Hi Folks,
I thought I'd push out for comment a thoguht which I've been pondering from time to time.
A laptop is pretty nearly immune to mains supply failure, since its battery provides a backup and modern laptops can shutdown on low battery.
Desktops, on the other hand, need an external UPS to guard it against mains failure; and of course a UPS can be configured to signal the computer to shut down if the power cut lasts more than a certain time.
But it has occurred to me that there's no obvious reason why a desktop can't have its own internal battery like a laptop, and I'm wonderign why they don't.
Of course, laptop batteries are notorious for not having a very long useful life -- you can be lucky, in some cases, if you get 5 years out of one. And they're even more notorious for being horrendously expensive to replace, if indeed you can still find one by the time yours fails.
But I'd have thought that the space available in a desktop would offer scope for a more robust and cheaper solution. (In passing -- I have an ancient laptop whose battery has been shot for years, which however works well if you plug 12v DC into its PS socket; so I've happily been using it in "away from the mains" situations by attaching it to a motorcycle battery which weighs less than 4 pounds -- 1.7kgm -- so is within my definition of "portable").
So a robust lead-acid battery of the appropriate voltage could be installed into a desktop, if the appropriate circuitry were present in the box's PSU, and would presumably last longer, be easier to replace if necessary, and cost much less.
Does anyone know of desktop boxes which do this?
Thanks, and best wishes to all, Ted.
-------------------------------------------------------------------- E-Mail: (Ted Harding) Ted.Harding@nessie.mcc.ac.uk Fax-to-email: +44 (0)870 094 0861 Date: 23-Jun-06 Time: 15:18:44 ------------------------------ XFMail ------------------------------
On Fri, 2006-06-23 at 15:18 +0100, Ted.Harding@nessie.mcc.ac.uk wrote:
So a robust lead-acid battery of the appropriate voltage could be installed into a desktop, if the appropriate circuitry were present in the box's PSU, and would presumably last longer, be easier to replace if necessary, and cost much less.
Does anyone know of desktop boxes which do this?
You used to be able to buy a replacement ATX PSU for desktops that had a battery module (that also fitted in the desktop case) it gave a limited supply (from memory about 5-10 minutes) in order for the machine to shut down.
It simply simulated a power button press on the plug on the MB front panel header (which for most modern ACPI aware operating systems causes them to start to shutdown)
But I haven't seen such a thing for many years (the last place I saw one for sale was in a bargain bin at Maplin).
The trouble is that with modern desktop machines the amount of current you would need from a 12 supply would require a fairly large battery for a reasonable runtime.
Back of fag packet calculations show that for my AMD64 given a DC-DC converter efficiency of say 85% (probably not far out for a consumer grade unit) I would need to draw 25A from a 12v supply. Which means that my laptop battery (10.8v at 2.5AH) would power my system for about 4 minutes...of course I am guessing that my system runs at an average of about 200-250 watts...which is pure guesswork...it may be lower, I've never measured it.
and of course the other limitation is that this PSU would only supply the PC itself...not any peripherals and would make the PC bulkier and more expensive...that being the case I think an external UPS makes more sense.
Building a DC-DC converter that would supply the different ATX voltages would be a more expensive beast than the switchmode PSU is in a PC anyway..and would be more complicated that the Inverter used in a UPS (and not necessarily that much more efficient)...you'd get a saving over a UPS because there would only be one stage of conversion not DC to AC and then AC to ATX voltages...12V 3.3V 5V and the - supplies (if they are all still needed..I know some of them were for legacy stuff like ISA slots)
You can't simply add the 12V battery supply to halfway through the existing switchmode circuit because SMPSU's don't work that way...you'd either have to completely redesign the SMPSU (so that in fact it wasn't an SMPSU but a high current single (switchmode derived) supply feeding multiple DC-DC converters or have two supplies (one a traditional SMPSU with an extra DC feed for charging) and the other being the DC-DC converter for battery supply and then add some clever fast switching logic
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Ted.Harding＠nessie.mcc.ac.uk -
Wayne Stallwood