J. Forster wrote:
When considering a battery backup you really have two ways to go:
Battery --> Inverter --> Load
Battery ---> Load
The first option is generally easy to implement, but a lot of the battery
capacity goes up as heat in the inverter. As others have pointed out,
inverters are not very efficient or electrically quiet, and they sop up
quite a bit of power w/o any load at all. For this reason, the second
option is better, IMO.
In a lot of cases, you'd be stepping up from the 12V, 24V or 48V the
batteries provide, then the PSU in whatever you're running would be
stepping that back down to 5V, 12V or a mix of voltages.
I've built a (relatively) low-power PC server out of a Mini-ITX
motherboard (Jetway NC92-230), two hard drives (a Seagate 'cuda 7200.10
and a 7200.11, both 500GB), 2GB of RAM... and a 120W PicoPSU. The
PicoPSU plugs straight into the ATX socket, takes 12V in, then converts
it to the +5V, +12V, -5V and -12V the motherboard needs.
Efficiency is supposed to be around 96%, but I haven't done any
measurements. As a bonus, it doesn't need any form of active cooling
(read: cooling fans) to keep it cool, so the whole machine is pretty
quiet. Well, aside from the Seagate Barracuda hard drives, which seem to
be designed to make as much noise as possible.
My plan is to get a couple of lead-acid batteries and rig up a
battery-backed AC-to-DC supply to run the server, external hard drive,
DSL modem and WiFi access point. That's still on the drawing board,
though -- the power to this menagerie has only been interrupted once
since it was installed, and that was down to a lightbulb blowing and
tripping the RCD...
As for the desktop... I'll probably end up buying a Back-UPS or
something along those lines. I figure I only need 5 minutes to hit
"Shutdown ==> Suspend to disc", and the laptops will run for a couple of
hours without AC power...
There are even purpose built battery backup supplies available for running
oscillators, receivers, and such. Sadly, the NiCds tend to be bad and are
quite expensive to replace.
In truth, most of the NiCd and NiMH packs I've used have been borderline
useless. The Varta Mempac memory backup cells are OK, but most of the AA
or AAA "off the shelf" cell above about 1600mAh have been dire.
The Sanyo Eneloops are pretty nice though. A bit more like the alkaline
AAs of old (as in, they don't go flat if you leave them for a week or
two), but rechargeable.
You can use diodes
to make an OR circuitto switch automatically between DC supplies when the
line fails.
But watch out for the voltage drop, and use suitably sized diodes.
A silicon diode drops about 0.7V (though big rectifiers are usually
closer to 1V). If you have a supply with 2A going through it, a diode
dropping 0.7V will be dissipating:
P = I * V
= 2 * 0.7
which works out at 1.4 Watts
At this point, you'll probably want something a little bigger than a
1N4001...
--
Phil.
[email protected]
http://www.philpem.me.uk/
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