensive. A good inexpensive one that I particularly favour comes from Battery Value, a commercial member on this site. It provides a 10A controller, suitable for up to 150W of panels, and ticks all the important boxes - it is a true MPPT controller providing multistage charging, and also provides over-current protection. Details are here.

With any MPPT controller, losses in wiring should be minimised by using heavy wiring, and the wiring should be kept as short as possible. (This is less important with a simple PWM controller since power will be discarded anyway.) Losses should always be avoided though between either type of controller and the battery, so the controller should be close to the battery, rather than close to the panels.

To achieve maximum power, the panels must be clean and aimed reasonably well at the sun, so must be moved a few times a day as the sun moves across the sky. (Purists might argue that it's the Earth that moves, but that's another story!)

According to our energy budget, if we could input about 40 Ah, we could replace one day's drain, so we'd gain an extra day's use. If we could do this every day from solar panels, we'd be fully self sufficient while ever the sun shone. Assuming 7 hours of strong sunlight per day, we'd need to push about 6 amps into the battery. To do this using a simple PWM controller we'd need a 100W panel, or with an MPPT controller, an 80W panel.

An 80 watt panel with a simple controller will supply close to 5 amps, which isn't quite enough to meet daily demand with 7 hours sunshine. With good sunshine though, it would take a week or more before the cumulative daily loss became a problem. A 60 watt panel will deliver only about 3.5 amps, or about 25 Ah per sunny day, so can meet only 2/3 of the daily demand. It will extend our stay from 2 days to 3 days before the battery is 2/3 discharged.

To harvest enough sunshine for a long term stay, allowing for some cloudy days or poor orientation, at least a 120W (or 2 x 60W) should be good. This is confirmed by our own experience. A 60W panel didn't help much, but adding an 85W panel to it to give 145W capacity was good if we had reliable sunshine.

What about 240V ?

The need to have 240V power when camping can be largely avoided. 12V chargers for cameras, phones, gps, laptop etc are readily available and are more efficient than using an inverter running from 12V to supply 240V to be changed back to some low voltage to charge the appliance batteries.

An inverter has basically two functions - to provide an alternating current (ac) voltage rather than the direct current (dc) available from the battery, and to raise the voltage up to an average of 240V. There are several classes of inverter. The most expensive ones provide a pure sine wave that is preferred for any sensitive equipment, especially laptops. The cheapest simply provide a square wave ac, which is satisfactory with most motors and some small chargers for cameras, phones etc, but not for most laptop computers. There are also intermediate types, "modified sine wave", which combine a number of square waves to approximate a sine wave shape. These are usually satisfactory for laptops, but, like the square wave types,can create a lot of radio interference. Some details of the three types may be found here.

Inverters are about 80% efficient. They come in different sizes. A 150W unit will handle most camp requirements, but may have trouble starting a laptop (even though the average drain by the computer is much less than this.) A 300W unit is probably a sensible minimum. Bear in mind that Watts = Volts x Amps, so, if we draw the full 300 watts, we will require 25 amps from the 12volt battery, plus 20% to account for inefficiency. That's 30 amps. This will draw from the battery in 1 hour about the same as our whole day's energy budget. There are also many larger inverters. A 2000W one will provide enough power to run power tools or even an electric jug, but at full output will draw from the battery about 150-200 amps. That's as much current as a winch when fully loaded, and way outside the comfort zone of any deep cycle battery. If drawing big power from these big inverters it is essential to run the engine so that the alternator can shoulder part of the load.

Something not to be overlooked - the 240 volts from an inverter or generator is just as lethal as the 240 volts in your home. To make matters worse, the devices fitted in our homes (ELCB's or RCD's) to disconnect the power if we get bitten by mains voltage, do NOT provide protection from generators and inverters. For an excellent discussion of the factors and physiology involved in the dangers of electricity , see The Human Conductor - Electric Shock and Electrocution.

So, finally, how much storage do we need?

First let's decide to have ample charging capability. That will probably call for a dc-dc charger capable of supplying at least 20A for travelling days, and at least 120W of solar panels (and sunlight!) to use on non-travel days. The dc-dc charger will take about 2 hours to meet our daily energy requirements, and the solar panels will need most of the day. If we have both and our 36 Ah per day budget applies, a 100 Ah battery should handle our requirements and allow for up to two days without sunlight and without running the engine. More storage or more solar capacity would provide a bigger buffer and more flexibility. If we were to increase our daily requirements by running the fridge as a freezer, or using a large fridge, we'd probably need to increase our solar capacity and our storage to maintain a reasonable buffer

There is another consideration. A minimal system would use a 100 Ah battery. A more substantial system might involve two of them. Some big rigs carry 500 Ah. Each 100Ah of storage weighs about 33 kg and costs around $150 to $400+. How many do we want to carry? Not too many!

Appendix 1

Measuring the state of charge of a battery

The question often arises "Can I measure how much charge is in the battery by measuring its voltage?"
The simple answer is NO. A slightly better answer is YES BUT.... where the BUT is like this - Battery voltage is very dependent on recent history. When a battery is being charged or discharged, the measured voltage depends largely on the rate of flow of current in or out, not just the state of charge. The battery voltage doesn't stabilise for hours after current flow ceases, so Yes, you can get some idea of the state of charge if you let the battery rest for a few hours before taking a measurement. Because of the different chemistries, different batteries will show different voltages at the same state of charge, so the measured voltage will give an idea, but not a good indication of state of charge. The curve in the graph applies to many wet cell batteries, but don't treat it as being reliable and note that it calls for a 5 hour rest period before measuring the voltage.

Well, can we measure the net flow of charge in and out of our battery to get some idea of the state of charge? Again.....Yes, BUT..... Batteries are generally about 90% efficient, so we need to put in at least 10% more than we take out. The efficiency gets worse if we take charge out fast - if we discharge at say 20 amps for a certain time, it will take much more than twice as long to recharge at 10 amps. So yes we can measure net in and out, but it isn't very useful.

OK, so how do you know when the battery is full, or getting low on charge?

Monitoring the battery voltage and current can give a good idea of the state of charge. If a charger is delivering say 14.4 volts to an AGM battery and the battery is only accepting a couple of amps, it is very close to fully charged. If the battery is delivering a few amps to the "house" system and the terminal voltage drops below 12V it's time to push some charge into it. If below 11.5V it's important to charge it. If it's delivering those same few amps and the terminal voltage has dropped below 10.8V, it is pretty close to being fully discharged and getting some charge into it is urgent and important! Suitable monitors such as this one are available too.

How do you pick that half to 2/3 discharged point? Watch those voltage and current meters, get to know just how much power you are drawing from the battery and how much you are putting back in. Aim to stay close to that 100% charged state, then you know you've got a day or two in reserve, and are comfortably inside the top 2/3.

Appendix 2

Wire sizing and resistive losses

What sized wiring should we use? These links refer to AWG sizing. B&S (more commonly used in Australia) sizing is the same as AWG. These sizes refer to the amount of copper in the cable, not the outside size of the insulation. Be careful - some wire sold as say 6mm is 6 mm diameter (outside the insulation!) not 6 square mm of copper. Best to buy by B&S (or AWG) sizing.
Wire gauges, sizing in mm, square mm, resistance per metre
Maximum recommended current as a function if wire size and length.

The voltage drop for a copper conductor can also be calculated easily using the equation:
Voltage drop = [cable length (in metres) X current (in amps) X 0.0164] divided by cable cross-section in square mm.
Remember to allow for the voltage drop in both the positive line and the negative (earth return) path.

Appendix 3

Some useful links and afterthoughts:

A very comprehensive article covering vehicle electrics and solar power may be found here on ExploreOz

An excellent source for detailed information on batteries generally is this one, and specifically for deep cycle batteries isthis one.

A good straightforward approach to wiring in an auxilliary battery may be found here.

Trailer wiring.

While there is no obligation to use the standard configurations when wiring a trailer connector, doing so can save a lot of frustration, especially when your mate wants to borrow your trailer! The standard colour coding and wiring configurations may be foundhere on Exploroz or or here.

J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein
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