Laptop external power from batteries (DC/DC)

[Drew Van Zandt]

The Ah of a battery may depend quite a lot on the current drawn from it; I'd
look for a datasheet, it probably has an Ah/current graph.

You need a pretty high-power switcher to feed that current at 19.5VDC, in
electronics-land that's a TON of power.  Definitely something with an
external power FET and a big-ass inductor.  Niftier switchers these days
might get the physical size of the inductor down a bit, but it has to be
rated at the max current you'll be drawing.

Do-able, but enough work that unless you can get a travel charger to solve
that part it might not be worth the headache - It Depends (TM).  One-shot,
or do you need 20 of them?

--DTVZ

On Mon, Aug 18, 2008 at 6:24 PM, Ben Scott <dragonhawk at gmail.com> wrote:

>  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
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