Understanding Solar Power

12v dc

I see a lot of very basic questions in forums about 12v DC electronics and often see them replied to with some very complex answers.

12v DC electronics is actually a very precise science and it’s really quite easy to understand once armed with some basic information. There’s nothing stupid about not understanding it. Most of us go through life never having to think about it. It’s only when we’re thrust into (or maybe gently led) into the world of off grid living that we have to understand a few basic rules.

Please note: information provided here is also applicable to bricks and mortar dwellings. You do not need to be a licensed tradesperson to undertake 12v installations. If necessary, cabling for 12v DC systems can run freely through dwellings. Basic 12v DC systems can dramatically cut home electricity bills. It doesn’t matter how many thousands of dollars you are told emphatically that you must spend, by solar power contractors; you CAN cut out the middle operator and install very inexpensive systems yourself. 

This guide does not offer schematics as setups can differ greatly. Do not hesitate to contact me for further advice.

Renewable energy is vital to our future, but already it has become an industry rife with operators who provide misinformation for the purpose of profit. Much of that misinformation pertains to the validity of more basic systems. Electrons are electrons – it doesn’t matter how you convert them from photons, just as long as you benefit. 

Some 12v stuff

Ohms Law Basics – It’s really simple. Honestly!

The main things we need to concern ourselves with are:
1. Power – Measured in Watts and abbreviated as P. This is the amount of work that an appliance or light globe does.

2. Current – Measured in Amps and abbreviated as I. This is the amount of electricity that’s drawn from your battery.
3. Voltage – Measured in volts (surprise, surprise) and abbreviated as E. This is the potential difference between two points, more simply described as pressure, like the water flow in a hose.

P and I are very variable for the purpose of calculations. V doesn’t vary much in a DC circuit. There are variations based on a batteries charge or depth of discharge (DoD), but it’s reasonably negligible, so we use 12v as the basis for our calculations. There is an exception to this rule that I personally apply when doing  calculations involving inverters, but that will be explained in due course.Some caravan RVs use 24v systems. They aren’t as common as they are in large commercial vehicles, but the same rules apply, substituting 12 with 24.

The most important equation that we can remember is:

E / V = I

In plain English: Power in Watts, divided by voltage in volts, equals Current in amps.

It’s important to be aware of how much current is being drawn, as that tells you how much you have left in your battery.

Example: 10 watt LED globe / 12v DC supply = 0.833 Amp current

Power is important for the purpose of calculating current. It’s also important to know how much power  devices consume, if you are using an inverter or generator. If the appliance consumes more power than is provided by a source of electricity from an inverter or generator, they just won’t work.

If you have a hair drier that produces 1,300 Watts, plugged in via a 1 KVA (1,000 Watt) generator, the generator will cut out. The same goes for inverters. 

Appliances that heat or cool via compression (air conditioners) consume a lot of power. That’s why items such as electric heaters, hair driers, electric kettles etc, are very impractical in terms of most small off grid 12v DC power set ups. 

Heating appliances are too powerful for small, practical inverters and generators, whilst large inverters / generators draw far too much current to be practical . In the case of inverters, they run batteries down quickly. In the case of larger generators, their power capacity is inversely proportional to their size, weight, noise level, fuel consumption and cost. 

Batteries

Types of batteries have been discussed in a previous post: The Best Things In Life Are Free Camps.
 
The vast majority of house batteries are AGM (absorbed glass mat) deep cycle batteries, so I’m basing information on AGM batteries. 
 
Deep cycle batteries are designed to deliver a steady current over a period of time and discharge / recharge slowly. These differ from cranking (car batteries) that are designed to deliver a large current in a short burst in order to start a car. They are recharged quickly by means of the car’s alternator.
 
Lithium batteries are becoming increasingly popular for many reasons. The downside is the cost. They’re one of my many pet hates for political reasons. More later. 
 
For the purpose of deep cycle house batteries, capacity measurements are generally rated in “Amp hours (Ah).” 
 
This is the measurement of how much a battery holds, based on how many hours it can supply a continuous current of 1 amp for. 
 
If a 100Ah battery is under a load of 1 Amp, technically it will supply power for 100 hours before it is totally discharged. If the combined load of your lights and appliances is 10 Amps (that’s very high by the way), a 100 Ah battery will fully discharge in 10 hours.
 
Now here’s the catch. It’s not a very complex catch, but it brings a lot of people undone!
If you gloss over everything else read this carefully:
Firstly, don’t think too much about the “hour” part in Amp hours. The time is fairly irrelevant to the battery itself, it’s more relevant to how long you run an appliance, or in techy talk; put a load on that battery.
 
Think of the battery as a fuel tank. Think of Amp hours as litres of fuel. A reasonable analogy is one that applies to older diesel engines. Before self bleeding diesel engines existed, running out of fuel could cost you dearly. An empty fuel tank would mean having the engine professionally bled!!!!
 
A similar principle applies to AGM batteries. If you let them run down to empty, it’s going to COST YOU. Every time an AGM battery falls below 50% capacity, think of it as an empty tank. By emptying it that much,  the batteries lifespan is reduced by a not insignificant amount. If you want your expensive AGM batteries to last (up to 7 years for some), treat them well and don’t discharge them below 50% DoD, because they’ll soon be DoA. 
 
The best way to meter your battery’s capacity easily is to read the voltage. Most “intelligent” chargers or solar regulators display the battery voltage. There are also some very inexpensive meters, widely available.
 
50% capacity for an AGM battery or bank of batteries is around 12.2v. Different battery manufacturers publish charts showing 50% DoD anywhere between 12.05v and 12.3v. I use 12.2v as a general guide, bearing in mind that a battery that is under load will show a lower voltage with the DoD at 50%. If a battery shows 12v first thing in the morning with a load on it, you’re fairly guaranteed that you haven’t done any serious damage to it. I tend to aim for a higher figure to be on the safe side. If my battery bank shows 12.1v first thing in the morning, under load, prior to the sun hitting the solar panels, I’m happy.  
 
The basic rule is to work out your daily consumption in Amp hours and at least double that figure in terms of battery capacity in Amp hours. If you are likely to consume 100 Amp hours per day, you will need at least a 200Ah battery bank, but that’s kind of pushing it really; leaving no latitude. In my set up, we consume around 40 Amp hours per day. Our battery bank is 240Ah. That’s more like the figure you should be working towards. It leaves you with a lot of capacity up your sleeve in case of low charging rates, such as solar charging during periods of low sunshine. 

 

Charging

There are three widely used methods of charging house batteries:
Solar panles
240v mains charger.
Tow vehicle alternator.
 
Solar Charging
 
I’ve covered solar charging in a couple of previous posts, so rather than bore everyone with the same information again. I’ll summarise the basics.
Solar panels are connected to a solar regulator, which regulates the charging of your batteries. It’s essentially a battery charger that’s powered from the sun. A group of panels connected to a single regulator is called an array.
 

Many forums advocate the use of MPPT solar chargers, claiming that you get more charge from less sun. I won’t get into the technical nuances of solar regulators, but will say that an MPPT regulator is far more effective if your array of panels exceeds 600w. A good quality PWM charger will suffice for smaller arrays and cost you much less money than a quality MPPT unit. I’m not saying that MPPT regulators don’t employ better technology, they certainly do. I just personally feel with smaller arrays, the cost can far outweigh the benefits. In some instances, high quality PWM regulators such as those made by Victron, are better than lower end MPPT regulators. On the other hand, low end PWM regulators are often bloody useless. 

The charge that a panel generates is generally measured in Watts. 

A 100 Watt solar panel produces approximately 6 Amps of current in full sun. That means that 6 Amp hours of charge would be absorbed by a battery in one hour of full sun, with the sun closest to the earth’s surface ie at midday. Given the movement of the sun from east to west from sunrise to sunset, a 100 Watt panel will produce around 30 Amp hours of charge on a cloudless sunny day. 

If you have a 100 Ah battery, given that it’s not advisable to discharge a battery below 50% DoD, no more than 50 Amp hours.should be discharged in a day. To regain 50Ah per day,  you,  would need a 200 watt array. 

It’s a commonly held belief that your array should be double in Watts,  what your battery bank is rated  in Amp hours.

100 Ah battery bank – 200 Watt array
200 Ah battery bank – 200 Watt array

 
It’s far from a firm and fast rule. Array size is also determined by daily consumption.  
I have a 240 Ah battery bank and a 460 Watt array. According to the above guide, I would have a 280 Watt array. However, I don’t draw anywhere near as many Amps from my battery in one day as is generated by my array. My array produces an average of 85 Amp hours in a day in good weather. I draw around 50 Amp hours from the battery bank, per day on average.
 
My setup is fine for sunny weather. One might even suggest a smaller array.  I could probably do with adding another 100 to 150 Watts to my array to compensate for cloudy weather.
 

However, I have a generator for topping up the batteries when necessary. Array size is to some extent, a case of “horses for courses.” An array can certainly be too small in electronic terms, but not too big. On the other hand, it can be too big in terms of size weight and cost. It’s about finding a balance. 

A Solar charge regulator

240v Charging

A 240v powered intelligent charger is, in my opinion, an essential part of any 12v set up. Regardless of how effective your solar charging is, there’s always going to be a need to plug into a 240v supply and put the battery bank through a very reliable, continuous charge cycle that doesn’t depend on available sunshine. This might be when the sun just isn’t providing and charging takes place by means of a generator. It might be simply when you’re on grid and plugged into a 240v supply, which is a great opportunity to give your batteries a good solid charge.

An intelligent multi-stage chargerter and an old  (now deceased) dicky inverter

Many intelligent chargers will analyse your battery and if necessary, put it through a regeneration cycle, which has the potential to repair a battery that has been damaged to some extent, by being over discharged or being discharged too quickly.

Most intelligent chargers allow selection of the charge rates up to 50 Amps. The charge rate can be explained as simplistically as the amount of Amp hours (litres of fuel) that are put back into the battery (fuel tank) in 1 hour. It isn’t quite so simple due to the nature of multi stage charging. To explain further: One would expect that a 25 Amp charger would charge a 100Ah battery from 50% DoD in in 2 hours. in realty it will charge the battery to around 75% full in around an hour. 25 Amp hours added to an existing  50 Amp hours, taking it up to a 75 Amp hours  or 75% of capacity. At approximately 75% full, the charger will automatically switch stages from “bulk” to “absorb” charge. The charging current is reduced in order to prevent the battery from over heating and over charging. As a result of the reduced current, absorb charging can take considerably longer than bulk charging; often around 3 hours to charge the last 75% of a 100 Ah battery.

 

Charging From A Vehicle

Many Australians say, there’s no such thing as a free lunch. I hope it’s not entirely true, because I’ve had hundreds and I don’t want to get a surprise bill, but the general gist of the euphemism is true.

If you’re going to gain from something somewhere, you’ll pay elsewhere. Charging a house battery from a vehicle is a bit like that. A vehicle’s alternator is a readily available source of a charging voltage, but a load on an alternator is inversely proportional to fuel consumption by the engine that drives the alternator. It’s not a huge consideration, but it is one to be aware of and should not be over looked.

From a personal perspective, we don’t charge our caravan house battery from the car’s alternator. We have a significant solar array on the roof of the caravan and that generally suffices as far as our charging needs go. We also have a second deep cycle battery system in the car that powers our Engel Freezer. That is charged from the alternator via a simple battery isolator.  The isolator ensures that the Auxiliary battery doesn’t charge if the car battery is below a nominal voltage. That prevents draining the car starting battery. It also ensures that charging ceases when the Aux battery reaches a certain voltage, thus preventing over charging. Battery isolators are a compromise in that they don’t offer multi stage charging as outlined above. They essentially cut off charging prior to the battery reaching full charge, where a multi stage charger drops the charging current in absorb charge mode, then continues to maintain the charge in float mode.

Multi stage charging is easily achievable via a vehicle’s alternator with a DC to DC charger. This type of device is an “intelligent” multi stage charger that’s powered by a 12v DC supply from the vehicle as opposed to a 240v mains supply. They are usually connected between the vehicle and a caravan or trailer via a significant cable of sufficient diameter and Anderson plugs. They usually charge at a rate of around 15 Amps, some up to 25 Amps.

Decent DC to DC chargers are reasonably expensive. They’re potentially an extravagance unless you spend a lot of time towing as opposed to staying in one place for long periods. However, the up side is that a lot of the better models also double up as a solar regulator, which makes them quite cost effective. In saying that, even the good ones aren’t necessarily the best solar regulators. 

Given that I prefer to hang around in places for a while and travel as infrequently as possible, it’s not worth me spending a lot of money on a DC to DC charger. A simple battery isolator serves my purposes well, given that we rarely travel for more than 4 hours in a day. This means that my battery is essentially bulk charged on the road, then plugged into a separate solar regulator for the later stages of charging. 

Inverters

 

Starting an online thread relating to inverters, can be to open a can of worms, which makes about as much sense as the non existence of a free lunch , but you know what I mean.
 
There are two main misconceptions about inverters: 1) That if you have a 600W inverter, it will produce 600 Watts of power, whenever it is switched on. 2) That they are magic! – Some people honestly believe that by plugging in a powerful inverter, they are magically transforming their 12v DC supply into a mains supply, providing and endless source of 240v AC electricity.
 
Debunking number 1 – Inverters only produce whatever power is required for whatever is plugged into it. Let’s say you have a 600W inverter and you plug a 300W stick blender into it. The inverter will consume 300W. Let’s say the stick blender operates for 3 minutes and use our important equation from earlier to debunk number 2.
 
P – 300W (Blender) /  V – 12v (battery supply) = I – 25 Amps
In one hour of continuous use the stick blender will draw 25 Amps from the battery.
 
This is where we get a little tricky. Inverters operate with an inefficiency of around 15 to 20%, that means. When calculating how much current is drawn using an appliance via an inverter, we can allow for it’s inefficiency by substituting 12v (V) in the equation with 10. There for V = 10.
So the equation would be 300W / 10 (V) = 30 Using a 12v DC supply via an inverter to power a 300W blender, it draws 30 Amps.
 
To calculate how much it draws in 3 minutes, is pretty simple 30 Amp hours / 60 (minutes) = 0.5 Amp hours in 1 minute x 3 (minutes) = 1.5 Amp hours
 
I (current) / t (60 minutes) = I per minute x t (minutes of use) = Current Drawn
 
In summary, using a 300W stick blender for a few minutes, doesn’t draw much from the battery. About 1.5 Amp hours. 
 
Inverters can be used in a couple of ways. I’ve taken a line out of the 240v outlet on the front of our inverter and wired that up, so that it can be switched into the caravan’s main 240v circuit. This allows us to use our power points and 240v lighting, which has all been upgraded to LED globes. It’s also possible to keep the invertor in a convenient location and plug appliances into its front panel as required. 
 
When an invertor is left on without actually providing power, it is in a state known as “idling.” Idling generally draws less than 1 Amp, however 1 amp over 24 hours is still 24 Amp hours. Good inverters run in standby mode, which means they idle for less time and power up as required.
 
I personally believe in buying a quality inverter, but not going over the top. Some can get into the thousands, quite unnecessarily. There really isn’t any point in buying a big inverter to run powerful appliances. They put a huge drain on battery banks and need to be hooked up with seriously heavy gauge cabling and breakers, in order to prevent the risk of fire. If you really need powerful appliances when you’re off grid, either get a generator or re-think your priorities. Do you really need the microwave that badly?
 
To buy a good quality (and I mean safe), high powered inverter, you’re looking at $1,500 plus. I had a cheap Chinese on that cost 80 bucks, for years. I wanted it to die so that I could justify buying a good one. I couldn’t kill the bloody thing, when it did eventually bite the dust, it took a few hundred bucks worth of appliances with it.
 
A very good 400 to 600W inverter can be purchased for around $300. Carry on reading to discover why it’s not worth having anything much bigger. There are 2 types of inverter in terms of how their supply voltage “cycles.” A pure sine wave invertor is the only type I recommend. It provides 240v supply that’s probably more stable than most mains supplies. A lot of technology, especially laptops require a pure sine wave inverter to operate. A “modified sine wave” inverter is good for lights and electric motors, but won’t run and could even damage complex technology. It’s not worth buying anything other than a pure sine wave model. 
 
Quality Victron inverter
 
NOW KETTLES!!!! (and other unnecessary things that heat)
 
One of the most commonly asked questions is “what size inverter do I need to boil a kettle?”
 
Let’s look at an average kettle as having a Wattage of 1,800W
Let’s look at the average boiling time to make a cuppa as 3 minutes.
 
We’ll assume that the person who possesses a fear of gas stoves, also possesses an inverter big enough to boil an average kettle. The inverter size is irrelevant. All that is relevant is that it can handle 1,800W.
 
So, strap yourself in for some simple maths with significant results!
 
1,800W / 10 (V) = 180 Amps – Boiling a kettle continuously would not only take a 100 Ah battery lower than it’s recommended DoD (Depth of Discharge 50%), it would actually flatten it in less than one hour!!!
 
What’s that I hear you say? “It’s only going to boil for 3 minutes!”
 
OK then, now that we have a figure of 180 Amps let’s look at the equation that calculates how much current is drawn in a given period.
 
180 (I) / t (60) = 3 Amp hours (I per minute)
3 x 3 (t – minutes of use) = 9 Amp hours
9 Amp hours drawn from the battery to make 1 cuppa. If you have a 100 Ah battery, 5 cups of tea in a day would discharge the battery to a suitable DoD. 2 cups a day would constitute over a quarter of available power.
 
There’s a very easy answer; use a gas burner and a stove top kettle. Appliances that heat, simply draw too much current.
 

Lithium

There’s is another option for running more powerful appliances, which is Lithium batteries. Lithium batteries are considerably lighter than AGM batteries, therefore you can have large banks without having a huge effect on the caravans weight. A 100 Ah AGM weighs about 30kg whereas a 100Ah lithium battery weighs about 10kg. The other huge advantage of Lithium batteries is that they can be discharged to over 80% DoD, therefor they’re considerably more efficient than AGM batteries.
 
Now the bad shit! Personally I don’t like the concept of lithium batteries being touted as the be all and end all of renewable energy. Lithium is a finite resource. A reasonably valuable finite resource. Currently $9,400 per metric Tonne. It has doubled in 4 years and isn’t going to get any cheaper. It’s more water intensive than lead in terms of extraction and unlike lead which is currently $2,300 per tonne, is economically impractical to recycle. 
 
Guess which country has the world’s largest Lithium deposits. Yes, it’s us again. We aren’t the biggest producer, Yet, but explorations have identified huge deposits. So, guess who’s backyard is going to be fucked up most by the increase of lithium battery use? Yes ours. 
 
So, think about those CMCA members in their big busses who feel entitled to exclusive rights to free camps because they are environmental saints for having fitted grey water tanks. Not only are they fucking the environment with their carbon emissions, a lot of them are helping the destruction with their bloody great lithium battery banks. 
 
I must add, I don’t have a personal problem with them, as long as they don’t cast environmentally based dispersions on people who pour a little bit of water on a tree. It’s hypocrisy of the highest order, based purely on financial elitism. It’s NOT environmentalism. They’re not fooling anyone. 
 
I have been accused of being “anti-big rig.” I suppose I am, but that doesn’t mean that I don’t like the people who own them. I generally get on with people. I’m also anti over investment in the housing market, yet many of my close friends have done it. 
 
My problem with “big rigs” is that they defeat the principal purposes and purpose principles of downscaling. Especially given that many owners don’t even live in them permanently. For me and many others, downscaling is political. It’s a means of survival. I just feel that in many cases “big rigs” display an over reliance on wealth. I don’t have a major issue with unnecessary wealth. I have an issue with wealthy people who regard those who struggle, as enemies of the state. The biggest problem in this country is insanely wealthy people who don’t pay taxes. Now, talking about Gina; back to mining lithium. 
 
It’s very interesting that Tesla one of the world’s biggest producers of Lithium batteries, is sneaking it’s way under the bed covers of both state and federal governments. Call me a cynic, but I have a sneaking suspicion that the sudden benevolence displayed by Elon Musk and Tesla towards Australia, might just have something to do with them getting their filthy mitts on our resources. 
 
We’ve seen the disastrous results of our national grid being sold off to private investors around the country. That’s only a small taste of the public being fucked over, in comparison to what’s going to happen if Tesla achieve world domination.
 
OK, there’s my political rant over. Well nearly. I have to add that it’s a very sad indictment on Western capitalism, when something as positive for our future as renewable energy, is allowed to be turned into a monster by government / corporate cronyism!
 
So, that’s the political bad shit about Lithium. Most people reading this probably won’t give a shit about that, so I’d better also point out that whilst an 100Ah AGM battery can cost as little as $200, a 100Ah Lithium battery will set you back over $1,000!!!! The charging equipment for Lithium batteries is also considerably more expensive than for AGM batteries. Also, if you’re going to have a huge battery bank, you’re going to need a hell of a big solar array and somewhere to put it. My personal advice to anyone, be they in a home, caravan or motorhome, is scale back your usage and make do with as small a system as you can.
 
Not all lithium stops me from being mad! Lithium batteries make me MAD!!!

Fuses, Breakers and Cable Ratings

Everything in a 12v system needs to have a fuse or breaker in line. A fuse contains a wire that burns out and breaks when a current greater than is required to supply an appliance passes through it. It breaks the circuit rendering it safe. Once the problem has been isolated the fuse can be replaced with a new one.
 
Breakers perform the same task as fuses, only they are somewhat more complex in that they are a switch mechanism that is activated, breaking the circuit when a current greater than it’s nominated value is detected. 
 
The most common reason for a fuse or breaker being activated is a short circuit. A short circuit occurs when there’s some sort of connection between the positive and negative sides of a DC circuit. This can result from wires or terminations making contact with each other, or contact being made through conductive material that is making common contact with each side of the same circuit. 
 
Whilst 12v DC circuits don’t carry the same risk of electrocution that 240v DC does, a 12v short circuit can easily result in a fire with catastrophic results. In my opinion, 12v DC installations can be more dangerous than 240v installations. You don’t have to be qualified to do the work. Terminations are not required to be of the same high safety standard as 240v AC and people fear 12v DC far less than 240v AC, due to the significantly lower risk of electrocution. As already stated, the risk of fire is indeed great, so BE CAREFUL. Read as much as you can and don’t attempt any installation until you are absolutely certain that you can do it safely and avoid short circuits. It’s probably a good idea to get your first few installations safety checked by an auto electrician. It’s a small price to pay for safety.
 
Personally I use breakers instead of fuses on everything. It’s easier to re-set than replace and they allow more convenient isolation of a problem. If a breaker trips after you have “fixed” a problem, you know you haven’t fixed the problem. The same applies to a fuse, however every time a fuse breaks, it has to be thrown away and replaced with a new one. You can go through a lot of fuses while isolating a problem.
 
I have also seen examples of blade fuses burning out but actually fusing open circuit as a result of the metal melting. This could result in an ongoing  short circuit, which could in turn result in a fire. 
 
I tend to use “blade breakers”, which look like a typical “blade fuse” that you fins in a car’s fuse box, only they’re black with a little red re-settable button in the top. They don’t seem to be very popular. I think there’s a common misconception that due to their less than imposing size, they’re somehow less reliable than seemingly more solidly built breakers. I’ve been using them in my caravan fuse box for nearly four years. They are extremely reliable. I’ve never had one fail. They don’t trip unnecessarily and I’ve tested their effectiveness on a regular basis. They’re excellent. They cost considerably less than seemingly more substantial breakers and take up considerably less space. They fit into a typical automotive fuse box.
 
Fuses and breakers are rated in amps. gauging a fuse rating isn’t rocket science. I work out the sum on the maximum current that can be drawn on a particular circuit and round off the fuse value to the next highest value. For instance, if the sum of all of your 12v fans in use at the same time is 4 Amps, the next highest value is a 5 Amp fuse or breaker. This allows for some latitude. They won’t trip the circuit if the current drawn surges slightly and goes above the nominal total current value a little bit, but if a short circuit occurs the fuse or breaker will trip safely. If the fuse or breaker rating is too high for the circuit, problems can occur without it tripping. This can result in damage to equipment.
 
The same principle applies to cable diameter, which in turn corresponds to current rating. However, if the current in a cable is too high for what it’s diameter allows for, it obviously won’t trip. Instead it will overheat with great potential to start a fire. Wire diameter which corresponds to current rating, should match fuse or breaker values. 
 
This post is designed as a brief guide. I seriously suggest reading considerably more on breakers, fuses, cable rating, schematics, terminations and joiners, prior to undertaking any installation work. 
 
Blade fuse box with blade breakers inserted

The 3-Way Fridge Trap

That sounds like some kind of bizarre criminal activity by a psychotic serial killer, but it’s not. At least not yet anyway. 
 
3-way fridges run on gas, 240v electricity and 12v electricity. 
 
I keep reading about people with the same problem; “my 3-way fridge is draining my battery quickly.”
 
This is very simple. This style of fridge runs by means of heat exchange, unlike others that run by means of a compressor. Compressor fridges, such as Engels and Waecos for instance, are very energy efficient. Heat exchanger fridges are very energy inefficient. Some  can draw up to 25 Amps, whilst a similar size compressor fridge will draw between 1.5 and 3 Amps.
3-way fridges are designed specifically to run on LPG when off grid and 240v AC when available. The 12v option is for the specific purpose of powering the fridge whilst the vehicle is moving, with a direct connection to the alternator. 
 
We have a 2-way fridge. We tend to use our freezer to pre-freeze ice packs prior to a long journey. We pack the fridge up with them for the first part of the journey, leaving the fridge turned off. We then connect the fridge to 240v via the invertor for the second half. Our fridge is more efficient than some and we are continuously charging the battery bank via solar panels on the roof as we travel. We usually end up with a reasonably full battery and cold beers when we arrive at our destination.
 
If you have a 3-way fridge, it’s meant to be powered by LPG gas when you’re off grid. Running it from a 12v battery will drain the battery very quickly. Don’t do it, or if you do, don’t whinge on the internet because you’ve been told.
 
Our kitchen with our 42 year old Electrolux 2-way fridge

Points to Remember

  •  Batteries and battery banks are measured in Amp hours
  • Amp hours are like litres in a fuel tan
  • P (Power) / V (Voltage) = I (Current) – That’s how to calculate how much “fuel” you’re using
  • A deep cycle battery is designed to deliver a low currently continuously over a long period of time, whilst a car / cranking battery is designed to deliver a high current in a short burst to start a motor.
  • You’re battery or bank should never fall below half full (50% DoD)
  • Voltage indicates how full a battery is – 12.2v is about half full.
  • A group of solar panels is called an array. Measured in Watts.
  • In full sun, 6 Amps is generated by every 100 Watts in an array.
  • Solar arrays are connected to solar regulators. Good solar regulators are intelligent multi stage chargers. They ensure a full charge and prevent over charging.
  • As a suggestion, an array should total in Watts, twice the size of a battery bank in amp hours. For example – 200W battery bank requires a 400W array. Not a hard and fast rule. 6
  • A good 240v intelligent charger is important for putting your batteries through a good continuous charge cycle when mains or generator power is available. They are also a very good means of regenerating batteries that have been over discharged.
  • Charging from an alternator / vehicle is very convenient if you travel regularly. Always bear in mind that gained energy is paid for elsewhere. Charging from a vehicle will increase fuel consumption.
  • DC to DC chargers provide multi stage charging from a vehicle. They are reasonably expensive and might be unnecessary for people who don’t move on from a location regularly.
  • Battery isolators can be used in place of DC to DC chargers. They are very inexpensive, but only provide a bulk charge. They will never completely charge a battery because a solenoid cuts switches charging out at a nominal voltage in order to prevent over charging.
  • Battery isolators also prevent a car / cranking battery from being discharged when a house / aux battery reaches a nominal DoD.
  • A house or aux battery can be bulk charged by a vehicle via a battery isolator whilst driving and then switched over to solar charging for final stage charging with a reduced current.
  •  Inverters transform 12v DC from a battery into 240v AC for powering domestic appliances.
  • When an invertor is left on but no appliances are being used, the invertor is “idling.” It does not use run at it’s rated power (wattage) when on but not powering an appliance. Idling draws a very small current.
  • Invertors are not magic. They are only as capable as your battery bank. Large invertors are often a waste of money. They can provide a 240v supply to appliances of a high wattage, but they’ll suck batteries dry in a hurry.
  • Lithium batteries are expensive, evil and the core of world domination by a US power giant. (Shit that’s going to cause some arguments – good job I enjoy being insulted by people who I’ve insulted because they just spent a fuck load of money on lithium batteries) Fuses, breakers and correct cable ratings can save your equipment or possibly your life by preventing a fire. They’re designed to ensure that a circuit does not carry a greater current than it’s supposed to.
  • Fridges that run on LPG gas are supposed to run off LPG gas when a 240v AC supply is not available. If you run one from a 12v DC battery, you will discharge it very quickly, so don’t do it! If you do, don’t whinge about it online, because now you know not to and it’s your own fault if someone calls you a goose. 
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