[PLUG] Battery backup issue

Keith Lofstrom keithl at gate.kl-ic.com
Mon Sep 30 14:21:18 PDT 2013


On Sun, Sep 29, 2013 at 11:06:14PM -0700, Dick Steffens wrote:
> I have an APC Back-UPS ES 550 UPS. 

Does the unplugged UPS provide enough power to start up the
machine?

That is probably an Offline/Standby UPS.  In normal conditions,
it feeds power straight from the wall to the load, bypassing
the charger and battery and inverter.  When the power goes
down, it twiddles its thumbs for a missing cycle or two, fires
up the inverter from the battery, throws a relay, and starts 
feeding the load, which has been without power for 30
milliseconds or more.  The better UPS units send a signal
over USB to the computer to signal "suspend NOW".

As you can imagine, a computer with a just-barely-adequate 
power supply, accustomed to a squirt of power 120 times a
second (plus and minus lobes of line frequency), might lose
hope and die during a very brief interruption of those 
squirts.  Inside the computer power supply are some
electrolytic capacitors which may have been full value when
the machine was new, but may have been cooked to weakness.

Same electrolytic problem for the UPS itself, which may also
have worn out batteries.  

And you may have added peripherals to the Gateway computer
that draw down the power faster.  A bigger third party 
power supply with larger internal capacitors might help.

The UPSs that Really Work are "online" UPSs.  Power from
the line charges the battery, battery power drives the
inverter continuously, which directly powers the computer. 
If the charger shuts down, the inverter and computer keep
running until wall power comes back or the battery is
exhausted.  If everything is functional, zero power
interruption.  Of course, online UPSs are more expensive.

Either the larger computer power supply, or the online
UPS, will make heat continuously that the offline UPS
and smaller computer power supply won't.  You can spend
Even More Money on high efficiency "green" power supplies
and UPS units.  There may even be computer power supplies
that connect to 370V batteries directly, supplementing
the electrolytics, which is more efficient still. 
This is what the big data centers are evolving to.

The power grid is becoming more unstable and less
reliable.  Utilities are strapped for cash and required
to absorb intermittent, unpredictable "green" energy.
And your neighbors are buying plug electric cars - when
their multikilowatt chargers turn on, the line power will
sag a little (or a lot, depending on charger design).

And if climate change expands the Hadley cell and moves
more tropical storms north, we will have more wind and
rain storms at 45N.  And we won't have as much glacial
melt to power hydro in the summer and fall.  So a good
offline UPS may be needed for the power grid of the future.

Encouraging versions of the Linux kernel that can recover
from instant power loss might be called for, too.  Spotty
power availability is the norm for most of the world, and
most future computer users.  We should help build software
for the precarious power world, and for our own future as
we reduce our own industrial capabilities.

Keith

-- 
Keith Lofstrom          keithl at keithl.com



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