Powering laptop etc in car from inverter?

I did with a dell lt, not again, i didnt like the feel of the laptop power module(230 to 18) it got very very hot compaired to warm on full fat mains power.

Lobster presented the following explanation :

A switch mode PSU (which is basically what an inverter is) has an efficiency of between 70 and 90%. Powering a laptops own PSU from one means that you would have two steps in voltage conversion, each with losses. A better way would be a purpose designed inverter which directly replaces the laptop PSU with one which takes in 12v and outputs whatever the laptops needs. Maplin do such an item with various output voltage settings, various plugs and at 3amps.

Agreed, but it is the 12v socket commonly fitted to most cars.

Remind me not to post while on the phone. ;-)

I meant the 20 hour rate, which would be around 3.5 amps discharge. A 150 watt invertor takes approx 12.5 amps, and the higher discharge rate means the battery won't deliver quite its full capacity.

But 3 hours from a 70 amp/hour battery driving a 150 watt invertor at max isn't a million miles out. ;-) Although theory says nearer 6.

Sorry for the rubbish post.

snipped-for-privacy@care2.com presented the following explanation :

Or carry a battery charger so you can plug that into the inverter to charge the battery back up :-)

That's just what Ian said.

Laptop PSUs (unless youve got one from the 80s) are quite happy with any of the inverters. However the direct converters from Maplin are cheaper & more effecient.

He did. Which bit of "assuming 100% conversion efficiency" don't you understand? It might not be a valid assumption but the method is perfectly clear.

MBQ

A lot of laptops need more than 12V so you would still need a DC-DC converter to run from the cigar lighter. It then depends on the relative efficiencies of the inverter and the DC-DC converter.

By "on the road" do you actually mean driving? If parked then just be careful your battery can withstand being discharged repeatedly.

MBQ

Indeed, and my point that he completely missed the point that asking "Was it 40W at 18V or 240V", is obviously referring to the conversion efficiency.

the current drawn at 40w on 18v is around 2.222222222222-amps // but 40w at 240v is about 6-amps this do not include the extra power that the inverter needs just to run its self,

also if you draw 40w at 18v then the inverter will need more than 6-amps input

sorry the 6-amps should read 0.6-amps (puts head under jumper in shame)

What do you reckon the capacity of the laptop's internal battery is?

watts amps = ----- volts

So 40w at 18 volts is as you say 2.2. But watts are sort of an absolute measurement of power, so increase the voltage and the current goes down, so 40 at 240 volts is 0.17 amps.

Thus the wattage drawn by the invertor will be the same as its load, but increased by the efficiency figure of the invertor. So if that's say 80%, the load of 40 watts will draw 50 watts from the battery.

Why is that relevant? The OP wants to power the laptop from a car battery. In the past I seem to have buggered a car battery by allowing it to discharge to much (courtesy light left on for a few hours). Maybe it was on it's last legs anyway.

MBQ

Plus the inefficiency of the laptop power supply, which may be another

Because it's likely to be be tiny compared to the car battery.

Most courtesy lights don't draw much current - or those that do often switch themselves off after a time.

Sounds rather like it.

Smaller, but not tiny. Many laptop batteries are >10% of the capacity energy-wise of the car battery.

The 40 watts was just following on from ab's post - not meant to be an actual figure for a laptop.