Factors Affecting Battery Life; Low Battery Warning Methods - GE 90-30 PLC Series Installation And Hardware Manual

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Factors Affecting Battery Life

Replacing your battery once per year is a good rule of thumb. However, no one can predict
precisely how long a backup battery will last because this depends upon what CPU is used, what
temperature it is subjected to, and how it used. Considering the following list of factors that affect
battery life will help you decide how frequently to replace the battery in your application:

Low Battery Warning Methods

There are three basic ways that the PLC warns of a low battery:
6-4 Series 90-30 PLC Installation and Hardware Manual – August 2002
A battery that is not in use has an estimated life (called its "shelf life") of 5 years at "room
temperature" (25 degrees C, or 77 degrees F).
A battery that is used continuously (supplying current to memory circuits with PLC power off)
if used at room temperature has an estimated average life as follows:
Model
CPU models 311, 313 and 323
CPU models 331—364
CPU374
As long as a PLC is powered up, its battery is not being used; so how often you power down
your PLC has a direct affect on battery life. Some users keep their PLC powered up all of the
time while others turn theirs off every night.
Temperature has a relatively large affect on battery life. Temperatures considerably above
room temperature (25 degrees C, or 77 degrees F), or below freezing (0 degrees C, or 32
degrees F) will appreciably shorten battery life.
The type of CPU has a small affect on battery life. Some CPUs have more memory than
others. Some memory types require more power. Also, some CPUs have a clock and some do
not. More memory requires more battery current to maintain its contents; and a clock requires
battery current to maintain its operation.
The red "BATT" LED on the Power Supply module lights when the battery is low. The
disadvantage of this method is that the PLC is often mounted in an enclosure, so this LED
might not be easily seen.
The PLC Fault Table is updated with a battery low message. Viewing the PLC Fault Table
requires that a programmer be connected to the PLC.
Certain System Reference bits are set to logic 1 when the battery is low. These are %SA011
(LOW_BAT), %SC009 (ANY_FLT), %S010 (SY_FLT), and %SC012 (SY_PRES). The most
specific is %SA011 (LOW_BAT). This bit could be used as a contact in your ladder logic
program to turn on an output that controls a warning light on an operator panel (as in the
example rung below), or to send a warning to an operator interface terminal.
%SA011
Estimated Average Life
at Room Temperature
2 years
1 year
1.2 months
%Qxxx
GFK-0356Q