Powering a laptop (or an ebike charger) in a van - all the issues in detail

Powering your laptop from a 12V campervan battery EDIT - EVERYTHING HERE ALSO APPLIES TO EBIKE CHARGERS - THERE IS AN EXTRA SECTION ON EBIKE CHARGERS AT THE END.

One of the most common questions I get asked online is about the best way to run a laptop in a motorhome or campervan, or any live-in vehicle. As with all things electrical, the answers can get a bit technical, but in this article I will cover the technicalities, but also try and express the main points in simple language, so please persevere!

Not everybody needs to run a laptop in a van. Many folk are happy to get away from computers in their van. If you only want Facebook, the web and email, then your phone or a tablet is often enough. For for some folk - especially those who work while on the move, powering a laptop in a van is a necessity.

The subject always seems to come down to two basic questions - Do you use the original laptop 220V charger plugged into an inverter? Or do you convert 12V DC from the battery to the DC voltage required by the laptop directly, either by obtaining a 12V charger, or by doing it yourself using a voltage converter.

Laptops and power in general

Laptops come in many shapes and sizes and their power requirements vary, but generally speaking small laptops consume about 50w and large laptops up to about 140w, and the average is about 80w. The way to find out is to look on the charger that came with your laptop. The charger is also often called a PSU (Power Supply Unit), or simply the "brick". It is usually a black plastic box with one wire that goes to a 220V plug and another wire that plugs into the laptop. The charger will always have a label on it that looks something like this.

It looks complicated, but most of it is just regulatory and legal stuff - there are only two things you are interested in and they are input and output. In simple terms - it takes one type of electricity in, and converts it to another type suitable for the laptop.

You may well ask, why can't they just make laptops that you can plug straight into the mains? The simple answer is that even with all this modern technology, there just isn't enough room inside a thin laptop to cram in the required circuits. Also, the safety regulations mean that a laptop that was powered directly from 220V would have to be engineered and insulated to a higher and more expensive standard. So all laptops have a brick - that's the way it is.

An Example

So looking at the this label, you can soon see that this Dell laptop takes an input of 100 - 240v AC, and gives an output of 19.5v DC. Hold on a minute! Where does it say AC and DC? Well it does and it doesn't. If you look closely at where it says 19.5v you will see a symbol that looks like a longer equals sign where the upper line is solid and the lower line is broken. This is the standard symbol for DC. For AC, the international symbol is usually a wavy line like this ~ and you can see this on the label, just after 240v.

An ebike charger will have a similar label

Modern power supplies are now almost always "universal" this means that they work anywhere in the world - whether it is America on 110V AC, Europe on 220V AC, or the UK on 240V AC. You can see this because it says 100 - 240v AC, which means that it will work on any voltage between these 2 numbers. If the input figure on your charger says just a single number like 220v only, then you know that it won't work in the USA.

On the output side, it says 19.5v 6.5A. 6.5A means 6.5 amps. And in the middle of the top row it says in big letters 130W. This means 130 watts. Not all charger labels display the number of watts, but most do. If you multiply 19.5 by 6.5 you get 126.75, which is rounded up to 130. This is yet another example of the universal laws of electrickery - watts = volts x amps. So if your charger doesn't display the watts, and you want to know it, then you just multiply the volts and amps together. I know that most people's eyes glaze over at this point, but if you are going to want to use your laptop in your van, and want to set it up correctly, you need to know the numbers in order to do it right.

One final point about charger specifications - the numbers quoted are maximums and have a built in safety margin. In real life the laptop will use a bit less than the max figures quoted. It will use more power when in use and on charge, and less power when in use with the battery fully charged, but for the purposes of choosing inverters or working out power requirements in your van, you should always use the maximum figure, so that the same margin of safety will be used in your van.

So now we understand the charger label, and in the case of this Dell, we need 130W.

So let's talk about inverters

For those that don't know what an inverter is, it's a box of tricks that takes 12V DC from your battery and converts it to 220V AC as you find at home. You can then plug in and use the charger that was supplied with your laptop. An inverter is specified in a similar way to a charger, but the other way round. It has an input voltage and an output voltage. A typical inverter takes 12V DC in, and outputs 220V, and the power of the inverter is specified in watts.

So in very simple terms, you connect your inverter to the 12V supply of your van, plug your laptop charger into the socket on the inverter and away you go.

Unfortunately it is not quite as simple as that if you want to avoid many of the problems that can arise if you don't understand the basic principles of power. Many inverters are supplied with wiring to connect to your 12V supply. Usually this will be a twin core wire, coloured red and black. One end is connected to the inverter, and the other end will either be bare, or with spade connectors, or with a cigar plug. It is the cigar plug that you have to be wary of.

But first - the dreaded cigar plug

Everybody knows what the cigar plug looks like - they have been around for years - they are a throwback to the days when all cars had a cigar lighter, and plugging into the cigar lighter socket was the only way to easily get a 12V supply from your vehicle. But the truth is that they are unsuitable for the job, and are responsible for the majority of problems that people report online.

Cigar plugs are inefficient. The majority of them are made very cheaply. If you look closely at one, you will see that it has a point, and two bits of metal on the side. When you push the plug into the socket, the point makes contact with the centre of the socket, and the side bits rub onto the sides of the socket. In technical terms, the point is the 12V positive, and the sides are the 12V negative. What you have to remember is that the cigar socket was originally designed for the cigar lighter, not to supply 12V to external equipment. The point only makes a very small contact with the centre of the socket, and the whole thing is completely open to the outside and is susceptible to dirt getting in. The bottom line is that it is an inefficient thing and unsuitable for powering expensive equipment. Many of you will have experienced the situation where you have something plugged into a cigar socket, and it doesn't work, so you wiggle the plug until it comes back to life. This is bad!

When mobile phones came out in the 90s, and we needed to charge them on the move, all that was available was a cigar plug adapter, so that was what we used. Then smartphones came out that are charged by USB, so we all bought USB cigar adapters. And generally, they always worked OK, and the occasional wiggle was OK and caused no problems. But it is a different story when you come to inverters and laptops - the reason being that phones don't need much power, but inverters and laptops do.

An average USB phone charger only needs about 10 watts. An average laptop, as we have already discovered, needs about 10 times more. The cigar socket, for all it's inefficiency and old fashioned design, has no problem delivering 10 watts, but it is a different matter trying to deliver  over 100 watts.

In pure technical terms the international standard for a 12V cigar socket is 10 amps, which is 120W (12 x 10). So you can see immediately that if you try and power a 130w device with it, it will be working at its maximum capacity. But you should also remember that it was only designed to power the cigar lighter - that hot thing that popped out back in the days when smoking was acceptable! But the cigar lighter would only take about 30 seconds to heat up and pop out. This is important - there is a world of difference between supplying 120W for under a minute, and supplying 120W constantly for many hours. The cigar socket simply isn't robust enough to supply its full capacity, constantly. And this is what gives rise to the problems. After many hours of use, things get hot and brittle, fuses can blow, and generally they can become unreliable and a pain in the bum.

So it's OK for phones, but no good for laptops. I have friends in the motorhome trade and they all say the same thing - cigar sockets are completely unsuitable for powering inverters and laptops and are a huge source of problems. In fact it is fair to say that their use should be avoided completely.

Motorhome and campervan owners have an additional problem on top of all this, and this is that in most vans, the cigar socket in the dash is supplied from the vehicle starter battery, and not the leisure battery, so taking a lot of power from the the cigar socket can run down the vehicle battery.

So now you know the technical reasons why never to use a cigar socket to power an inverter/laptop setup. Phones are OK, because they are low power, but even a couple of phones can run down a vehicle battery in a few days.

Back to inverters

So what is the correct solution? The inverter must be connected directly to the battery, through a suitably sized and fused cable. A good inverter will be supplied with a suitable cable, and the fuse may be in the cable, or installed in the inverter itself. If the supplied cable isn't long enough, then it is OK to extend the cable using a wire of equal or thicker dimensions. If the ideal cable length for your particular installation is longer than about 2m, then you should use an even thicker cable, because longer cables at 12V DC can suffer from voltage drop. This is the reason that most inverter cables are quite short, and also the reason that most van inverters are located quite close to the battery.

So having connected your inverter properly, you are good to go, and you should never suffer from any of the problems just described.

You now have the power - or do you?

The next thing you need to know is just how long your battery will last. A laptop is probably going to be one of the most powerful devices running off 12V in your van. It may sound strange, but it is true. Van lighting takes very little power, neither do phones and tablets. A 12V TV is a medium power device, usually about 30W or so, but a decent sized laptop, like the Dell we are using as an example, can take around 100W while charging, and around 50W while in use, plugged in, but fully charged. Anything that uses more than 20 or 30W of power, has the potential to run down your battery quite quickly.

The way to work it out is this. You divide the watts by the supply voltage - which is 12V. Yes I know that the charger label says 19.5V, but the actual supply voltage is the voltage of your battery - which is 12V. So taking 100W as the average, divide 100 by 12 and you get 8.3. That is 8.3 amps. And when your laptop has finished charging its internal battery, then the power will drop to about 50W while you are using it, so that is about 4 amps. The exact numbers will vary a little bit depending on your laptop, but these figures are a good average.

You will also know how long your laptop takes to get fully charged from flat - this is usually about 2 hours, sometimes more, sometimes less. So you know that to fully charge your laptop will take about 2 hours at 12 amps, which is 24 amps in total.

Now you need to know the capacity of your battery. The commonest van battery is a 100ah leisure battery. The AH stands for amp hours. A 100ah battery should supply 1 amp for 100 hours, or 10 amps for ten hours - it's a simple calculation. So in our laptop case, 24 amps represents roughly 25% of the battery capacity. But normal 12V leisure batteries are generally only capable of delivering roughly 50% of their capacity before the voltage drops to below 12V. So the real world figure is more like 50%. So charging your laptop from flat could easily use 50% of a 100ah leisure battery. So you can see that a laptop can be a hungry beast, a beast capable of eating a battery! For this reason, people who need to have their laptop available at all times should pay close attention to power management. They usually need bigger batteries, solar panels and monitoring devices so they can plan their activities accordingly.

And here is the point about laptops in a van - it doesn't really matter whether you use an inverter or some other method, like a DC converter - people argue for hours online about whether inverters are efficient or not, and how it is daft to convert 12V to 220V and then back to 19v again. The truth is that all methods are better than 90% efficient, and that 10% isn't really relevant in the grand scheme of things. 10% doesn't make much difference - it might mean you can get an extra 10 minutes of use on top of the 2 hours. Purists (and trolls!) love to argue the point to show how clever they are, but based on my experience, people just want things to work reliably.

Use the original charger for peace of mind

This brings me onto another issue about using an inverter, and that is that using an inverter enables you to use the original charger that was supplied with your laptop. Even though a purist would say that it's inefficient to invert 12V to 220V to plug in a charger that then converts 220V to 19v, I would say that this is still the best method because a modern laptop charger doesn't just convert voltage - most modern chargers are "smart" chargers - they communicate with the laptop and are designed specifically to give the laptop just what it needs. There is also the matter of warranty. Try explaining to the Apple Shop why your £2000 Macbook is broken because you used a different, non Apple, charger. It's just not worth the risk.

This is the main reason I generally advise normal non technical people to use an inverter and the original charger - not because it is "technically" a few percent more or less efficient, but because it is convenient and safer for the laptop.

Apple MacBooks

Talking about Apple … A quick note about Apple MacBooks. They have white chargers that have no labels - however the specs are online. MacBook chargers are called "Magsafe Adaptors" and are between 60W and 90W. The electronics inside the adaptor are tightly integrated with the MacBook itself so it is generally impossible to use any other adaptor. Apple used to offer a 12V car charger for their laptops - but they don't any more - they disappeared several years ago. The story went that they were experiencing too many warranty claims. So generally it is easiest to use an inverter for a MacBook. And safer in terms of warranty and the wellbeing of the MacBook.

More on inverters

There are a few more points about inverters that i would like to touch on.

You will see references and online discussions about "pure sine" inverters, "modified sine" inverters and "quasi sine" inverters. For years now there has been much discussion about which is best. It gets technical, but for all intents and purposes it doesn't matter. It used to be that the "pure sine" inverters were much more expensive than the other type, but this is no longer the case. Technology has moved on and prices have come down, and in purely laptop terms, a laptop charger is happy with either type. Despite all the online discussions, I have yet to see anybody tell a story about having a major problem with either type. There are a couple of items of equipment that occasionally give problems with the wrong type of inverter, but these are rarely found in a van. "Pure sine" inverters are usually the most popular these days because they are about the same price. Some cynics say that many cheap inverters are advertised as "pure sine", but are the other two types. I have never come across this myself, but you need an oscilloscope to know for sure, and these are expensive.

The next discussion point is what inverter to buy. Ebay and Amazon are chock full of a bewildering array of inverters of all shapes and sizes and at prices from £25 upwards. Again, lots of folk have opinions and love to make statements on Facebook on what is best. A favourite opinion is that cheap "chinese" inverters are "crap", and you get what you pay for. My answer to this is that while there is always a possibility that any electronic product can break, it is bad business to sell a faulty or unreliable product - Ebay and Amazon function on the principle of feedback, and nobody survives very long if half the stuff they sell comes back for refund. The reality is that chinese stuff isn't all that bad, and all you have to do is look at a range of prices, check that the supplier has decent feedback, and choose the middle way - not the cheapest, not the most expensive.

However it is worth bearing in mind that there are a couple of manufacturers who are famous for the quality of their products and the most famous of these is Victron. Victron are a Dutch company who have a vast array of quality products, and are generally regarded as the best available. There is another company called Sterling, who are UK based and have a similar reputation, although as with most things, opinions always differ. Victron make a wide range of inverters, and their small inverters are priced similarly to the bigger models of other manufacturers. When asked, my standard answer is usually, I would rather have a small Victron than a big chinese one.

Don't over-specify!

A common mistake that many people make when choosing an inverter is over specifying. You will see ads that offer 1000 watt inverters, or more, and it is easy to think that more is better. However this isn't the case. You should size an inverter for the purpose you need it for. We are talking laptops in this discussion, and we have agreed that an average laptop needs about 130W. So why buy a 1000w model when you only need a tenth of that? Big chinese inverters are less efficient than small quality ones, and even though we have already discussed that efficiency isn't everything, there is another aspect to efficiency that is quite important in a van, especially if you are a "normal" owner with a single leisure battery. This is about "no load" or "standby" power. This is the amount of battery power an inverter uses while switched on, but doing nothing. A big 1000w inverter can use 1 amp of your battery every hour it is switched on. This may not seem like a lot, but that 24 amps if left on all day and night. So a good rule to obey when using an inverter is - switch it off when not in use. I have seen many a flat battery because of this - people think that if nothing is plugged into an inverter, then it is using no power. This isn't true.

So generally speaking, if the main purpose of owning an inverter is to power your 130W laptop, then a 200w or 300w inverter is more than enough, and smaller high quality inverters use less power on standby than bigger cheaper ones. Victron inverters take this one step further by having "auto sensing" - they take hardly any power at all until they sense the need. This is a useful feature to have if you forget to switch it off as it won't drain your battery. Other manufacturers also offer auto sensing.

Of course there are many other reasons why you might need a big inverter, but as we are talking laptops - small is better. If your budget allows, a small Victron is a good choice, I have installed many, and can testify to this fact. But I have also installed many cheap chinese inverters and know many more folk who have, and they can also give good service. The main thing to remember is not to over specify - very few van owners have a valid reason to own a 1000w inverter.

At the time of writing in early 2019 I have started to notice a new type of chinese inverter on the market that not only supplies around 400w reasonably efficiently, it also has built in USB sockets to charge phones and tablets, and it also has a digital display so you can keep an eye on your battery voltage - and all for £40.

So having discussed inverters, and why I think they are the best method, for the sake of completeness let's discuss the other methods.

12v laptop chargers

The first of these is to use a dedicated laptop charger that takes 12V as its input and outputs the required voltage to the laptop (usually, but not always. 19v). This cuts out the need for an inverter. Even though I think an inverter is the best bet in general terms, there is no real technical argument against it. The only problem I have with it is that, as stated above, you are replacing the warrantied charger with a third party charger that is not warrantied by the laptop manufacturer. This means that if you do get a problem, and need warranty, you will have no choice but to hide the fact that you were using a third party charger. However, it is unlikely that a third party charger will actually damage a laptop - not impossible, but unlikely. Any problem will more likely be with the charger itself ie it stops working, leaving you stranded with no laptop. Ebay and Amazon have plenty of 12V chargers for most laptops, but at the end of the day you are trusting an unknown supplier. I have seen plenty of folk online who have suffered this way.

At the end of the day I think it is swings and roundabouts. The cost of an extra third party 12V charger will be about £50 - less for a cheap one, more for a quality one. The same money will buy you an inverter, allowing you to use the original charger, plus have the added convenience of being able to use the inverter for other uses.

But in terms of efficiency - there is very little in it, as discussed above.

DC to DC converters - the DIY way

There is another method of powering a laptop from 12V, and it is the cheapest of them all. I include it here for completeness, but with the caveat that it is really only for folk that have basic DIY skills and a basic understanding of 12V electrics. Do not attempt this unless you know what you are doing.

It is based on the fact that, as discussed above, most laptops run on 19v DC, and most vans/vehicles run on 12V DC, so why not just convert 12V to 19v and plug in the laptop? No reason at all, especially if the laptop is out of warranty and you don't mind a bit of DIY.

On Ebay and Amazon you will find devices that cost around £10. They are called DC to DC converters, also known as "buck converters" - please don't ask why! They comprise a circuit board with a few transistors and chips and have a set of terminals to attach a 12V supply wire, and another set to attach the wire to supply the laptop. This wire must have the same plug on it that fits the laptop socket - they are available online, but you have to make sure it is the right plug. You may have to solder the plug to the wire. Another method is to find an old broken charger and cut the wire and plug off it and use that - like I said it's a DIY job, there is no off the shelf product that saves all this work. In effect you are home brewing your own 12V charger.

The main thing to remember if going down this route is that you must make sure you buy the right converter - it must be capable of delivering a constant output voltage for a varying input voltage. The reason for this is that a fully charged battery gives about 13v, which reduces as the battery discharges down to 12V and below. But a laptop must have a rock solid and constant voltage. The other thing you have to look out for is to make sure that the unit you choose has the correct power and cooling.

Like I said - it's a home brew DIY project that needs basic skill, but there is no reason for it not to work if done properly. The problem is that guys who have the skill to do this love to go online and advise others to do the same, saying it's easy and that you are daft to go to the expense of an inverter. It is! As long as you know what you are doing. But not everybody knows what they are doing - they just want to plug it in and it works.

And finally ...

So those are the 3 basic methods of powering a laptop in a van. Here are some bullet points in conclusion.

• An inverter is the easiest method in terms of ease of use for non technical people
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• An inverter allows you to use the original charger
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• An inverter is a useful thing to have around anyway
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• Cigar sockets to supply power to a laptop are a no no
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• Third party 12V laptop chargers are OK, but don't offer any real benefit over an inverter
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• You can homebrew your own 12V laptop charger with basic DIY and electrical skills
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• Laptops take more power at 12V than most people realise

Any questions please leave a comment or contact me directly.

EDIT - NOTES FOR EBIKE CHARGERS

Everything above applies equally to ebike chargers, except that ebike chargers are generally more powerful - generally they are about 180w, so they will draw about (180 / 12) 15amps, roughly 50% more than a decent laptop. And if the battery is empty, it needs about 3 to 4 hours. This means that if you have a fully charged 100ah leisure battery, it will be pretty much all used up, although generally it is not as bad because most riders won't use a full battery on a ride.

So it makes sense to charge your ebike batts while you are driving or on hookup. If you do need to charge while off grid, then the only other charge source available is solar. So you need enough solar both to charge the ebike battery, and have enough left over to put enough charge in your LB for your normal use.

In a UK summer, a 500w solar system with 200ah of battery should cope with it, but you have to monitor it closely - a few cloudy days would be problematic. Again, as suggested above, the best tool for this is a decent battery monitor, so you know what you have left and can plan accordingly.