Anonymous said:
I read about Google's clusters, that they run them off 12 volt batteries
rather than the usually UPS setup. I could use such a power supply
for a system I am concocting. Where do I get them?
This is just to illustrate the concept for you, not to pick one
ready to buy. You have to know a fair bit about what you're getting.
http://www.mini-itx.com/store/?c=10#dcdc
For the kind of usage you envision, you'd want something with
a wider input range. A battery is not "regulated" and the
voltage varies between full charge and discharged. Some of
the pico regulators in the above list, require a closely regulated
input, such as the output coming from a laptop AC adapter as
a power source. Operating from a car battery, is not considered
a "regulated" source. That's why I selected the WI model as
a starting point.
http://www.mini-itx.com/store/?c=10#p1843
The user manual provides more detail.
http://www.short-circuit.com/manuals/picopsu-125-wi-25-manual.pdf
"Power Ratings (Max Load = 120 Watts)
Volts (V) Max Load (A) Peak Load (A) Regulation %
5V 6A 8A +/- 1.5%
5VSB 1.5A 2A +/- 1.5%
3.3V 6A 8A +/- 1.5%
-12V 0.1A 0.1A +/- 5%
12V 6A 8A +/- 3%
At max load, forced air ventilation is required. For fanless
operation and/or high V(In) de-rate the combined output of the
3.3, 5V and 12V rails by ~20-40%. Peak load should not exceed 60 seconds."
There is no mention in the manual, of "undervoltage cutoff", which is
desired when used with a car battery. So the unit in question,
may still be inappropriate for usage with a lead-acid battery.
If it discharges the battery too low, it reduces the battery life.
(Car batteries don't really like to be discharged too low.)
*******
You also have the choice, of using a 12V inverter, boost to 115VAC at
a couple hundred watt rating, then connect a regular ATX supply (preferably
one without power factor correction PFC). That might be a more normal
route to doing something like that. The advantage of using a 12V lead-acid
battery inverter, is it's designed for the application, and it will shut off
the output when the lead acid battery voltage drops too low. If you don't
have such protection, the lead acid battery won't last very long at all.
You'll get very few charge-discharge cycles if you insist on "deep discharging"
a car battery. With a lead-acid car battery, you might only want to use a fifth
of the amp-hours, before recharging. Do shallow discharges, and use
a larger capacity battery, to help compensate for the lack of deep
discharge. They make lead-acid batteries in deep discharge format, but
they're more expensive, and may not be available at your local retail.
You can also use other battery types, but they still need protection
against discharging below a safe level. I think I've seen Lithium systems,
where the package includes builtin charger and discharge protection, so
you just use the thing, and when it wants to protect itself, your power
goes off. You don't have to use car batteries. There are other solutions,
but they're going to be more expensive. Car batteries are for situations
where you don't care about the weight. (Like, they're just sitting
in the back of your pickup truck.) Every battery types, benefits from
an intelligent charger design. A lot of the leaking batteries you find,
end up that way because the charger for the battery was a piece of crap.
(Think "cordless screwdriver" if you need an example of this. Stupid
things are designed to ruin their batteries.)
Paul
