A> A few weeks ago I chucked around the idea of backing up the
A> power to a friends Linux web-server box (he lives in
A> Zululand where power cuts are quite common). We did some
A> back-of-the-virtual-envelope calculations for a mains
A> charger -> 24Vbattery -> DC/DCconverter -> straight into the
A> system:
See my earlier comments. I especially don't see the need for a full-blown
DC/DC converter when a simple regulator will suffice. Converters are handy
when stepping up in voltage, but that's not the issue when you are starting
with a 24V supply.
A> For any serious up-time you need serious batteries (this
A> applies to the commercial UPSes too of course - some of the
A> £100 types only give you 5 minutes). By hoping that all
A> sorts of important factors will cancel out we might hope to
A> run a 200W PC for an hour or so on a pair of 12Ah batteries
A> (12Ah/(200W/24V)*75% eff = 1hr). My catalogue has sealed
A> lead acid gel types for about £36 a peice. Car batteries
A> weren't really an option because they had to live in his
A> house with him and his kids.
This makes a fundamental error in assuming that the "200W" power supply in a PC
uses anything approaching 200W. In general, you can expect a motherboard to
consume about 10W at most, and then only when loaded up with a fairly fast CPU
and lots of RAM. Sure, there are horrible exceptions such as the Cyrix 6x86
that eats something absurd like 27W all by itself, but obviously you don't use
that. Even the largest and most massive of the old full-height hard drives
take about 1.5A at 12V once started, although they can draw as much as 4A at
12V to start.
A good rule of thumb is that a basic PC using a reasonable CPU, no more than 32
MB RAM, and a relatively modern hard drive would consume about 20W at most
without the monitor. If the PC has power-saving features such that it supports
Advanced Power Management or can spin down the hard drive, you could probably
keep running on a quarter of that. Also remember that CMOS devices, including
the CPU, will consume power linearly proportional to their clock rate, so
simply popping the machine out of turbo mode might give you hours more run time
if you have a hungry CPU.
-- Mike