[Soekris] long-run DC UPS

[Richard Gooch]

Mark Phillips writes:
> 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.

Um, no, a 650 VA UPS could last longer than a 3 kVA UPS. The "VA"
rating just tells you what the maximum load it can sustain, it doesn't
tell you how long it can sustain that load. It's possible (though
unlikely) to have a 650 VA UPS with a monster battery and a 3 kVA UPS
with a tiny hand-held lead-acid battery. The VA rating is just a
pseudo-power rating.

You often have to look hard to figure out what the energy rating is.
Manufactures are often shy telling you that. They'll quote bogus "20
minutes!!**!! run time" for a "standard PC" (that could be a 486 with
a 14" LCD). Many "20 minute" UPS's will only run for 5 minutes at full
load.

> 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.

Er, you want to follow the E=P*T (Energy = Power * Time) rule.
Most car batteries can deliver hundreds of amps (for a short time),
but their energy rating is typically 50 to 75 AH (amp hours, i.e. 75
amps for 1 hour).

Since a car battery delivers ~12 V, a 75 AH battery has ~900 WH (watt
hours) capacity (3.24 MJ (mega joules)). A 10 W load would last for
~90 hours.

</pedant>

				Regards,

					Richard....