If your laptop runs on 12 V, the device can be very simple; if the
computer can tolerate a somewhat irregular voltage up to 14 or 15 V,
all you need is a cord with proper plugs.  In some cases you might
need a regulator to keep it down to a steady 12V., still pretty
simple.
  David was talking about an HP that needs a higher voltage.  This
means a device that takes the automotive voltage and increases it.
This equals or exceeds the complexity of an inverter and since it's
not a mass market item, the cost is even higher.

Larry A.

On Fri, Feb 27, 2009 at 7:49 AM, pdooley <psdooley@verizon.net> wrote:
> David, did you mean to say DC notebook adapters are quite "in"expensive?
>
> You can get a DC adapter that plugs into the cigarette lighter for less than
> 11 bucks with free shipping.
> Doesn't get much cheaper than that.
> http://tinyurl.com/agfkzd
>
>
>
> -----Original Message-----
> From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
> David Kao
> Sent: Friday, February 27, 2009 12:32 AM
> To: vanagon@GERRY.VANAGON.COM
> Subject: Re: Invertor ?
>
> Notebook computer vendors normally have DC power adapters for
> use in cars to power notebooks. They take DC 13.8 volt input
> and generate DC 19 volt (for HP notebooks) 3 - 4 amp output to
> power notebooks. Such DC power adapters are quite expensive.
> There are aftermarket DC inverters that are cheaper to get.
>
> Using an inverter (13.8 volt DC in, 120 volt AC out) to power
> a notebook AC adapter then power the notebook is indeed less
> efficient. That's two stages of inverters.
>
> If the setup is used only for charging the notebook you probably
> will lose maybe 3 - 5 watts of power due to the inefficiency.
> This will not be too much a concern really. But the reliability of
> the inverter will be one to worry. Most cheap ones are over rated.
> When they go bad they can cause damage to the device they power.
>
> David
>