[Nut-upsuser] UPS Battery internal resistance matters.

Arjen de Korte nut+users at de-korte.org
Tue Dec 11 08:29:39 UTC 2007

> For any who don't know:
>    Voltage is an insufficient indicator of { health, time to support
>    load, of if batteries will even support the load at all } . Internal
>    resistance is important to calculate, but imperils load while
>    testing to calculate (***).

Thanks for the reminder.

> I mention it 'cos ~2 years ago when I was using NUT, I didn't notice
> much if anything about internal resistance, but its's important.
> & people I suspect often think about just battery voltage/ charge
> as affecting performance / load time.
> I have an APC, & another brand, & another same APC as Charlie, &
> have an electrical engineering background & replaced batteries in
> various UPS, & took measurements. Below is based on APC 2200 &
> 2000, but similar principles apply generaly.
> Maybe some UPS _might_ measure both open circuit voltages & voltages
> under load at test times, & current, to calculate state of batteries,
> I dont know, but I suspect UPmany/most UPS may go for a simpler
> voltage measurement, perhaps mapped through a hysteresis curve,
> then fed to NUT as some percentage figure.

Good practice is to periodically test a UPS under load. This should be
done regularly (about once every week to month) for a brief moment to
check if the UPS is capable to provide enough power to sustain the load.
To limit wear on the batteries, this test shouldn't last longer than a
couple of seconds. Another thing is, to test the amount of charge the
batteries hold. This can only be done through a complete discharge cycle
under load (often referred to as deep discharge test), which should not
take place more often than about two to four times a year.

As you also indicate, there is always the risk that the tests result in a
failure and that the load is switched off immediately. Therefor (and
especially for the deep discharge test) you want to do this at a moment
that is convenient for you. You definitly don't want to be surprized by
this through an automated test, which might hit you at the worst possible
moment. Generally (in larger installations) a maintenance window will be
created/used to perform the deep discharge test and the short test will be
run off hours. I doubt that we will ever automate this, simply because
there is no way for us to determine when is a good time to run these

Best regards, Arjen

PS  If your grid is sufficiently poor, these tests may be performed for
you by the utility company and running them yourself might not be needed
at all... :-)

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