Inverters


Someone in PC World was trying to sell me an inverter, telling me that I can run not just my laptop but a whole host of 240v mains appliances off my 12v car battery using an inverter.
I have seen these before but wondering how good are they? How fast do they run the car battery down and how much damage to a car battery can the cause? Seems to me there's a big drain being put on the 12v to up it to 220-240v
--
Ron O\'Brien


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My PC world laptop won't run from an inverter, as I discovered on my canal boat hols last year! The inverter and power pack got quite warm though!
Haven't bothered to investigate further as I'm normally within reach of mains:-)
Peter
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Peter Andrews wrote:

I have an inverter bought for exactly that reason. It happily ran my Tosh laptop. I now have a Dell one with a 12V-compatible PSU - you can run it straight off the car. Mine is "Modified Sine wave" - which isn't very sine like, but not square either.
Andy
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Ron O'Brien wrote:

but a large inverter (say 1000w) will take about 100 Amps. Your battery will not last long before it is flat, and not much longer before it is damaged/destroyed.
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Much more sensible would be to run the laptop directly off the 12 volt supply as that is (I believe) what quite a lot of laptops require. Saves all the losses of converting up to 240 volts and back.
--
Chris Green


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Typically a car battery is 50Ah - which means it can deliver 50 Amps for 1 Hour before it is flat.
Your laptop power supply should have a label which will give you an idea of the current the laptop draws - mine is 4.7A.
If conversion were perfect a 50Ah battery could supply that current for 50/4.7 = 10.6 hours.
But conversion isn't perfect - assume 70% efficiency and that gives you 7.4 hours.
That's for a standard car battery - if you have a heavy duty you may be able to get twice that.
Also I imagine that 4.7A is a peak current rather than continues so again you will get more.
So figure 7.4 hours worst case.
Also remember your car is charging the battery at the same time, or you are stopped and you are falettening your battery.
Cheers,
Nigel
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Nigel Heather the-heathers.co.uk> <nigel@ wrote:

the car battery voltage. my laptop for example needs 20 volts at 4.5 amp which is 90watts assuming perfect conversion, 90 watts from a 12 volt source is 7.5 amps so your 50A/h battery might last 50/7.5 about 6.5 hours However discharging at 7.5 amps won't give you all the theoretical 50AH so say 5 hours. Then allow for less than perfect conversion, 70% your figure, we get to 3.5 hours worst case.
Bob
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True but that is based on a linear convertor which are hardly ever used because they have to disipate volatge difference as heat (and lost energy).
Instead they using switching technology which does not suffer from this problem.
Cheers,
Nigel
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Nigel Heather the-heathers.co.uk> <nigel@ wrote:

converter. Linear, switching, rotary, whatever other type you care to cite. Electrical Power out = Power in x efficiency
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Bob Minchin wrote:

Also car batteries are NOT designed for deep discharge, so to avoid damage do not discharge much below say 60% charged (ie 40% discharged). So now we are looking at about 90 mins battery life. Of course if the battery is a few years old it may work fine in your car but it's capacity may have reduced quite a lot. Final point, which is obvious, is a flat battery will not start the car.
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the inverter output meets the requirements of the laptop power supply. It won't in all cases - caveat emptor - the input requirements of the supply are probably not clearly stated, and the output of the inverter is probably described optimistically. Only way to be sure is try it. If the shop won't let you try, then don't buy.
As to how long it will operate ... watts are the important thing. If the laptop takes about 50 watts, then in car battery terms that is about 5 amps (allow a bit for conversion efficiency). If the battery is 50 AmpHour, then it "should" be able to put out 5 amps for maximum 10 hours - but as it goes flat, the inverter will stop sometime before the power runs out depending on the how the battery voltage drops. This depends on the battery (size, age etc) and the inverter.
Only realistic way (without knowing a lot more than me about the relevant battery and the relevant inverter) is to try it and see ...
hth Neil
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