Laptop external power from batteries (DC/DC)

[Ben Scott]

On Thu, Aug 21, 2008 at 1:47 PM, Bill McGonigle <bill at bfccomputing.com> wrote:
> ... must cut speaker wire!

  For most of the APC Smart-UPS line, pressing the "On" button briefly
will silence the "On Battery" alarm (but not the "Low Battery" alarm).
 For the ones with only a single "On/Off" button, I think pressing and
releasing it very quickly might do it.  (I recall that you need to
hold it down for about 1 second to actually turn it off.)

  Most of the Back-UPS models are "dumb" and can't be silenced without
wire-cutters.

> I'm curious how noisy of an inverter APC puts in those things.

  Electrical or audible noise?  In my experience, audible varies from
unit to unit; the manufacturing criteria, if any, must be fairly
forgiving.  For electrical: The Smart-UPS product line is supposed to
output a "pure sine" wave.  I haven't verified this myself, but I'd be
surprised if they were getting away with a lie on that front.  Most of
the Back-UPS models use a step wave approximation that's nominally
"good enough" for most things.

> I'm also now curious about using a bigger battery with the COTS UPS
> since they've already done most of the work for me.

  If the battery is already charged, I think that would generally
work.  The only potential problem I can see would be that the DC->AC
electronics might start to overheat in an extended run.  A meltdown
with that kind of battery amperage has the potential to be seriously
dangerous.  This isn't idle speculation, either.  One of the
rack-mount APC Smart-UPS models, the XL variant took up more rack
space, and that was supposedly solely for heat dissipation reasons.

> they've done most of the hard engineering work I'd be unlikely to
> replicate.

  Hmmmm.... good point.  While 12 VDC -> 19 VDC might be more
efficient than 12 VDC -> 120 VAC -> 19 VDC in theory, the UPS industry
has put way more engineering into the former problem than we can with
the latter.

-- Ben