Lobster presented the following explanation :
Sounds good in theory, but are there any gotchas? Eg, safety? Does it run a
car battery down more to power up a laptop via a 240V inverter than directly
to the cigar lighter? etc etc

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.


Can't help feeling an inverter would be much better if wired or plugged into
the car some other way than via the god-awful universal cigar-lighter plug...

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

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk