Laptop external power from batteries (DC/DC)

[Jim Kuzdrall]

On Monday 18 August 2008 18:24, Ben Scott wrote:
>   Bringing this thread over here from gnhlug-org...
> According to the sticker on the 
> bottom, it's rated for 19.5 VDC at 4.62 A.

    That number may be considerably higher for several reasons.  One is 
government regulation for the safety (fire resistance).  That would be 
the maximum power the thing could draw if everything went wrong.  The 
other factor is that the startup power or some operating surge may 
require a peak power much higher than average.

    If you have access top a variable laboratory DC supply, you can 
measure the turn-on and running characteristics.

    An HP f1703 display I have says 3.5A at 12V on the name plate.  It 
draws 2.1A running.  It will start with the current limit set as low as 
2.3A.  Generally, you can expect a lower power draw than the name plate 
rating.
 
> 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.

    The AC and DC converters are completely different circuits housed in 
the same module.  They should have both met the minimum rating, of 
course.  One possible excuse for them is that the DC voltage was too 
low after going through whatever wiring connected it.  The car battery 
is actually 2.2V*6= 13.2V at "room" temperature.

> Assuming 7.5 A, and again doing the math, that is anywhere from 3 to
> 13 hours of operation at full power.

    Don't forget that DC-to-DC converters are 85% to 95% efficient, so 
there is some additional loss.
>
>   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 (?).

    Make sure it isn't the battery you need to start the car when you 
want to go home!  You can put a similar sized car battery in parallel 
with your regular battery, and the idling car engine will charge it - 
at great expense in gasoline.

    I looked at www.mpja.com for something DC-to-DC, but they don't seem 
to have anything at the moment.

Jim Kuzdrall