I agree. How about, using a 12V and a 6V battery in series, giving 18V
in total. That would still be enough to step down to 12V regulated,
without wasting quite as much power.
> 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
I agree with all this. The box I would be putting this on only consumes
13W, so I can't see any point designing one that can supply 200W, as
long as I know its limitations, and don't try to power ten full height 5
1/4" MFM hard drives from it ;)
-- --------------- Linux- the choice of a GNU generation. -------------- : Alex Holden (M1CJD)- Caver, Programmer, Land Rover nut, Radio Ham : ---------- http://www.geocities.com/CapeCanaveral/Lab/1532/ ---------