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[TheBoy]

Doz, post up pic of label side of PSU. Ithink some of what is suggested here may be bollyox.

I can't imagine an XP era laptop is less than about 60W.  If more than 6 years old (remember Vista came out 6 yrs ago), I'd expect it to be closer to 100W.

I would aim for an inverter at least twice the capacity, ideally 3x the capacity, and go for a decent one, as the cheap ones produce far from perfect sine waves (the good ones aren't perfect, but a lot better).

Also, using over simplified maths, 120W will be pulling 10A from battery - assuming 100% efficiency - so a 50AH battery will last 5hrs. Only deep discharging like that, the battery will be destroyed quickly.

Going back to my earlier post, maybe a more efficient laptop, possibly with multiple batteries...   ...or a tablet.

[dbdb]

Something like this should do the job then?

http://www.ebay.co.uk/itm/500W-Power-Inverter-convert-12v-DC-to-230v-AC-/400234658288?pt=UK_Campervan_Caravan_Accessories&hash=item5d2fd839f0

No that's a modified sine wave inverter.  I have lost 2 chargers to these type.  A pure sine wave will be better eg http://www.ebay.co.uk/itm/121042946004 . Cheaper too. If it doesn't say pure sine wave it isn't.  Almost all inverters are made in China now, last UK one I had was 15 years ago and that was modified sine. My laptop charger is 1 amp at 19V, your 1.7amp doesn't surprise me but yes do check the charger label.  100W inverter will be plenty, it will probably only put out 60W  but a charger will not have a high start up like a motor or compressor.  Don't forget you are only charging a battery, its the battery that starts up the computer unless you want to run without a battery at all. Inverter prices rise exponentially with output. 

Inverters are either modified sine wave or pure, yes a pure is not perfect but it is entirely different to any modified sine wave.  Yes some modified sine waves are better than others but all bear no comparison to a pure sine wave inverter. 

[Abiton]

As an experiment, I'm running a Vostro 3500 laptop here, on Windows 7, power-brick plugged into the mains via an "Ecosavers" Wattmeter*.
Battery fully charged (normal for me), screen brightness at maximum (not normal, but more likely when camping), mains load varies from 20W minimum to 27ish briefly while opening a new webpage etc.  When processor fan starts up periodically, it goes up to about 31W for a few seconds.
I dare say that if I played games that used graphics intensively it might go higher; but I don't. DVD-drive usage would surely push it up more too; but I don't use that either.

Quite impressed at how little juice it needs TBH.

*May well not be spot-on accurate, but gives an idea.

[Kevin Wood]

No that's a modified sine wave inverter.  I have lost 2 chargers to these type.  A pure sine wave will be better eg http://www.ebay.co.uk/itm/121042946004 . Cheaper too.

Personally, I wouldn't believe that was pure sine wave unless I'd put a 'scope on the output.

The reality is that a laptop power adaptor will be switched mode, so the first thing that happens to the incoming AC is that it gets rectified to DC. The shape of the incoming waveform is therefore not likely to matter much in this application, as long as the voltage doesn't stray outside safe limits.

Also bear in mind that a lighter socket is rated at about 10 amps, so 120-140 watts maximum anyway.

I have a cheap 150W Maplin job that happily powers any laptop I've ever thrown at it, including some seriously old junk.

If it's only the one laptop you need to use, a better way would be to buy a universal laptop DC-DC adaptor which will take the 12v supply straight to the voltage required by the laptop instead of via 240v. More efficient and less clutter in the car. Again, Maplin, etc. will have them.

[Marks DTM Calib]

You cant get a pure sine wave from any such a device (or the mains socket on your wall in reality!)

As Kevin says, the first thing that will happen in the SMPS is the AC will be rectified and hence the output quality is not that important assuming the rec diodes can withstand the slightly great dV/dt and consequental di/dt of the stepped ac waveform (bloody poor design if they cant!).

You could even feed 100V+ DC into it and it would work

[biggriffin]

most of the boys now days in there trucks are running 24v 300w inverters these will run a laptop and a 19" flat screen tv,those who have toasters  are using 1000w,(engine running) when using the toaster.
so 150w is  plenty to run a laptop.

[TheBoy]

As an experiment, I'm running a Vostro 3500 laptop here, on Windows 7, power-brick plugged into the mains via an "Ecosavers" Wattmeter*.
Battery fully charged (normal for me), screen brightness at maximum (not normal, but more likely when camping), mains load varies from 20W minimum to 27ish briefly while opening a new webpage etc.  When processor fan starts up periodically, it goes up to about 31W for a few seconds.
I dare say that if I played games that used graphics intensively it might go higher; but I don't. DVD-drive usage would surely push it up more too; but I don't use that either.

Quite impressed at how little juice it needs TBH.

*May well not be spot-on accurate, but gives an idea.

Yeah, modern lappys with proper mobile processors are great.

My works lappy, first gen Core i5, with 4G ram, and a traditional HDD, pulls between 12-25W, not including screen.

When I rebuilt my media centre with a 3rd gen i5, I purposely went for a 35W chip to keep power usage down...

[tidla]

Did you buy one/ have a leaning towards...

[dbdb]

You cant get a pure sine wave from any such a device (or the mains socket on your wall in reality!)

As Kevin says, the first thing that will happen in the SMPS is the AC will be rectified and hence the output quality is not that important assuming the rec diodes can withstand the slightly great dV/dt and consequental di/dt of the stepped ac waveform (bloody poor design if they cant!).

You could even feed 100V+ DC into it and it would work

You can't get a pure sine wave from anything just like you can never get a perfect circle.  However it you put a scope on mains supply or on a Chinese pure sine wave inverter and then on any even top quality modified sine wave inverter you will see they are apples and oranges, completely different output.  To say a sine wave isn't pure is irrelevant really when you look at a stepped wave. In fact it's not even a wave.

It's not the laptop you should be concerned about it is the laptop charger.   I have had a toothbrush charger and a screwdriver charger both blown by a quality modified sine wave inverter.  Plus pumps would make ominous sounds.  Since switching to 'pure' sine I have had nothing go and that is on a Powerjack budget pure sine inverter from China.   

[TheBoy]

I use a craplin special, some Jap sounding name, that I picked up 19 or more years ago when craplin had a sale.

Its 300W, was about £30 back then.

Does most thinks I ask it to, though I stuck a ciggy lighter plug on it, with 10A fuse, limiting it somewhat. Never tried a lappy in it - I always go for low power lappy, with spare batts.

[Marks DTM Calib]

You can't get a pure sine wave from anything just like you can never get a perfect circle.  However it you put a scope on mains supply or on a Chinese pure sine wave inverter and then on any even top quality modified sine wave inverter you will see they are apples and oranges, completely different output.  To say a sine wave isn't pure is irrelevant really when you look at a stepped wave. In fact it's not even a wave.

It's not the laptop you should be concerned about it is the laptop charger.   I have had a toothbrush charger and a screwdriver charger both blown by a quality modified sine wave inverter.  Plus pumps would make ominous sounds.  Since switching to 'pure' sine I have had nothing go and that is on a Powerjack budget pure sine inverter from China.

And again we come back to the brick design, the two items you describe that failed will most likely have a standard magentic core transformer.

This is a very different setup to that used on a laptop power supply as if it used the same design then you would not be able to carry it.

Hence the statement still stands.

The modified sine wave units fire FET's multiple times per phase to create the pseudo sine wave (still some very fast edges), the lesser ones will have a centre tap transformer and a couple fo FET's to create a squarer version (can be easily made to look good with the correct filters).

So again, for the laptop solution, it does not matter. 

[dbdb]

I think we may be confusing protecting the laptop with protecting the charger here.  The two chargers I mentioned will be almost identical to a laptop charger.  Putting a digital stepped 'wave' (not actually a wave at all) into any device designed for an analogue wave could damage it, chargers and motors especially.  Agreed that if the charger survives then what it puts out to the laptop will be fine so the laptop is not at risk at any time.  Just the charger.

[Marks DTM Calib]

I think we may be confusing protecting the laptop with protecting the charger here.  The two chargers I mentioned will be almost identical to a laptop charger.  Putting a digital stepped 'wave' (not actually a wave at all) into any device designed for an analogue wave could damage it, chargers and motors especially.  Agreed that if the charger survives then what it puts out to the laptop will be fine so the laptop is not at risk at any time.  Just the charger.

If you understand the opertion of a laptop battery charger then you will find that the output makes no difference due to the input stage of the charger being a rectifier and not a transformer primary or motor winding 

[dbdb]

transformerless chargers are listed as items that may be damaged by a modified sine wave see http://www.samlexamerica.com/support/faqs/faq02.aspx.  As I say I've lost two such chargers to modified sine.  They worked fine until they didn't.  You may get away with it for a while until they fail.  I wouldn't use modified 'sine' inverters now you can get 'pure' sine for not much more cost.  And definitely not for chargers , motors fridges or microwaves.

Where you see my quotes round  'sine' and 'pure'  I mean the modified is stepped and not a wave at all (it's digital) and for the pure sine although analogue no wave is is ever pure.

[Marks DTM Calib]

Lets put it this way shall we, as somebody who has designed and had approval testing carried out on switchmode supplies for sensitive equipment, I would have no concerns over using one with a laptop power supply (as there is no technical risk to it)