Motorhome 12v basics for beginners.
I get lots of questions on Classic Hymers about power - so here we go!
First you need to grasp the basics of how batteries work.
Think of a 12v system this way - it is really no different to a simple irrigation system, with a tank of water, and pipes leading to the plants. If the tank holds 100 litres and the plants need 1 litre an hour, then if you don't get some water back into that tank within 100 hours, then the plants will start to die - or in our case, the lights will go out!
If you can get 50 litres back in the tank, then you gain an extra 50 hours. In reality the tank will get refills at random times and from different sources, but the basic arithmetic is always the same.
So if you have a normal 100amp/hour battery, fully charged, then you can use 10 amps an hour for 10 hours, or one amp and hour for 100 hours - it's that simple.
Having got amps worked out - you now need to know about volts. You know that your system works on 12 volts - that's just the way it is designed. 12v is pretty universal - it's the optimum voltage for economical storage, and 12v won't kill you if you touch it.
What is important to learn about 12v is that fractions of a volt are important when diagnosing problems. A healthy system will show between 11v and 14v depending on what is happening, so it is important to be able to measure voltage accurately.
So you need equipment to measure it. To measure volts you need something called a multimeter.
This is a device that looks like a meter(!), and has two wires coming from it - red and black, that go to two things that look like weird pens, but have metal tips instead of ink nibs! They are called probes, and what you do is put the metal tips on anything that contains electricity, and the meter will tell you what's going on. In our case we are almost always looking for 12 volts DC, so you set the dial on the meter to DC V or volts - you will see lots of other symbols on the selector dial - AC, Ohms and lots of numbers - don't worry, just concentrate on DC Volts. Some multimeters have a dial and a needle, others have a digital display - the digital ones are best.
A multimeter is an essential tool for anybody who wants to check their electrics. Plus a basic tool kit - we all have one of those don't we!
To practice measuring volts - find yourself a battery, identify which terminal is positive (+) and negative(-), and then touch the terminals with the tips of the probes. Make sure the meter is set to DC Volts. The meter should read something between 11 and 14 volts - usually more like 12 or 13, and to 2 decimal places - so 12.56v for example.
Here you should remember that when taking readings on 12v systems, points of a volt are very important - so remember that the difference between 12.2 and 12.8 could well be significant.
The next tool you need is to measure the amps. Unfortunately it is more difficult to measure amps also called current, than it is to measure volts. This is because current flows through a wire only when power is being used, so you have to measure the current flowing through the wire you want to measure.
Luckily modern technology now makes this easier than the good old days, when only electricians could measure current. There is now a device available called a clamp meter. It has two jaws that open and close - see the picture. You open the jaws and place it so that the wire you want to measure is between the jaws. Clever stuff in the meter then measures the current flow - it senses the magnetic field caused by current flow in the wire - so you can measure the wire without disconnecting it.
The best place to measure current is from the main wire that goes to the positive of your battery. If the clamp doesn't fit, it makes sense to alter things a bit so you can always get the clamps round - so in future you can always do it again.
Remember - when a wire is between the clamps - you are measuring current - not volts. To measure volts you need the probes - the red one on positive (usually red, but in Hymers also blue) and negative (black, but in Hymers also brown) - and the selector dial on the meter must be in the right position - "DC Amps" for current and "DC Volts" for volts.
And even better news - you can now buy a combined meter which does both|! See picture.
Remember - you check VOLTAGE with the probes in the DC V position, and you check CURRENT by placing the object wire - the wire you want to measure, between the jaws of the meter, and select the DC Amps position.
Armed with your meter I suggest that you experiment. While measuring voltage and current at your battery - switch things on and off in your van, and see how the readings change. Play around so that you can get a feel for how you use your meter to measure voltage and current.
once you can measure these two things accurately and with confidence, you will be able to troubleshoot properly.
Current flows both ways down a wire - so if your battery is being charged, then the current is flowing into it - if the battery is powering your stuff in the van then the current is flowing out of it. The clamp meter will display a minus sign to help you decide which way is which.
OK so now we know what the voltages are, and where the amps are flowing, now we can get serious.
Lets deal with a real world example.
Lets say you have a normal van with a single leisure battery, and no solar panel. This means that your battery will only receive power (charge) when either the engine is running - power from the alternator, or when you are hooked up.
Lets assume that you arrive on site - there is no hookup, so your only source of power is your battery.
The first thing to do is check the voltage. It should be between 12v and 13v. Now start the engine. It should now be around 14v. This is the way it should be - the voltage of a battery that is "on charge:" will be higher than a battery that isn't. Switch the engine off now.
Now get the clamp meter around the wire that delivers power from the battery to the van - all vans are different so i can't tell you which one, but once you know it - you know it!
With everything switched off in the van - the current flow should be zero, or near zero. Now switch on things - you will see the current go up to 2, 3, 4 or more amps. Switch things off, and it goes down. So now you see how you can easily work out how much power something takes.
Now switch on the heating fans, the TV, plug in your laptop, and switch on the lights you use at night - and anything else you use regularly. Now take a reading. Lets assume it's 5 amps. make a note of that figure - this is the amount of power you use in normal life in the evening. Now you can calculate that if you spend 6 hours in the evening before bed, using 5 amps - then by bedtime you will have used 6 hours times 5 amps - 30 amps - so you will have used a third of your battery - assuming a standard 100amp hour leisure battery.
So it is fair to assume that if you use no power in the day, then you will get three nights ..... it's a fair assumption, but unfortunately a characteristic of the system is that you don't usually get every last amp out a battery - so best to be conservative - but by Sunday you will have to watch out!
On the first night - take a voltage reading at the start of the evening - lets say you see 12.8v. Now take another reading before bed - 12.4v. Where did that .4v go? This demonstrates a fundamental characteristic of batteries - as theyu deliver their current - the voltage drops gradually.
Knowing this little fact is very useful for troubleshooting.
measure it the next day - you might see 12.4v and 12.0v on the second night....still OK!
The third night it will start to show 11 point something ...... any 12v battery that is reading under 12v is telling you that it is coming to the end of its charge. But don't worry - down to 11.3 or so is still OK, but you are on borrowed time.
But we want to stay 2 more days! What to do? Well obviously you need to get more juice back into the battery - you need a charging device. First of these is your engine. If your battery is getting flat - start the engine and take the readings. The voltage will creep back up to 13 or 14 ..... but look at the current! You will see quite a high figure of between 20 and 40 amps, flowing into your battery.
Using the same maths - you can now work out how long you have to run the engine. 20 amps for 1 hour puts 20 amps back in. You are using 30 amps on an average night....so an hour on tickover will probably give you another night. If the neighbours don't complain!
Or lets say you drive to a new site with hookup - and the drive takes 2 hours. You will arrive at the site with 40 amps already put back in - almost half! Then you hookup, and test again. Your van has a built in battery charger that converts mains 240v to 12v to charge your battery. So when hooked up you will see 13 to 14v .... but only about 10 amps going in. This is because the charger isn't as powerful as the alternator - don't ask - it just is!
So you calculate that your battery will be fully charged in 6 hours....6 hours at 10 amps - to replace the missing 60amps after the drive. This is only true if it is daytime and you are not using any power. If it is at night - you are using 5 amps an hour, so only the difference - 5 amps is going back into the battery. But no worry, as soon as you go to bed, the difference will be made up, so in the morning your battery will be fully charged, and will stay that way all the time you are hooked up.
Remember - when a wire is between the clamps - you are measuring current - not volts. To measure volts you need the probes - the red one on positive (usually red, but in Hymers also blue) and negative (black, but in Hymers also brown) - and the selector dial on the meter must be in the right position - "DC Amps" for current and "DC Volts" for volts.
Now lets see what happens when things go wrong.
You expect your battery to last 3 days or so - but on the second day it is reading 11v....what happened? because you are taking accurate measurements, you know you have only taken out 30 amps in one night - so how come it is reading 11v? Well that's a pretty sure sign that your battery is knackered, especially if it is old. They don't last forever. An old batteries don't die overnight - they die gracefully. So they might perform at 50% for a while before finally giving up.
Here's another common situation - you are hooked up but the battery is only reading 11 or 12v - your charger isn't working - probably!
Or the engine is running and the voltage is low - 11 or 12 - there is a problem between the engine and the battery.
Or how about this? Everything is switched off...but the meter is still reading 1 amp or more, or less. This means that something, somewhere, isn't switched off at all! You need to find it! A 1amp drain will flatten your battery in 3 or 4 days.
Now I hope you can see how critical is to be able to take these measurements - even if you don't understand them - they are needed by the people online who will try to help you.
Now if your system is working OK, but doesn't have enough power for your lifestyle....now is the time to consider upgrading it.
This is when people consider solar panels.
People like solar! It puts juice back in all day long while you are out partying, or walking, or working.....And solar works on exactly the same principle as chargers and alternators. Except that with solar panels, they are rated in Watts. We haven't mentioned Watts so far have we! Watts are not something you can measure - they can only be calculated! And it's really simple - Watts are the product of amps multiplied by volts! And if you have only 2 of the three numbers - volts amps or watts, then you can ALWAYS work out the other one - basic school algebra. So we know that for us with our 12v battery systems the volts are always 12 ..... so if we have a 100 watt solar panel, how many amps will it deliver? So it's W = V x A, this can also be written A = W / A, or even V = W / A. So we know V is 12 and we know the panel is 100 W - so the amps is found by dividing 100 by 12 - which is 8.3A. So we know that a 100w panel should deliver just over 8 amps.But this is solar! To that statement you have to add the words - "In bright sunshine with the sun overhead"! If you ever do buy a panel, and you attach your trusty clamp meter and measure the amps flowing from the panel into the battery, you will see a big variation as clouds go over! So you have to take an average, and you have to just feel that on a cloudy day you are not going to get much juice from solar panels.So despite the hype, solar panels are not the be all and end all for off grid van living, especially in a country like the UK with big seasonal variations.
But the calculations are just the same, and it is easy to work out - you just have to make some average assumptions. If you use an average of 30 amps a day, and only rely on solar, then you need a system that will put in an average of 30 amps a day. In reality the batteries will even out the differences. But the numbers have to be positive - if not then one day, you will be out of juice.
But to get back to our example - if you have a 100w solar panel on your van, and it's a summers day, you can reasonably expect that by the time you get back from a day out then maybe 8 hours of sun at a max of 8 amps - that's 64 amps - so it's a fair bet that your system will put back in every sunny day what you take out at night.
I read on a solar website that for the UK climate, on average you should double the spec of your solar system to average out the cloudy days - and triple it if you are fulltiming in winter as well. Most motorhome solar panels are between 80 and 200 watts.
Finally I will discuss inverters. Most of the stuff in a van is 12v....but at home you use 240v. Depending on your lifestyle, sooner or later you will wish you could plug in some of your 240v stuff. Well you can if you install a piece of kit called an inverter. Don't ask why it is called an inverter - it just is! It is a box that you put 12v DC battery power in one end, and 240v AC comes out the other, just like in a house. Sounds simple! And it is, as long as you know the numbers - and after reading this article - you do know the numbers!
But now the volts are 240 - not 12!! So the amps and the watts will be different! Let's take a common appliance - a hairdryer. Look on the label and it will tell you how many watts. A small hairdryer will be about 750w. A big one 1500w - and this will be written often as 1.5kw . Just like 1500 grammes is 1.5kg.
So using the numbers - divide 750w by 240v to get the amps = answer 3.1amps. Not a lot.....but hang on - that is fine at home, but if you plug your hairdryer into an inverter that is connected to your 12v battery, then you MUST do the calculation at 12v - not 240v!! Because you are in reality powering it from a 12v battery, via an inverter which converts 12 to 240v - but the power is still 750 watts! So out with the calculator! 750 watts divided by 12 volts = what? 62 amps! That's the highest number of amps we have ever seen! Our battery is only 100 Ah, and we know that in reality it is only really happy at half of that. But even so it should deliver 65 amps for about an hour - and indeed it will! But only if your inverter can handle it - you need at least a 750 w inverter. And there are other considerations - for lots of amps you need very thick wires - have you seen how thick the wires are that go from your vehicle battery?
So is it worth it? Probably not - better to use the plug in the campsite shower block. Or a towel!
The rule here is this - any device that consumes more than 100w is generally considered to be heavy duty as far as 12v inverter systems are concerned. And this is usually means anything that generates heat. So fan heaters, toasters, hairdryers, microwaves, all the things that you take for grated at home, are out, unless you are ready to invest in heavy duty wiring, a big inverter, and willing to do the numbers.
The sort of things you can use with a normal inverter are laptop power supplies, small TV's and music systems and almost anything that is small and doesn't generate heat - or that the label says is less than 100w - a rough rule of thumb is that every 10 watts needs an amp of current at 12v - the formula is the same, every time!
So if you want to fit an inverter so that you can run your (non heating) home appliances in the van, then do the numbers. Most laptops need about 70 watts, and it's always best to double up for good measure, so a 200 w inverter will do nicely, and that 200w inverter will draw about 6 amps from your battery - per hour - while charging your laptop.
If you want to run more powerful appliances at 240v then just remember that at 12v the amps will be a LOT higher, so the 12v wiring has to be heavy duty and properly installed.
Here is how my own van system is set up. I decided I needed to be able to survive a full week in cloudy weather, never plugging in, or starting the engine, or a generator. I want a decent music system, a TV and 2 laptops. My lighting is LED so takes hardly anything. I calculate and then check by measuring that on an average day I need 50 amps average of 12v battery power (I am greedy!). So 7 days at 50 amps is 350 amps. But batteries roughly only deliver half their rated capacity....but commercial expensive batteries do actually deliver a bit better than cheap and nasty retail leisure batteries. Plus I am willing to take the risk, because I know that 7 dark days of heavy consumption is a rare event!!
So I bought 4 heavy duty ex commercial 100ah batteries off Ebay for £75 each. That's 400 amp hours in total. I bought a 1000w inverter - mainly because I like toast! It is so much easier to use a toaster than a grill, and it only uses power for a few minutes...and in an emergency my wife needs the hairdryer, again only for 5 minutes or so.
Having taken juice out of my batteries - I need to fully charge them plugged in for one night only. So I installed a 50 amp charger - this will recharge me in about 10 hours.
I also know that my alternator will deliver 50 amps when I am driving, so if I drive 4 hours before plugging in, then I only need 4 hours of being plugged in, and if I drive for a total of 8 hours, I am fully charged and don't even need that!
On top I have a 200w solar panel. It messes up all the calculations, but everything it puts in is a bonus. In summer I know I have more to play with, and in winter I assume it doesn't help much. But I do know, because I was there, that down in Spain with 12 hours of high sunshine a day, the solar was putting so much in that I reckon I could have stayed permanently off grid, as long as I didn't party too hard. My 200w panel was putting in 8 amps for 8 hours minimum, so as long as I used less that 64 amps a day I was good.
And then finally I have a permanent display of voltage - this is really useful - you just give it a glance every now and again. You get a feeling for it. So if it's been a while...and the number is creeping down towards 12v, the I know I am on a downward curve. Remember that points of a volt matter! If you start the week at 12.8v, and two days of use later it's 12.5, and the next day 12.4, then you know that you are roughly half way there, and make plans to re-charge. Sunny days will slow the decline - cloudy days and partying will accelerate it!
So after 10 years, 3 systems and a lot of pain, I now have a van system that hardly ever lets me down, no matter what the weather.
In my opinion the minimum system for fulltiming is 2 x 100ah leisure batteries, a small solar panel of about 60 to 100w. A Battery charger that will deliver 30 amps when hooked up, and a small inverter of between 200w and 600w for your small home appliances - laptop etc.(No heaters! But curling tongs and straighteners are OK!)
So....there you have the basics - a decent meter, a simple formula, and an image of a tank of water drip feeding plants!
So finally, in no particular order here are the basic rules....
You must be able to measure voltage and current to within 1 decimal place.
You need to understand amp hours - 100 amp hours equals 1 amp for 100 hours or 25 amps for 4 hours.
Amp hours work both ways - you can put it back in the same way. If your charger delivers 10 amps, then it will put back in 40 amps in 4 hours. If your engine alternator delivers 30 amps then 90 in 3 hours!
Remember to calculate power in as well as power out - everything in from solar, driving and hookup, has to equal or exceed everything you take out, averaged over time.
Decimal points of a volt really matter. The numbers to remember are as follows - on charge the battery voltage will be over 13v. When not charging but delivering power, it should be over 12v - 12.5 is a good figure to see! At the end of the day, just before bed, if you are reading more than 12.0v day after day, then you know you have got it right.
Battery voltage will drop gradually over time as it delivers it's power. Very roughly expect to see it down around 12v after delivering half it's power. Under 12v and you are running out of power!
It will carry on down to 11v, but will get worse quicker, so start to switch things off - most appliances will work down to 11v...and lights will just get dimmer and dimmer.
It is not a good idea to completely flatten a battery repeatedly. Get a bigger system!
When specifying and installing - double up! You won't be sorry. Think you need one battery? Buy 2! think you use 20 amps a day? Allow for 40!
Learn the calculations - A = W / V.....W = A X V.....V = W / A - It's called Ohm's law, it's universal, and never wrong!
Remember that less volts means more amps, and more volts is less amps. More amps needs thicker wires. If anything in your system is going to need more than 20 amps, then the wiring needs to be thicker. (just look how thick the cable is on your starter battery!)
A battery on charge will take on average about 10% of it's rated capacity an hour. So it takes roughly 10 hours to charge up a 100Ah battery - and I mean roughly! In reality it will not be fully discharged so will take a lot less. And as it get's up to charge, it will take less and less.
A 50 amp charger will not charge a 100Ah battery any faster! But a 10 amp charger will take ages to charge a 400Ah battery - remember the 10% rule.
Most batteries are around 100Ah - so if you want 400Ah you will usually end up installing multiple batteries. Try and site them close together - never a good idea to have 1 in the back and one in the front with a long wire between them.
People say that multiple batteries should be of the same size rating and make - true in principle, but if it means throwing out a good battery - then at least try it.
A fully charged battery will not take any more amps - so don't worry if there is no current flowing into the battery - check the voltage and if over 12.5v then all good.
An "intelligent" battery charger is one that has a computer inside it and not only charges your battery in the most efficient way, you can leave it connected and it keeps the battery in good condition during long periods when not in use - also called float charging. They are a good investment.
You can buy an inverter and a charger in the same box - if you want!
12v will not shock you - 240v certainly will. Don't mess with 240v unless you have been properly trained. But you can install a basic small inverter because the 240v bits are shielded - all you have to do is plug your stuff into it. But inverters that feed sockets inside your van through wiring MUST be installed properly.
12v is not completely harmless - bad wiring can cause fires. Too many amps flowing through too thin a cable means heat, and heat means fire. You do have a proper smoke and gas alarm in your van don't you!
When out on the road and having 12v problems - you don't only need an auto or caravan electrician - boatyards by the sea, rivers and canals are just as good - boats use the same systems as vans.
Good luck and feel free to ask questions.
Meters....
If starting from scratch best to buy a combined unit - and make sure it does DC current - cheap ones only do AC, which is useless in a van.
At time of writing June 2015 - this is the easiest to find, available at Maplins .... the "Uni Trend 203 DC/AC Current Clamp Multimeter"
