Laptop external power from batteries (DC/DC)

  Bringing this thread over here from gnhlug-org...

  Someone brought up the idea of powering a laptop from DC sources,
rather than the traditional AC power bricks.  This is strictly about
external power, not the laptop's built-in batteries.

  Obviously, many laptops come with "travel adapters", designed to
plug into 12 VDC automobile "cigarette lighter" power sockets.  If you
have one of those, and it works, use it.  But things are not always so
easy.

  Case-in-point was a Dell Vostro 1700.  It has a 17" display and
sucks power like a black hole sucks... everything.  Dell doesn't offer
a travel adapter for it.  According to the sticker on the bottom, it's
rated for 19.5 VDC at 4.62 A.  Doing the math, that's 90 watts.
Dell-branded travel adapters seem to top out at around 65 watts for
some reason.  90 watts at 12 VDC is only 7.5 A, which doesn't seem
that big to me.  I believe my car cig lighter socket is rated at 12 A.

  I know laptops can vary their power draw quite a bit.  In
particular, running the laptop while also charging the internal
battery is a major pig.  So that 90 watts may be overly high.

  I know a lot of travel adapters are misleading in their specs.  A
travel adapter rated at 90 watts may only be able to deliver that when
running on AC.  On DC, they may provide considerably less.  This was
not mentioned in the published specifications.  Again, I don't know
the reason for the limit, but I know it's caused me grief.  When I
attempted to use such adapters to power a Gateway laptop rated at 80
watts, the laptop would continuously switch between line-power and
internal-battery, at about 0.5 Hz.  Apparently the adapter was going
in and out of over-current shut-off.  Happened with two different
adapters, one of which I remember was Kensington brand.

  Inverters are an option, of course, but the DC->AC->DC conversion
tends to waste a lot of energy.  Not a big concern if you're running a
car engine anyway, but if you're running solely off the battery, it
matters.  Given that the discussion arose from a situation where a car
wouldn't be convenient (but a car battery in a box would be), I'm
curious about the battery-only scenario.

  Looking quickly, I can't seem to find amp-hour ratings from car
battery makers/sellers.  Google finds various third-party claims, but
they're all over the map (25 to 100 Ah on the first page alone).
Assuming 7.5 A, and again doing the math, that is anywhere from 3 to
13 hours of operation at full power.

  I know if one was planning on doing this on a regular basis, a deep
cycle battery would be highly recommended.  For a one-shot, though,
pulling the battery from a car might be feasible (?).

  Anyone have additional information they'd like to share?  Inquiring
minds... probably don't care.  But I'd like to know.  

-- Ben