Dual-voltage battery charging system

A battery charging system for a dual-voltage motor vehicle electrical system that has a pair of series-connected batteries that can energize an electric cranking motor. One of the batteries is charged from a diode-rectified alternating current generator. A voltage regulator regulates the output voltage of the generator and includes a switching means connected in series with the field winding of the generator to control field current. The other battery is charged by field discharge current that is produced by the voltage induced in the generator field winding when the switching means is switched to a nonconductive state.

This invention relates to a battery charging system for a dual-voltage 
motor vehicle electrical system where a pair of series-connected batteries 
are utilized to provide a voltage that corresponds to the sum of the 
terminal voltages of the batteries for energizing an electric cranking 
motor, and where one of the batteries feeds the accessory loads on the 
vehicle. 
Dual-voltage motor vehicle electrical systems that utilize two 
series-connected batteries are known, examples being the systems disclosed 
in the U.S. patent to Huntzinger et al U.S. Pat. No. 3,671,843 and to 
Raver U.S. Pat. No. 3,863,127. In these systems, one of the batteries, 
which may be termed the "accessory battery", is charged directly from the 
engine driven battery-charging generator. The other battery, which may be 
termed a "cranking battery", may be used only for engine cranking and it 
is charged by auxiliary apparatus energized by the vehicle generator. In 
the Huntzinger et al patent, the auxiliary apparatus comprises a DC-to-DC 
converter, and in the Raver patent the auxiliary apparatus comprises a 
transformer-rectifier. 
It is an object of this invention to provide a dual-battery charging system 
that utilizes the energy stored in the field winding of the 
battery-charging alternator to charge the cranking battery. In carrying 
this object forward, a circuit is provided for charging the cranking 
battery by field-discharge current that is produced when a switching 
device of a voltage regulator that is connected in series with the field 
winding turns off. The circuit connecting opposite ends of the field 
winding to the cranking battery comprises a diode, an ignition switch, and 
a generator tell-tale lamp. 
Another object of this invention is to provide a system for charging the 
cranking battery by generator field discharge current that includes means 
for sensing the terminal voltage of the cranking battery and diverting 
field discharge current from the cranking battery when the terminal 
voltage of the cranking battery reaches a value indicative of a fully 
charged condition. This prevents overcharging the cranking battery. More 
specifically, a switching device, such as a controlled rectifier, is gated 
conductive when the terminal voltage of the cranking battery attains a 
predetermined value. The controlled rectifier is connected to the 
generator field winding in such a manner as to provide a path for field 
discharge current that does not include the cranking battery.

Referring now to the drawing, the reference numeral 10 generally designates 
an alternating current generator which has a three-phase Y-connected 
output winding 12 and a field winding 14. The field winding 14 is carried 
by the rotor of the generator in a manner well known to those skilled in 
the art, and the rotor is driven by the engine of the motor vehicle (not 
illustrated). The three-phase output winding 12 is connected to a 
three-phase full-wave bridge rectifier 16 which is comprised of six 
diodes. The bridge rectifier 16 has a positive direct voltage output 
terminal 18 and a negative direct voltage output terminal 20 which is 
grounded. 
The output winding 12 is connected to the anodes of three diodes, each 
designated by reference numeral 22. The diodes 22 form a so-called "diode 
trio" and the cathodes of the diodes are connected to a junction 24. A 
direct voltage is developed between junction 24 and ground which serves to 
energize the field winding 14 of the generator in a manner which is well 
known to those skilled in the art. 
The motor vehicle electrical system has an accessory battery which is 
designated by reference numeral 26. The accessory battery 26 is a 12-volt 
battery and the positive terminal thereof is connected to a junction 28. 
The negative terminal of battery 26 is grounded, as illustrated. The 
electrical system also has a 4-volt cranking battery 30 which is utilized 
only for energizing an electric cranking motor. The cranking battery 30 
has its negative terminal connected to junction 28 and its positive 
terminal is connected to a conductor 32. In the drawing, the batteries 26 
and 30 have been illustrated as separate batteries, but it is to be 
understood that a single battery case or container could be provided which 
would include both batteries 26 and 30 and in such an arrangement the 
battery case would have a terminal corresponding to the junction 28 as 
well as positive and negative terminals. 
The accessory battery 26 is charged from the direct voltage output 
terminals 18 and 20 of the bridge rectifier 16 and to this end, the 
positive direct voltage output terminal 18 is connected to junction 28 by 
a conductor 34. The conductor 34 is connected to a conductor 36 at 
junction 28 and the conductor 36 feeds the 12-volt accessory loads on the 
motor vehicle. One of the 12-volt accessory loads is designated by the 
reference numeral 38, and it is energized whenever a switch 40 is closed. 
It is to be understood that there are are a plurality of vehicle accessory 
loads that can be connected to the conductor 36 so as to be energized 
thereby. 
The output voltage of the alternating current generator 10 is regulated to 
maintain a desired regulated voltage between conductor 34 and ground by a 
voltage regulator which has been generally designated by reference numeral 
42. The regulated voltage may be, for example, 14 volts in a 12-volt 
system. The voltage regulator 42 can be of a type disclosed in the U.S. 
patent to Harland et al U.S. Pat. No. 3,579,654. The voltage regulator 
comprises an output transistor 44 having a collector connected to junction 
46 and an emitter which is grounded. The transistor 44 may actually 
comprise a pair of Darlington-connected transistors, as is illustrated in 
the above-referenced Harland et al patent. The voltage regulator 42 
further includes voltage regulator circuitry 48 of the type disclosed in 
the above-referenced Harland et al patent which is connected to the base 
of transistor 44 and which biases transistor 44 to conductive and 
nonconductive states. The circuitry 48 responds to the voltage applied to 
a voltage-sensing lead which is designated by reference numeral 50 and 
which is connected to a junction 52 so as to sense the voltage applied to 
the accessory battery 26. Another conductor 54 is connected to the voltage 
regulator circuitry 48 and this conductor is connected to junction 56. The 
junction 56 is connected to a junction 58 located between the field 
winding 14 and the direct voltage output terminal 24 of the diode trio. 
The conductor 54 provides an input voltage to the voltage regulator 
circuitry 48 in a manner disclosed in the above-referenced Harland et al 
patent. 
The reference numerals 60 and 62 respectively designate an ignition switch 
and an accessory switch for the motor vehicle electrical system. These 
switches are arranged such that when the ignition system for the engine of 
the motor vehicle is energized, both switches 60 and 62 are closed. The 
switch 60 is connected in series with a generator charge indicator lamp 
64, and the switch 62 is connected in series with a resistor 66. When 
switches 60 and 62 are closed, the indicator lamp 64 and the resistor 66 
are connected in parallel and between junctions 56 and 68. 
As will be more fully described hereinafter, the voltage regulator 42 
senses the voltage appearing between junction 52 and ground and operates 
to cause the switching transistor 44 to switch on and off to thereby 
control field current in the field winding 14. 
The reference numeral 70 designates an electric cranking motor which is 
energized when the switch 72 is closed to crank the engine on the motor 
vehicle. One side of the cranking motor 70 is grounded and when switch 72 
is closed, the opposite side of the cranking motor is connected to a 
junction 74 and, hence, to the positive side of the cranking battery 30. 
This applies the combined terminal voltages of the batteries 26 and 30 to 
the cranking motor 70 so that it is energized with 16 volts. 
The cranking battery 30 is charged by the field discharge current developed 
as a result of the voltage induced in the field winding 14 when the 
transistor 44 is biased nonconductive. The circuit for charging the 
battery 30 can be traced from junction 46, through conductor 76, through 
diode 78, through conductor 80 to junction 82, through conductor 32 to the 
positive side of battery 30, through battery 30 to junction 28, and then 
back to the opposite side of the field winding 14 via conductor 50, 
junction 68, closed switches 60 and 62, the parallel-connected indicator 
lamp 64 and resistor 66, and then to junction 58. As will be more fully 
described hereinafter, each time the output transistor 44 goes 
nonconductive, a voltage is induced in field winding 14 that develops a 
field discharge current that is utilized to charge the battery 30. 
The battery charging system of this invention includes a voltage-responsive 
control or voltage regulator for terminating the supply of charging 
current to the cranking battery 30 whenever its terminal voltage reaches 
some value indicative of an adequately charged condition, for example, 5 
volts. This control includes a Zener diode 84, one side of which is 
connected to junction 82 by a conductor 86. The opposite side of the Zener 
diode 84 is connected to a junction 88. A resistor 90 and a variable 
resistor 92 are connected in series between junction 88 and junction 94 
located on conductor 36. The control circuitry further includes an 
opto-isolator 96 which comprises, in a single package, a light-emitting 
diode 98 and a silicon NPN phototransistor 100. When the light-emitting 
diode 98 is energized, it applies light energy to the phototransistor 100 
to cause it to conduct in its collector-emitter circuit. The opto-isolator 
96 is known to those skilled in the art and may be, for example, a 
Litronix type 4N25. 
The light-emitting diode 98 is connected in series with a resistor 102, one 
end of which is connected to the junction 88. The collector of the 
phototransistor 100 is connected with a conductor 104, while the emitter 
of phototransistor 100 is connected with a conductor 105. The conductor 
104 is connected to one side of a circuit comprising the 
parallel-connected resistor 106 and capacitor 108. The opposite side of 
this circuit is connected to a junction 110 which, in turn, is connected 
to a conductor 112. The conductor 112 is connected to one side of field 
winding 14 at junction 46. 
An NPN transistor 114 has its collector connected to junction 110 and has a 
base connected to conductor 105. The emitter of transistor 114 is 
connected to junction 116 and a resistor 118 is connected between junction 
116 and ground. The junction 116 is connected to the gate electrode of a 
controlled rectifier 120 via the resistor 122, diode 124, and conductor 
126. The anode of controlled rectifier 120 is connected to conductor 112 
at junction 123 and the cathode of controlled rectifier 120 is connected 
to conductor 34 at junction 127. 
The operation of the battery charging system will be described first under 
a condition of operation in which it is assumed that the battery 30 is in 
such a condition of charge, as represented by its terminal voltage, that 
it requires charging current. As previously mentioned, the voltage 
regulator 42 controls the field current in field winding 14 so as to 
maintain a desired regulated voltage between conductor 34 and ground. The 
voltage regulator 42 senses the voltage between conductor 34 and ground 
via conductor 50 and when this voltage exceeds the desired regulated 
value, it causes the transistor 44 to switch off. The output voltage of 
the generator 10 now decreases and when the voltage between conductor 34 
and ground decreases below the desired regulated value, the transistor 44 
is switched back on to complete a circuit for the field winding 14 between 
junction 24 and ground. The transistor 44 continuously switches on and off 
in response to changes in the voltage between conductor 34 and ground to 
regulate the output voltage of the generator 10. Each time the transistor 
44 switches off, a voltage is induced in the field winding 14 which has a 
polarity that is positive at the junction 46. In normal voltage regulating 
systems, the field winding 14 is shunted by a field discharge diode in 
order to prevent destruction of components connected with the field 
winding 14 by the voltage that is induced therein whenever the output 
switching transistor 44 goes nonconductive. In this invention, however, 
the current that is developed due to the voltage induced in the field 
winding 14 is utilized to charge the cranking battery 30 through the 
circuit that has previously been traced that includes the diode 78, the 
conductor 80 and the conductor 32. Thus, each time the transistor 44 goes 
nonconductive, a pulse of current is supplied to the battery 30 and the 
frequency of these pulses of current will depend upon the switching 
frequency of the transistor 44. This charging mode for charging battery 30 
by field discharge current continues as long as controlled rectifier 120 
remains nonconductive in its anodecathode circuit. The controlled 
rectifier 120 is biased conductive in a manner to be more fully described 
hereinafter whenever the terminal voltage of the cranking battery 30 
attains some predetermined value. 
The terminal voltage of the cranking battery 30 is sensed by the circuit 
connected thereacross including Zener diode 84, resistor 102, and the 
light-emitting diode 98. When the terminal voltage of battery 30 exceeds 
some value, for example, 5 volts, the Zener diode 84 conducts in a reverse 
direction to energize the light-emitting diode 98. The resistors 90 and 92 
shunt some of the input current from the opto-isolator 96 to allow 
adjustment of the control or regulator circuitry for the battery 30. 
If the terminal voltage of the battery 30 is high enough to cause the 
light-emitting diode 98 to be energized, the phototransistor 100 will be 
biased conductive in its collector-emitter circuit. Assuming now that the 
transistor 44 has just turned off, a voltage is developed at junction 46 
of, for example, 20 volts, which is coupled to junction 110 via conductor 
112. This voltage is applied to the base of NPN transistor 114 through the 
parallel-connected resistor and capacitor 106 and 108, through the 
collector-emitter circuit of phototransistor 100 and conductor 105. This 
voltage will bias the NPN transistor 114 conductive so that the voltage on 
conductor 112 will now be applied to the gate of controlled rectifier 120 
via the collector-emitter circuit of NPN transistor 114, diode 124, and 
conductor 126. When the controlled rectifier 120 is gated conductive in 
its anode-cathode circuit, the voltage on conductor 112 is applied to the 
junction 28 and the field discharge current now developed in field winding 
14 is applied to the accessory battery 26. The path for field discharge 
current, when controlled rectifier 120 is conductive, can be traced from 
junction 46, through conductor 112 to the anode of controlled rectifier 
120, through the anode-cathode circuit of controlled rectifier 120 to 
conductor 34, through the accessory battery 26 and accessory load 38 to 
ground, through the three diodes of the bridge rectifier 16 connected to 
grounded output terminal 20 and then through the diodes 22 to the opposite 
side of the field winding 14. Whenever controlled rectifier 120 is biased 
conductive, current is no longer supplied to the cranking battery 30 since 
the field discharge current is diverted away from the cranking battery 30 
and into a circuit that includes the accessory battery 26 and accessory 
load 38. When the transistor 44 switches conductive, the voltage at 
junction 46 decreases, to lower the gate voltage to the gate of the 
controlled rectifier 120, and it shuts off since the voltage of its 
cathode will go higher than the voltage of the anode to thereby 
reverse-bias its anode-cathode circuit. As long as the terminal voltage of 
battery 30 exceeds a predetermined value, the controlled rectifier 120 is 
gated conductive each time the switching transistor 44 is biased 
nonconductive to provide a path for field discharge current that does not 
include the battery 30. Accordingly, the battery 30 will not receive field 
discharge current as long as its terminal voltage is above the 
predetermined value. 
Each time the cranking motor 70 is energized, it utilizes energy that has 
been stored in the cranking battery 30, and eventually the terminal 
voltage of battery 30 will drop to some point indicative of the fact that 
battery 30 should be charged. When this occurs, the system reverts back to 
a mode of operation in which field discharge current is supplied to the 
cranking battery 30 via the circuit that includes diode 78 and the 
conductor 80. 
The lamp 64 lights up when the generator output voltage is below a 
predetermined value indicative of low or no generator voltage. When 
generator output voltage is normal, the voltages at terminal 18 and 
junction 24 are substantially equal and accordingly the lamp 64 has 
substantially equal voltages applied to opposite ends thereof, and it is 
extinguished. The pulsating field discharge current that is at times 
supplied to battery 30 through the circuit that includes the lamp 64 does 
not cause the lamp to light. 
By way of example, and not by way of limitation, the average direct current 
supplied to battery 30 may be approximately 0.5 amp where generator 10 is 
a type 15-SI generator manufactured by the Delco-Remy Division of General 
Motors Corporation. This average current may vary between approximately 
0.3 to 0.7 amp, depending upon the magnitude of the 12-volt load 38 and 
the speed of the engine that drives the generator 10. There is a slight 
reduction in generator output current when field discharge current is 
being supplied to battery 30, for example, 1.0 amp when output transistor 
44 is operating in a switching mode. At loads where transistor 44 remains 
continuously conductive (not switching), there is no loss of alternator 
output current since no field discharge current is coupled to the 4-volt 
battery 30. 
The cranking motor 70 may be a 12-volt cranking motor, and the system of 
this invention provides 16 volts to the cranking motor so as to cause the 
cranking motor to develop a higher cranking speed as compared to a system 
that would energize the cranking motor with 12 volts. This is accomplished 
while maintaining 12-volt energization for all the other electrical 
accessories on the motor vehicle.