[time-nuts] UPS for my time rack

[Bob Camp]

Hi

Unless you live in an unusual location, long term power outages are going to be pretty 
rare. At the house I’m now in, we had a high voltage feed that was on it’s last legs. We
had short outages on a “many times a week” basis if the wind was blowing at all. We had
rare outages in the > 5 minute range. The short / frequent blip stuff is what most light weight UPS’s
are designed to handle. Not everybody has this problem. I no longer have it, they 
ripped out 10 miles of old feeder and the new one works fine. 

Indeed there are locations that experience multi hour outages on a fairly regular basis. 
The combination of bars closing late on Saturday and a long straight road with an abrupt
turn in it was particularly hard on a feed line I once had to cope with. In that case gas turbine
generators were the answer. 

If you have a case where long outages are common, rotary machines are often the better 
answer than batteries. In the case above, the power company was the one footing the bill
for the gear. Fair in this case since they were the ones that *could* have moved the line. 

If a > 10 minute outage is a “less than once a year” sort of thing, and OCXO’s are your only concern, 
let them shut down. The net impact to your lab will be relatively small. The cost to fix the problem will
be relatively large. Short blips often, are well worth fixing. 

The hidden issue with running a UPS is the relatively short life of the batteries.
Sealed lead acid is low cost up front, but they simply do not last when charged the way
a typical UPS charges them. Before you go into “can’t be true” mode … plug a 100W light 
bulb  load into your UPS and see how long it runs. Battery still *shows* as good on the indicator.
Gizmo only runs for 1/4 the time it should … hmmmm …. It’s very common to go into these
projects with a  reasonable budget, and then find out that the budget to keep it going is 
not quite so generous.

Bob



> On Oct 11, 2015, at 4:57 AM, Kasper Pedersen <time-nuts at kasperkp.dk> wrote:
> 
> On 10/11/2015 12:07 AM, Dr. David Kirkby (Kirkby Microwave Ltd) wrote:
>> Essentially the charging circuits are not designed to run as long as needed
>> to charge big batteries. Even on ones designed for external batteries,
>> there's a recommended limit on the size of them. So if you think you might
>> want to increase runtime by adding some batteries, buy one designed for
>> that service.
> 
> I have gone down that route, so I have some real data to share:
> 
> My (soon to be replaced) backup is an old back-ups CS 500, with a
> rewired battery pack out of an RT3000 UPS. So instead of 7Ah, the UPS
> has 40Ah. With plenty of fuses.
> 
> When charging the standard 7Ah battery, the UPS delivers about 0.7A
> (from memory) for many hours, and sits at about 14C above ambient.
> 
> When charging the 40Ah, the current is the same, the temperature is the
> same, just for longer, as it should be, since the thermal time constant
> is much shorter than the time it takes to charge the 7Ah.
> 
> Where this has problems is during discharge:
> I have about 55W load on it, which in turn is at least 5A on the
> battery. After 2 hours a timer in the UPS shuts it off, regardless of
> battery voltage.
> Also, if you run the UPS at high load where the standard battery lasts
> shorter than the thermal time constant, then there might well be trouble.
> 
> 
> 
> The replacement, a back-ups pro 1500 behaves differently.
> It has support for external battery packs, and will happily run for at
> least 5 hours, even when the external-pack-present signal is not connected.
> The external-pack-present signal does make a difference when charging;
> Without it, it charges at 0.7A. with it, it charges at 1.5A, and the fan
> is on continuously.
> 
> 
> /Kasper Pedersen
> 
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