[Nut-upsuser] UPS Battery internal resistance matters.

Julian H. Stacey jhs at berklix.org
Thu Dec 13 17:37:23 UTC 2007

"Arjen de Korte" wrote:
> > 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.

Suggestion: we know, but not all NUT package users, especially those
not on mail list may, so if something's not yet in NUT docs about
internal resistance & load checking, Something could be added ?

> > 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
> tests.

Yes, some of that could go in a NUT doc (if not already?)

What suprised me was this table entry from APC ASTE-6YWRZE_R0_EN.pdf 
	Smart-UPS 2200/3000 VA 100/120/230 VAC Tower User Manual Page 9
] Function:		Automatic Self-Test
] Factory Default:	Every 14 days (336 hours)
] User Selectable Choices: Every 7 days (168 hours);
] 			 On start up only;
] 			 No self-test
] Description:	Set the interval at which the UPS will execute a self-test.

APC are less conservative than we are !
I agree NUT should not do self test as shipped by default.
I think NUT by default should change the APC default,
	(I'll look later to see what NUT does, as I'm new back to NUT).
I agree UPS users should do periodic battery tests, at times they
peronally decide appropriate for their installation.  
   However, rather than doing it by front button, 
	( which merely shows pretty but misleading voltage LEDS, &
	one may not get scared enough by eg my button test just
	now, where one 60 Watt light bulb after a few seconds self
	test took the batteries down drom 5 LEDs to 3 LEDs ...)

   Instead NUT could offer a template script, callable by human or
   crontab, which can measure Voltage before during & after And
   load, & thus resistance.  If NUT doesn't already include this
   I'll see what I can do & send ant diffs later.

Sorry for the IFs, I'm new back to NUT, & doing
UPS hardware before grabbing latest NUT sources.

PS minor point Re my:
>    ( On the APC Smart 2200 there's 2 connections between battery
>      level & UPS level, so maybe APC can measure 2 x 24 V (I'll
>      unscrew case again later to see wiring ) or maybe it too is
>      just 1 serial measurement of 48V. )
The 2 x 2 Batteries sets are in series, no intermediate cable, so
can not eitherrun UPS for half time on one set while changing other
set, or measure 2 resistances, only total.

Julian Stacey. Munich Computer Consultant, BSD Unix C Linux. http://berklix.com
	Ihr Rauch = mein allergischer Kopfschmerz. Dump cigs 4 snuff.

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