Rockwell Automation 1757-BEM ProcessLogix Battery Extension Module Installation Instructions - Manual (Page 8)

8 ProcessLogix Battery Extension Module
Publication 1757-IN093B-EN-P - April 2004
Recommended Configuration
There are no restrictions on the number of 1757-BEMs that may reside in a given chassis.
This gives the user an option for increasing backup capacity. Also, coupled with RIUP, this
facilitates continued backup protection in the event a 1757-BEM must be removed from the
rack. The recommended configuration is to have one BEM unit for each module requiring
memory backup power. This ensures that enough battery capacity is available for extended
periods of time, such as a four day Thanksgiving weekend.
Battery voltage is unregulated on the 1757-BEM. Sink modules must provide surge limiting to
prevent voltage dips if they are installed with the controller power off - i.e., while the battery
pack is supplying power. Sink modules should also be prepared to withstand voltage dips
potentially caused by other modules. Battery power is protected on the 1757-BEM against
damaging current drain rates.
Switchover Circuitry
There is no switchover circuitry on the 1757-BEM. Switchover to battery power is
accomplished on the sink module(s). The switchover circuitry must be designed so that it
does not impose a surge on the battery pack when the sink modules are installed under power
(i.e., it must recognize chassis power before battery power) to protect against nuisance “low
battery” alarms.
Understanding the Electrical Design
Charge Control
The battery charging circuit is a quasi-constant current, trickle charge design. Constant
current is the preferred method for charging NiCd batteries. Quasi-constant current is a
simplified variant. It is accomplished by making the charging source voltage much greater
than the battery voltage and limiting the current flow with a series resistance. The series
resistance value is large compared to battery internal resistance (usually a fraction of an ohm),
hence making the effect of battery voltage changes less significant to circuit operation. The
current becomes relatively constant with respect to small battery voltage changes (as the
battery charges) and source voltage variation.
In trickle charge, the battery is continuously charged at a very low rate. A NiCd cell, when left
open circuit for long periods of time, will gradually lose charge or capacity. A trickle charge
replenishes these losses (and recharges a spent battery). The trickle current for the BEM is
40mA. This provides for a 1/30 capacity charge rate for 1200mAh cells.
Charger power is drawn from the +24 VDC main supply. The battery pack is connected to
the charging power supply with the load in parallel. Normally, power flows from the DC
source to the load, but when power is interrupted from the main source, power is discharged
from the battery. A blocking diode in series with the charging source prevents back-flow of
battery current into the +24 VDC plane.