On Thu, 2010-01-07 at 10:18 +0000, Wayne Stallwood wrote:
On some makes (even more so with cheaper ones) a completely dead battery will cause the UPS to go into a state were you can't power it up, even some APC's suffer from this, although most modern APC units test the batteries even when on a reliable supply and therefore should have been telling you the batteries were shot for ages.
Interesting.
A basic design for a UPS has a converter that converts mains voltage AC to the correct DC voltage to keep the batteries charged and an inverter that converts that DC voltage back to mains voltage AC, either all the time or when the mains fails[1].
In a modern unit I would expect both the converters to be switch mode designs and, from what I remember, one of the difficulties of switch mode PSU design is that one generally wants the control electronics powered from the low voltage DC which isn't available until the unit has started up thus necessitating some extra components for a "startup mode". My guess is that in the case of those cheap UPSs they miss out the extra components and use the battery to power the control electronics until the input SMPS has got going in which case if the battery is dead it won't start.
Reading some articles I found on Google it seems the batteries are generally regarded as the weakest link and, if the point at which one finds the UPS is broken is when it fails to take over when the mains fails this makes this even more likely.
Steve.
[1] There are also some delta conversion units where the inverter can be used to top-up the mains rather than completely replace it.