[Soekris] long-run DC UPS

[Jonathon Exley]

> -----Original Message-----
> From: soekris-tech-bounces at lists.soekris.com 
> [mailto:soekris-tech-bounces at lists.soekris.com] On Behalf Of 
> Mark Phillips
> Sent: Tuesday, 14 August 2007 12:36 p.m.
> To: soekris-tech at lists.soekris.com
> Subject: Re: [Soekris] long-run DC UPS
> 
> Surely it's a simple matter of mathematics and then you'll 
> end up with the size you want?
> 
> For example; if the device draws 10 watts an hour then a 
> standard 650va ups would last 6.5 hours. So it follows that a 
> 3Kva ups would last 30 hours.
> 
> What we haven't allowed for is the UPS' own current 
> requirements. I would make a stab at 3Kva lasting about 24+ 
> hours with a 10W load on it.
> You might get longer with a different type of UPS.
> 
> For example, you could build a charger/battery solution 
> whereby a charger keeps a car/sealed lead acid/gel cell 
> charged and in turn the
> 5501 is connected to the battery. When the volts die the 
> battery takes over and runs for as long as it runs for based 
> on it's size.
> 
> If you follow the P=IV rule should be good. A standard 75amp 
> car battery should last you about 100 hours with a 10W draw.

The kVA rating of a UPS refers to how much power it can supply, not the
battery capacity.
The approach is right - have a rectifier supplying a car or motorbike
battery so that when mains dies, the battery cuts in.
You may also want a more intellegent device that tests the battery
capacity from time to time to make sure it's still ok.
Check out http://www.innovative.co.nz/nobreak-dc-overview.asp for
something that does this.