Laptop external power from batteries (DC/DC)

[mike ledoux]

Probably more than you wanted to know, but you asked.  :)

On Mon, Aug 18, 2008 at 06:24:32PM -0400, Ben Scott wrote:
>   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.

Car battery manufacturers don't typically advertize amp-hour ratings
because you don't really care about that for a starting battery; all
you really care about there is the CCA rating.

Also, the math really isn't as simple as what you are doing.  Those
amp-hour ratings are at a specific current draw--if you are drawing
at a higher rate, you'll get less out of the battery (Peukert
Effect).  Typically for a deep-cycle battery the amp-hour rating is
for 100% discharge over 20 hours, though some manufacturers rate
over 100 hours to make their batteries look better.

So, presuming your 100Ah battery above is a 20-hour rating, it will
provide 100Ah at a constant discharge of 5A.  Since you want to
draw more than that, you'll get noticibly less than 100Ah.  Without
seeing specifications for that battery, I'd anticipate a 10-20% drop
in capacity for the 50% higher discharge rate.  So, ~10-11 hours,
not 13.  That's to 100% discharge, which does damage the battery.

Of course, that presumes your equipment will operate on the battery
at all states of charge, which is likely not to be the case.  Below
about 50% charge on any 12V LA battery, the voltage will be <12V,
and your adapter (which was probably designed with the expectation
of being attached to a runing car, at ~14V for the nominal 12V
system) may not be able to cope.  So, presuming your adapter needs
>=12Vdc to operate, that 10-11 hours becomes more like 5 hours.

So far, all of these numbers are based on 100% efficiency, which
isn't realistic.  A good (and sadly, expensive) DC-DC voltage
converter is only about 85% efficient, so to get the 19.5Vdc @ 4.62A
you need, you will draw ~8.63A @ 12Vdc.  ~5 hours becomes ~4.5.

>   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 (?).

I wouldn't do it.  Definitely don't do it if you need that battery
to start the car to get you home after the event.

Car batteries are designed to supply a very large current for a short
period of time.  They are not designed for continuous draw, or to be
deeply discharged; doing either will severely reduce the lifespan of
the battery, and may kill it outright.  The thin lead sponge used in
these batteries to maximize surface area (and therefore current) is
easily damaged by deep discharge. For maximum battery cycles, don't
discharge more than 5-10%.

Deep cycle batteries have much thicker solid lead plates, which
gives less surface area (and lower maximum current), but these
plates will not crumble on deep discharge.  You can discharge a good
deep-cycle battery to 50-80% without harming the battery.


If you're going to do this, save yourself a lot of headache and
buy/borrow a deep cycle battery & charger.  You'll need a pretty
big one for the load you're talking about, one rated for 250-300Ah.
More if you need to use an inverter, to account for the extra
efficiency losses there.  A portable generator might be easier, if a
long outdoor extension cord is not possible.

-- 
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Holder of Past Knowledge           CS, O-
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