On 9/25/20 11:07 PM, Poul-Henning Kamp wrote:
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Bill Notfaded writes:
Can't I just use a high quality APC backup power system like we use to
power racks of gear in our Telco and compute closets?
Very few UPS's are good at long-run applications, they are typically
built to run a heavy load for minutes, not a tiny load for hours
or even days on end.
That means low efficiency, 75% net efficiency is considered good,
and it goes totally south the further you are from the name-plate load,
because the constant overhead is large.
Some UPSs dont even have a thermal design allowing 24*365 operation.
As people have found when modifying those inexpensive $100 UPSes to use
an external battery. They're *cost sensitive* so the electronics just
gets to maximum temperature as the designed in battery runs out. It
might be cheaper to add mass than a (noisy) fan, for instance.
If you want to power mains kit from batteries, it is usually better
to get a "real" inverter which is built island-grid applications.
They do make UPSes for long term running but not as a static inverter -
typically you can identify them because they come with options for
external batteries. Or,you choose which battery pack to add.
But for powering small loads, OCXO's, GPSDO's, Rb's, fire alarms,
emergency lighting etc, the overhead of going from battery voltage
to mains voltage and back is just a unnecessary loss.
There is, also, the Tesla PowerWall approach..
13.5 kWh, 7kW peak, 5 kW avg, $6500
You can keep a lot of oscillators humming for quite a while with 13.5
kWh, even at reduced efficiency.
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