Battery saving switching mechanism

An intelligent switching mechanism for the backup batteries of an alarm system is disclosed. When AC input power has failed but no alarm has been declared, the batteries are switched into a parallel configuration, which is sufficient to supply the control panel, so that the batteries are drained in tandem. When AC input power has failed and an alarm has been declared, the batteries are switched into a series configuration to supply power at the higher voltage required by the alarm annunciators.

TECHNICAL FIELD 
The present invention is a battery saving switching mechanism for 
implementation in an electrical system having voltage requirements that 
vary with system state. 
BACKGROUND OF THE INVENTION 
The present invention finds its most immediate application in a fire alarm 
system having many sensors and alarm annunciators, such as bells or 
sirens, communicating with a fire alarm control panel. In most countries, 
including the United States, such a system is required to have backup 
batteries that can provide electrical power for a specified time span when 
the regular supply of electrical power (AC input power) fails. 
In a fire alarm system it is typical for the logic circuitry of the control 
panel to operate at a relatively low voltage, for example 5 VDC or 12 VDC, 
and for the alarm annunciators to operate at a higher voltage, for example 
24 VDC. The annunciators operate at a higher voltage because in a large, 
centralized alarm system, an annunciator may be 100 yards or more away 
from the control panel. It is, therefore, desirable to use annunciators 
that require a relatively high voltage and consequently draw less current 
to permit the use of thinner, less expensive wires. 
Generally, the backup batteries consist of a pair of batteries connected in 
series across the fire alarm control panel. The lower voltage needed for 
the control panel logic circuitry is derived by a voltage regulator or by 
voltage dividers. These devices tend to be quite inefficient, wasting a 
large portion of the energy delivered to the control panel. 
The backup batteries should have sufficient energy storage capacity to 
power the system when it is in a nonalarm state for a specified time span. 
They should also have sufficient energy storage capacity to power the 
system when it is in an alarm state for an additional, typically much 
shorter, specified time span. The energy storage requirements of the 
backup batteries may be determined as follows: 
Total backup battery energy storage requirement=T.sub.cp *I.sub.cp 
*V.sub.cp +T.sub.a *I.sub.a *V.sub.a 
where: 
T.sub.cp =battery backup time span requirement for control panel 
I.sub.cp =current draw of control panel 
V.sub.cp =voltage required by control panel 
T.sub.a =battery backup time span requirement for annunciator 
I.sub.a =current draw of annunciator 
V.sub.a =voltage required by annunciator. 
It is typical for T.sub.cp =24 hours, I.sub.cp =175 milliamps, V.sub.cp =24 
volts, T.sub.a =0.167 hours, I.sub.a =6 amps and V.sub.a =24 volts, 
yielding a charge storage requirement for each of a pair of 12 volt 
batteries connected in series to equal 5.2 amp hours. 
SUMMARY OF THE INVENTION 
The present invention is a backup battery saver switching mechanism for use 
in an electrical system (such as a fire alarm system) drawing AC input 
power and having first and second subsystems. The first subsystem (such as 
a control panel) is designed to require a first voltage; and the second 
subsystem (such as a set of alarm annunciators) may be in an "on" state 
(when an alarm has been declared), requiring a second input voltage that 
is greater than the first input voltage, or be in an "off state," 
requiring no input voltage. When the second subsystem is in its off state 
and the AC input power has failed, the switching mechanism configures the 
batteries in parallel so that they jointly provide power at the first 
input voltage. Otherwise, the switching mechanism configures the batteries 
in series to jointly provide power at the first input voltage and to 
accept a battery charging input, also at the first input voltage. A first 
one of the batteries is configured so that it continues to supply power at 
the first voltage to the first subsystem when the second subsystem is in 
its on state and AC input power has failed. 
If the present invention were implemented in the system postulated in the 
"Background of the Invention" portion of this application the batteries 
would be connected in parallel when there was no alarm condition and AC 
input power had failed. If, as is typical, I.sub.cp does not increase as a 
result of the implementation of the invention, the two batteries would 
require at most 3.1 amp hours of charge storage capacity. This reduction 
in charge storage requirement results in the ability to use 4 amp hour 
batteries rather than 7 amp hour batteries. This reduction of charge 
storage requirement produces a savings of about $10.00 at typical 1997 
battery prices. Similarly, where there is a 60 hour requirement or for 
larger systems requiring more zones the monetary savings would typically 
amount to about $30.00 to $45.00. 
Additional objects and advantages of this invention will be apparent from 
the following detailed description of a preferred embodiment thereof which 
proceeds with reference to the accompanying drawing.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 shows a fire alarm system 10 having a 12 VDC first backup battery 12 
and a 12 VDC second backup battery 14. Batteries 12 and 14 are dynamically 
configured by a switching mechanism 16 that is constructed in accordance 
with the present invention. When system 10 is in not in an alarm state and 
when the AC input power is unavailable, switching mechanism 16 configures 
batteries 12 and 14 in parallel so that they are both drained equally by 
alarm control system 10, which requires 12 VDC. Otherwise, switching 
mechanism 16 configures batteries 12 and 14 in series. While the usual 
supply of power is available, the series configuration allows for the 
convenient charging of batteries 12 and 14. When the usual supply of power 
is not available and system 10 is in an alarm condition, the series 
configuration is necessary to supply power to a set of alarm annunciators 
18, which require 24 VDC and which are switched on in response to an alarm 
condition by a set of alarm annunciator switches 20. 
A power supply 22 of system 10 receives AC input power. Power supply 22 
produces 12 VDC, 24 VDC, and 27.6 VDC from this source. System 10 may 
include a separate printed circuit board for power supply 22. 
A power good sense block 24 issues a "power not good" output when the usual 
supply of power is inadequate. An alarm condition logic block 26 examines 
a set of sensor signals received from a suite of external sensors (not 
shown) and emits a "no alarm" signal when indicated by all of the sensor 
signals by sending the "no alarm" output of block 26 high. A two-input AND 
gate G1 comprises a logic circuit for designating the system state of 
alarm system 10. The inputs of AND gate G1 are electrically connected with 
the "power not good" output of block 24 and the "no alarm" output of block 
26. A "power not good" signal and a "no alarm" signal drive the output of 
AND gate G1 to its high state, which commands a transistor Q1 to its "on" 
state. If the alarm output of block 26 is low, this constitutes an alarm 
signal, which drives gate G1 low. 
In this state, Q1 drives in tandem a first relay 30 to contact its "b" pole 
and a second relay 32 to its closed state, thereby placing batteries 12 
and 14 in a parallel configuration. Whenever there is either an alarm 
signal or a power good signal, the output of AND gate G1 goes low driving 
transistor Q1 to its "off" state, thereby causing first relay 30 to 
contact its "a" pole and second relay 32 to open. This places batteries 12 
and 14 in a series configuration. When batteries 12 and 14 are in this 
configuration and AC power is unavailable, first battery 12 provides 12 
VDC to power a 5 VDC power supply 34 that powers alarm condition logic 
block 26. 
A diode D2 prevents battery 12 from draining power from the 12 VDC output 
of power supply 22 during normal operation, and a diode D4 prevents 
batteries 12 and 14 from draining power from the 24 VDC output of power 
supply 22 during normal operation. Batteries 12 and 14 are charged from 
the 27.6 VDC output of power supply 22. A 5 ohm current limiting resistor 
R1 protects batteries 12 and 14 from a harmfully large battery charge 
current than manageable demand for current from batteries 12 and 14. 
It will be obvious to those having skill in the art that many changes may 
be made to the details of the above-described embodiment of this invention 
without departing from the underlying principles thereof. The scope of the 
present invention should, therefore, be determined only by the following 
claims.