[time-nuts] UPS for my time rack

[Bob Benward]

Dave, 
You could use a 120V relay and switch the high capacity battery from its own
charger to the battery pack in the UPS.  When power comes back, the relay
automatically switches the battery out and back to its own charger.

Bob

>>> -----Original Message-----
>>> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Dr.
>>> David Kirkby (Kirkby Microwave Ltd)
>>> Sent: Saturday, October 10, 2015 6:07 PM
>>> To: Discussion of precise time and frequency measurement
>>> Subject: Re: [time-nuts] UPS for my time rack
>>> 
>>> On 10 October 2015 at 14:20, Chris Waldrup <kd4pbj at gmail.com> wrote:
>>> 
>>> > Hi,
>>> >
>>> >
>>> > I have decided I'd like to get a UPS to put on the rack containing my
>>> > Thunderbolt, the laptop that runs Lady Heather, and frequency counter.
>>> >
>>> 
>>> There's one issue with them that I don't see anyone mention.
>>> 
>>> I was thinking of doing the same a while back, and intended getting a
UPS
>>> and adding a large external battery pack, so if the mains failed late at
night, I
>>> could run the GPS receiver and a few other things overnight, and
consider
>>> starting the generator in the morning.  I contacted a dealer on eBay,
who
>>> specilaises in UPSs. He told me that the smaller units with built in
batteries
>>> will die if you put large external batteries on them.
>>> 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've had two here which were HP/Compaq 5 kW units. These were different
>>> to the normal, in that the batteries added up to over 300 V, so could
produce
>>> 240 VAC with no need to step it up. Both these blew up on me, for
reasons I
>>> never worked out. The load was never anywhere near 5 kW.
>>> 
>>> Lots of people mention sine wave. Of course, if you keen enough, you
could
>>> make a class A amplifier and sine wave oscillator. The problem is that
the
>>> pure sine wave inverters tend to be very inefficient.
>>> 
>>> As with most things, there are a lot of things to balance - runtime,
cost,
>>> quality of output, audio noise, RFI  etc etc.
>>> 
>>> Dave
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