Battery voltage monitor and disconnection circuit

A battery saver circuit compares the battery terminal voltage to a programmable reference voltage and disconnects the battery from the load when the measured battery voltage falls below a predetermined minimum value. The reference voltage is programmable by a transfer function generator which varies the reference voltage in accordance with the current drawn by the load to account for the voltage drop in the line series wiring. A fault indicator is included to prevent chatter in the relay by raising the battery voltage necessary to reconnect the battery to the load. The fault indicator is reset when the battery is recharged.

This invention relates to electronic circuits for monitoring storage 
batteries. More particularly, it relates to circuits which monitor storage 
battery output voltage and disconnect the battery from the load when 
output voltage falls below a predetermined value. 
BACKGROUND OF THE INVENTION 
Excessive use of lights or accessories such as cellular phones, radios and 
the like without appropriate re-charging can rapidly reduce the output of 
a storage battery. In the case of an automobile storage battery or the 
like, the battery can readily be discharged to a level where it contains 
insufficient output energy to crank the engine. Accordingly, attempts have 
been made to automatically monitor battery output voltage and disconnect 
the battery from the load when the measured output voltage fails below a 
predetermined minimum level. Apparatus for this purpose (known as 
intelligent battery guards) are usually designed for universal 
applications having varying current requirements and have especially poor 
performance histories. One reason for most failures is the simplistic 
designs of the previous devices. Another reason is the inaccurate 
assumption that the voltage measured at the accessory port (usually a 
cigarette lighter socket) is an accurate measure of the battery's 
discharge condition. The most accurate measure of battery condition is 
accomplished by measuring the specific gravity of the electrolyte. 
However, this is not practical in many cases and not physically possible 
with sealed batteries. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a circuit is provided which 
monitors battery terminal voltage under load conditions. When the circuit 
determines that the battery has reached a predetermined minimum level 
(such as the minimum storage capacity required to crank the engine of a 
vehicle in which it is installed) the load is disconnected from the 
battery. Furthermore, the load is not reconnected until a predetermined 
event (such as engine restart occurs). The load, of course, may be 
reconnected when the battery voltage is found by the circuit to be above 
the predetermined minimum level. 
The apparatus of the invention develops a reference voltage to which 
battery voltage can be compared. The reference voltage is programmable by 
a transfer function generator which makes the reference voltage 
proportional to the load current to account for voltage drops, in wiring 
and other components. When the battery voltage (as determined by a 
comparator in the circuit) is found to be below the predetermined minimum 
voltage level represented by the programmable reference voltage, a relay 
disconnects the battery from the load. Further discharge is thus 
prevented. 
In the preferred embodiment the programmable reference voltage generator is 
a programmable zener diode and the transfer function generator by a 
current to voltage converter and an inverting amplifier. The current to 
voltage converter generates a voltage proportional to the current drawn by 
the load. This voltage is fed to the inverting amplifier which acts to 
alter the value of a voltage divider which sets the reference voltage 
generated by the zener diode. 
The preferred embodiment also includes a visible status indicator of the 
state of the battery saver circuit. The visible status indicator is formed 
by a dual output light emitting diode (LED) which has both a red and a 
green light. The red and the green lights are used individually or in 
combination to indicate (i) the battery voltage is above the predetermined 
minimum level and the load is not drawing current; (ii) the battery 
voltage is above the predetermined minimum level and is drawing current; 
or (iii) the battery has been disconnected from the load because battery 
voltage is below the predetermined minimum level. 
To prevent relay chatter, the battery saver circuit is preferably provided 
with a fault indicator which raises the voltage of the battery necessary 
to reconnect the load. The fault indicator in the preferred embodiment is 
a thyristor which latches on when the battery is disconnected from the 
load to place a resistor in parallel with the input voltage divider for 
the measured battery voltage. This raises the battery voltage required to 
reconnect the battery to the load while the fault indicator is "on". The 
fault indicator may be reset by engine restart and reconnecting the 
battery to the load.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A circuit diagram of the preferred embodiment of the battery saver circuit 
of the invention is illustrated at 10 in the drawing. All parts shown 
readily available are standard electrical components. The anode and 
cathode of the battery to be monitored are connected to anode input 12a 
and cathode input 12c, respectively. The input voltage is connected to 
circuit 10 through relay switch 24 which also connects the battery to the 
load through output anode 66a and output cathode 66c. (The heavy lines at 
the output indicate high current traces necessary to handle the currents 
drawn by the load.) 
Zener diode 14 is connected across the battery terminal through resistor 16 
and is used to set an accurate programmable voltage reference against 
which the battery input voltage can be compared. This reference voltage 
can be modified by a transfer function which is set by op-amps 18 and 20. 
Op-amps 18 and 20 provide a control voltage which controls the voltage 
drop across zener diode 14. The control voltage is proportional to the 
current through fuse 26 as the current varies. 
Op-amp 20 acts as a current to voltage converter which converts the current 
through fuse 26 to a voltage with the gain function of op-amp 20 set by 
resistors 28 and 30 when small currents are flowing through fuse 26. When 
sufficient current flows through fuse 26, diodes 92, 94 and 96 become 
forward biased and resistor 98 is placed in parallel with resistor 28, 
thereby altering the gain of op-amp 20. Resistor 30 is connected between 
cathode input 12(c) and the negative input of op-amp 20. Resistor 28 is 
connected between the negative input of op-amp 20 and the battery cathode 
to set the gain of op-amp 20. Resistors 32 and 34 are connected between 
anode input 12a and the output of op-amp 20 and initialize the reference 
voltage modification to approximately a level representing a zero current 
flowing in fuse 26. 
Op-amp 18 acts as an inverting amplifier to amplify the voltage provided by 
op-amp 20. The gain of op-amp 18 is set by resistors 36 and 38. Resistor 
40 is connected between the output of op-amp 18 through diode 42, and 
zener diode 14. Resistor 40 modulates the rate and range of the reference 
voltage generated by zener diode 14. Resistors 44 and 46 form a voltage 
divider which, in conjunction with resistor 40, controls the voltage used 
to modulate the drop across zener diode 14. 
Comparator 52 compares the reference voltage across zener diode 14 with the 
battery voltage. Resistors 48 and 50 are connected to the positive input 
of comparator 52 and act as a voltage divider for the battery voltage to 
provide an accurate scaled input to comparator 52. The reference voltage 
is connected to the negative terminal of comparator 52. The output of 
comparator 52 and pull-up resistor 56 are connected to the base of 
transistor 54. Transistor 54 is connected in series with relay coil 60 
across anode input 12a and cathode input 12c and to control relay coil 60. 
Diode 58 is connected in parallel with relay coil 60 between anode input 
12a and the collector of transistor 60. Resistor 62 (in series with diode 
64) is connected to the positive input of comparator 52 to provide 
hysterisis feedback and prevent chatter due to voltage noise. 
To latch the circuit in the "on" state when relay coil 60 has disconnected 
the load from the battery, thyristor 68 is connected to capacitor 70 
through resistor 72. Discharging of capacitor 70 latches thyristor 68 "on" 
which brings resistor 76 and diode 78 in parallel with resistor 50. 
Resistor 76 in parallel with resistor 50 alters the voltage divider 
arrangement formed by resistors 48 and 50 to input the voltage associated 
with the battery to comparator 52, thus raising the voltage needed to 
reconnect the battery to the load. 
Transistor 80, which has resistor 84 connected between its base and emitter 
and is connected to output anode 66a through capacitor 82, operates to 
unlatch thyristor 68 when the battery is reconnected to the load by 
restarting the car, and thereby allowing the alternator to charge the 
battery. When the relay coil reconnects the load, transistor 80 is turned 
"on" through capacitor 82. When transistor 80 is "on" it shorts resistor 
76 by connecting resistor 86 and diode 88 directly to ground. Shorting 
resistor 76 drops the current through thyristor 68 to essentially zero, 
thereby unlatching thyristor 68. 
Dual output light emitting diode 102 usually indicates the various states 
of battery saver circuit 10. Dual output LED 102 contains both a red light 
104 emitter and a green light 106 emitter. Green light 106 only is used to 
indicate that the battery is in safe condition and the load is not drawing 
current. Green light 106 and red light 104 are "on" at the same time to 
indicate that the battery is in safe condition and the load is drawing 
current. Red light 104 only indicates that the battery is in an unsafe 
condition and no load is connected. 
Comparator 100 monitors the current through fuse 26. Resistors 112 and 114 
are connected to the respective inputs of comparator 100 and set the 
minimum current necessary to activate comparator 100. When the minimum 
current is exceeded, comparator 100 turns "on" and activates red light 104 
of dual output light emitting diode 102. When relay switch 24 is 
connecting the battery to the load, green light 106 is connected to 
battery anode 12a through resistor 108 and is, therefore, turned "on". If 
the battery has been disconnected from the load by relay coil 60, red 
light is connected to battery anode 12a through resistor 110 and is "on", 
indicating that the battery has been disconnected from the load. (Green 
light 106 is disconnected from the battery and is necessarily "off"). 
Battery saver circuit 10 is designed to protect a battery by disconnecting 
any external load in series with circuit 10 at a point before the battery 
is discharged to such a level as to prevent restart of a vehicle engine. 
Circuit 10 is also designed to provide visual indications via dual output 
LED 102. An accurate programmable reference voltage is set by zener diode 
14 which is fed, along with the battery voltage scaled by the voltage 
divider formed by resistor 48 and 50, to comparator 52. When comparator 52 
finds the scaled voltage from the battery below the reference voltage, 
comparator 52 turns transistor 54 "off" which turns relay coil 60 "off", 
thereby disconnecting the battery from the load by disconnecting relay 
switch 24. 
To prevent chatter in relay coil 60 and relay switch 24, thyristor 68 acts 
as a fault indicator and is latched "on" by capacitor 70 when the load is 
disconnected. Measured battery voltage, however, tends to increase when 
the load is removed because of the voltage drop caused by connector 
wiring, ignition switch and relay contacts. Accordingly, thyristor 68, 
when "on", places resistor 76 in parallel with resistor 50 and alters the 
voltage divider circuit, thereby increasing the value of the battery 
voltage required to restore the load connection. 
Thyristor 68 is reset (and the voltage divider is restored to its initial 
value) by action of transistor 80 which shorts resistor 76 and thyristor 
68 when the load is reconnected by starting the vehicle and engaging the 
alternator. By shorting resistor 76 and thyristor 68, the current in 
thyristor 68 is reduced to zero or nearly zero, thereby unlatching 
thyristor 76. In the preferred embodiment, battery saver circuit 10 is set 
to disconnect the battery from the load when the battery voltage falls to 
twelve (12) volts. Thyristor 68 resets this voltage upward, requiring the 
battery voltage to be thirteen (13) volts to reconnect the load until the 
circuit is reset by starting the engine. 
The reference voltage provided by zener diode 14 is adjustable to account 
for the line series voltage drop caused by different load currents. This 
adjustment in the reference voltage is provided by op-amps 18 and 20 and 
is proportional to the current drawn by the load. Op-amp 20 is a current 
to voltage converter which converts the current in fuse 26 to a voltage. 
The voltage from op-amp 20 is then fed to op-amp 18 which is configured as 
an inverting amplifier. As the voltage output of op-amp 18 decreases, the 
value of the voltage divider formed by resistors 44 and 46 changes as 
resistor 40 is placed in parallel with resistor 46. Altering the 
characteristics of the voltage divider formed by resistors 44 and 46 
alters the reference voltage generated by zener diode 14. 
Various applications including different makes and models of vehicles with 
various battery and load conditions can be accommodated by battery saver 
circuit 10. These variations can be accounted for by varying the gain 
factors of op-amps 18 and 20. Varying the gain factors is accomplished 
simply by changing resistors 30, 32, 36, 38, and/or 40. 
While the invention has been shown and described with particular reference 
to a preferred embodiment, it will be understood by those skilled in the 
art that various changes in form and detail may be made therein without 
departing from the spirit and scope of the invention as defined by the 
appended claims.