[Tfug] UPS

[Kramer Lee]

Just some thoughts:

Well you could replace the battery in a commercial UPS box with a car
battery if the charging circuit could handle charging the car battery
without burning out, and if the charger circuit could charge the
battery to the correct voltage.  If the original battery is a 16V
battery, then putting in a 12V car battery would probably blow up the
charging circuit in a while, as it would never get the battery up to
16V but it would keep trying.  If the charging circuit was set up so
it never puts out too much current, it might not burn out, but then
the car battery may not provide enough voltage for the inverter so it
wouldn't put any AC out, so it just wouldn't work.

I guess you could get a couple car batteries, and a regular car
battery charger, then get a regular inverter that is made to run from
the car electrical system already, and put them all together.  Just
run the computer and maybe an energy efficient display from that.
Then there wouldn't be any issues with trying to synchronize the
inverter output with the input AC power.  Most the time you would be
running from the charger current, filtered out by the car batteries,
running the inverter (hopefully the inverter puts out a mostly
sinusoidal output waveform so the computer actually works well from
it), which provides the AC for the computer.  The inverter would have
to be able to handle the computer (and whatever else) current for
years without damage, and probably many car inverters are depending on
not being on 24 hours in a row, they believe they will only be on just
during driving so the designers cut corners and save production costs.

So I guess I would get an inverter rated for a couple hundred watts,
and two car batteries, and an at least 10A contnuous battery charger
if you actually need 120W continuously.

UPSs don't usually work this way, they let the regular AC go through
to power the unit, and charge the batteries (trickle most the time) at
the same time.  When power is lost its inverter comes on and gives a
few minutes or more of power, depending on their rating.  They seem to
just be for a few minutes to allow the computer to shut down
gracefully instead of dying immediately in the middle of a write to a
sector, which we don't want of course (the incomplete write to the
sector and perhpas head crash).

OK, how long would two car batteries last powering a computer?  Well,
I believe a set of headlights draws about 60W, and you can drive a
couple hours with the headlights on before the car dies usually (if
the battery is still good and well charged at the start of the
driving).  So if you have one 60W computer and a 30W monitor, and two
batteries it might last a couple hours.  That is my guess.  If the
monitor is in standby and the computer is not processing much, maybe
it would last quite a bit longer, maybe a day.  If the inverter
doesn't keep working below 12.6V, then it is going to stop a lot
sooner.  If the inverter will still work OK putting out enough voltage
and current AC wise, it will keep working a lot longer.  I never tried
varying the input DC voltage on an inverter running a computer (and of
course different computers will work differenty themselves.

We had a quite short power failure in the monsoons the other day, and
it was long enough to reboot 2 out of 4 of my computers, make the AC
die but it didn't seem to bother the refrigerator.  I unplugged that
anyway and shut off the AC to let the pressure bleed off.  So, some of
the computers have more capacitance in their power supply outputs than
others, even though 3 of them are the same brand.

I guess if you really want to survive through a power failure, a
laptop is ready made for that.  On AC power it keeps the battery
charged, and when power dies it just runs off the battery.  I just
looked at a laptop charger and it says it puts out 19.6V and 6.7A.
Well, keep a charged car battery and a boost voltage regulator (to
take its output up to 19V).  Keep the battery charged up, and the
output of the boost regulator connected up to the same connector as
the output of the laptop charger, diode isolated, and when the AC
power goes out, it will start charging the laptop battery from the car
battery, and at 6.7A max it will take probably more than a day to
discharge the car battery.  If you really want a long life laptop,
just pack a charged up car battery (with a boost regulator and
modified power connector) around with your laptop.  You can easily
work an 8 hour day on that even with wireless going.  Your cell phone
would last a long time on a motorcycle battery.  Just a little bulky,
that is all.