In article ,
dg wrote:
(Gordon Henderson) wrote in message ...

I've found it easier to use the invertor, then just plug the devices
standard mains adapters into the invertor to run/charge them.

Gordon

On the subject of inverters, isn't it very inefficient to convert 12v
to 240 v, and then back to say 12v for the appliance?

If say, I wanted to power a laptop this way (mains transformer = 12v
and 4.16A) how long could I expect the car battery to last? Would it
just be better to get a 12v cigar adaptor - or would the current drain
to the car battery be the same either way?

No, it's not that efficient, but it's convenient - especially if the
appicance requires a regulated external power source to charge internal
batteries. If the appliance comes with a 12V lighter-socket adapter,
then by all means use that instead, but it might be cheaper to use the
invertor than buy lighter-adapters for every bit of kit you want to power.

They aren't that innefficient either, and the one I have claims to switch
off if the voltage drops to below 11V, but I've never been able to run
anything that long to find out.

For battery calculations, you need to know the rated AmpHour capacity. But
note that they are usually stated at a particular amp rating. Eg. a typical
car battery might be 40 Amp Hours at 10 Amps. That means it will last
for 4 hours (down to some set voltage, maybe 10V, I don't know, would
have to check spec.) at a current drain of 10 amps, but it wouldn't last
one hour at 40 amps (usually a lot less!)

So drawing 5 amps, you might expect it to last 8 hours. Most car batteris
will have their rating printed on the labels.

Gordon