Subject: UPS Batteries Internal Resistance
From: UPS@
Sender: UPS@

Q:
UPS Battery charge did not go below 70% when the box died after
about 5 minutes. No shutdown scripts, no warnings of shutdown,
nothing.  one might suspect a bug somewhere.

A:
Start with basics: Suspect the battery. Even the microprocessor
inside the UPS that NUT talks to, needs good power, if the battery
suddenly drops, the microprocessor may not have time to warn NUT
of an approaching or actual failure, & NUT might not have time or power.

As I wrote to someone else recently on this list: A battery can
appear to have a good charge state (= voltage) even when it's gone
bad & developed a high internal resistance.  That happened to me a
while back when I wasn't even running any software to control UPS.
UPS had been running for years, with charge state good (LEDs). I
pulled power, & it dropped out after maybe 10 secs ! 2 of the 4
batteries had good voltages but high internal resistance.

Test each of your batteries' internal resistances, run each battery
separately into eg a few car 12V headlamp light bulbs, Use Ohms
law: V = IR.  Measure 
  Voc = Open Circuit no load Voltage (about 13 or 14)
  I   = Current through bulb[s]	(as many as you can in parallel)
  Vload = While bulb burns bright, measure voltage (but at the battery not
	 at the bulb, to avoid voltage drops in thin wire) 
Rinternal = (Voc - Vload) / I 

My really bad batteries had 2 & 3 ohms.  The less the better, I've
not measure a new one, but I guess not more than a few tenths of
an ohm (& a lot less than that for eg car starter batteries).
(Battery manufacturers have copious info in PDF on web).

Also calculate capacity in Watt Hours, by running batteries some
hours till they grow dim. (You'll need to log current each half
hour, as it varies, as batt voltage drops, as resistance of light
bulbs depends on brightness.