explains how to calculate the battery run time you should expect for
a specific brand and model of UPS Battery.
For the purpose of this exercise
there are some items that we need to know including the UPS inverter
efficiency, allowing us to calculate the kWb, the DC endpoint or cut-off
voltage and finally the average room ambient temperature therefore we
will make certain assumptions.You
can easily change these assumptions to suit your application.
Let’s make the following
-Inverter efficiency 90%.(Industry norm is 90%-98%)
-End point voltage 1.67.(Typical reference used
in battery specifications)
-Ambient Room Temp 77F.(Typical reference used in battery
A UPS system has one string of
batteries, consisting of a quantity of 30 each, Northstar Model NSB12-475 and
the rating of the UPS is 40kW.The
battery has 6 cells per unit.
Calculate the Actual Battery Load for a 40kW UPS Load
40kw load assuming a 90% inverter efficiency is
actually 44.44kw battery load
ups load) DIVIDED BY .90 (90% inverter efficiency) = 44.44kw (actual
load on the batteries)
Calculate Watts Per Battery
Each battery will see 1481 watts
load on the batteries) DIVIDED
BY 30 batteries = 1481 watts per battery.
Calculate watts per Cell
Each cell of each battery will see 247 watts
1481 watts per battery DIVIDED BY 6 (cells per unit)
= 247 watts
Estimate Your Expected Run-Time
The specifications for this battery show that at 247
Watts per cell and 1.67V your run time is between 45 and 60 minutes.