[time-nuts] UPS for my time rack

Kasper Pedersen time-nuts at kasperkp.dk
Sun Oct 11 04:57:04 EDT 2015

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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