Method and circuit for controlling charging in a dual battery electrical system

A control circuit for controlling the recharging of two batteries in a dual battery vehicle electrical system includes a charging circuit (20) for stepping up the voltage of a reserve battery (RES.), so that it can be applied to a starting battery (HPB), an alternative circuit path (18, 19) to allow charging of both batteries by the alternator (A) when the vehicle engine is running, and a controller (30) for switching the connection of the batteries through the charging circuit (20) and the alternative circuit path (18, 19) under various combinations of conditions.

TECHNICAL FIELD 
The present invention relates generally to motor vehicle electrical systems 
and more particularly to a dual battery electrical system for supplying 
electrical energy to the motor vehicle and for maintaining the charge of 
the dual battery system. 
BACKGROUND ART 
Typically, vehicles powered by internal combustion engines have an electric 
starter motor for starting the engine. The starting motor is electrically 
connected to a starting circuit which receives electrical energy from an 
electric storage battery. When an ignition keyswitch is operated, power 
from the battery is supplied to the starting motor to turn over the 
internal combustion engine. In common vehicle applications, devices such 
as engine control electronics, lighting systems, and vehicle accessories, 
which present an electrical load to the battery, or to alternator, when 
the vehicle engine is running. 
Traditional batteries are often referred to as starting, lighting and 
ignition (SLI) batteries. In design and construction, these are 
multi-cell, lead-acid batteries, which are constructed from lead plates 
carrying active material and arranged into stacks. The stacks are inserted 
into partitioned cell compartments of a battery container, electrically 
connected, and flooded with dilute acid electrolyte. 
Starting requires high power output for a short time period. SLI batteries 
of this construction are more than adequate for providing the relatively 
high power demand required for engine starting. 
Maintaining electrical loads in the vehicle both during vehicle operation 
and during periods of non-operation requires a relatively lower power 
demand than starting. Therefore, SLI battery design is difficult as an SLI 
battery must be to optimized to perform, both short duration high-power 
output and long duration low-power output. An additional drawback of SLI 
batteries is relatively low specific energy (kilo-watt hours/grams 
(kWh/g)) as compared to other battery constructions owing to the weight of 
the lead plates and the liquid electrolyte. 
More recently vehicle power systems have incorporated two batteries. A 
first battery in the system, a starting battery, is optimized for engine 
starting, that is, designed specifically for short duration, high-power 
output. A second battery in the system, a reserve battery, is optimized 
for operating and maintaining non-starting electrical loads. An advantage 
of such a system is that the starting battery may be made smaller and 
lighter yet capable of provide a high power output for a short period of 
time. In addition, the reserve battery may be made smaller and lighter, 
yet capable of satisfying the relatively low power requirements of the 
vehicle accessories. In combination, the two battery system can be 
designed to occupy less space and weigh less than a single traditional SLI 
battery. 
Dual battery systems require control circuits to maintain the charge of 
both batteries in the system. Typically, the vehicle includes a regulation 
device which regulates the output of the alternator in response to the 
charging needs of the SLI battery and the vehicle electrical loads. In a 
dual battery system, each battery type delivers power and accepts charge 
at a different rate. For example, the starting battery delivers power at a 
very high rate and likewise accepts charge at a high rate. In contrast, 
the reserve battery delivers power at a lower rate and accepts charge at a 
lower rate. Each battery will typically exhibit a different 
state-of-charge, and hence require different charge maintenance. 
Additional advantages may also be attained by selectively coupling or 
decoupling the batteries during non-operational, starting and operational 
periods of the vehicle. However, careful management is required so as not 
to damage either the vehicle electrical system or the dual battery system. 
Therefore, a dual-battery system for vehicle starting and operation that 
provides the advantages of reduced size and weight and includes power and 
charge management is needed. 
Dougherty et al., U.S. Pat. No. 5,162,164, discloses a dual battery system 
in which two batteries are contained in a single casing. Thus, it should 
be understood that the identification of two batteries means, without 
limitation, containment in either separate casings or in one casing 
SUMMARY OF THE INVENTION 
The present invention provides a control circuit for a dual energy supply 
storage system for a vehicle. While the invention is described in terms of 
two batteries, it also contemplates systems in which a starting battery is 
replaced with a starting capacitor. The term "charge energy source" shall 
be used to include batteries, capacitors and other types of equivalent 
charge energy sources. 
In various preferred embodiments of the present invention, battery control 
electronics, vehicle control electronics and combinations of the these 
electronic control devices are utilized for battery charge management and 
enhanced system performance. For example, the system is adaptable to 
automatically determine charge status of the batteries in the system and 
to couple, as appropriate, the battery or batteries with sufficient charge 
to operate essential vehicle electrical loads and to provide energy for 
starting. 
In addition, a preferred charge management strategy reduces the potential 
for overcharging one or more of the system batteries yet maintains each of 
the batteries at a ready state of charge. 
These and other advantages and objects of the invention will be appreciated 
from the description that follows, in which reference is made to the 
accompanying drawings, which are a part of the teaching of the invention, 
and in which a preferred embodiment of the invention is illustrated. The 
description and illustration of the preferred embodiment is by way of 
example and not by way of limitation. For the various embodiments of the 
invention within the scope of the invention, reference is made to the 
claims which follow the description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a vehicle electrical system 10 includes a starting 
battery, also referred to as a high power battery (HPB). This refers to 
the high instantaneous or short burst of power which is needed to start 
the vehicle. The starting battery HPB is connected through a pair of 
switch contacts 12 to an electrical starter motor (S) 13. The starter 
motor 13 is mechanically coupled to the internal combustion engine (not 
shown) through an output shaft (not shown), and when starter motor 13 is 
rotated, it turns over the internal combustion engine to begin combustion 
of fuel in the presence of a spark. 
The starting battery HPB is preferably a battery of recent technology which 
provides the necessary high starting power in a smaller package than 
traditional SLI batteries. Such a new type of starting battery is shown 
and described in copending U.S. Pat. App. Ser. No. 08/870,803, filed Jun. 
6, 1997, assigned to the assignee of the present invention and entitled 
"Modular Electric Storage Battery," the disclosure of which is hereby 
expressly incorporated herein by reference. Such a starting battery should 
typically provide 800 amps of current at a cranking voltage of 10.0 volts 
for startup, and have 4.5 amp-hours of reserve energy capacity. 
The vehicle electrical system 10 also includes a reserve battery (RES.) 
which is preferably of the absorptive glass mat (AGM) type construction 
having high reserve capacity. The reserve battery RES. is adapted to 
provide a relatively low rate discharge for an extended period of time. 
The reserve battery typically may have a rating of twenty-three amp-hours 
of energy storage capacity, and a rating output of twenty-five amps at a 
nominal voltage of eleven volts. 
Next to the reserve battery RES. is a conventional alternator (A) 14 which 
is a type of electrical generator driven by the internal combustion 
engine, to supply rectified DC power for charging the batteries and for 
operating other loads in the vehicle. 
An ignition keyswitch with two switch positions "On" and "Start" is 
represented by two switches ON and START with corresponding labels in FIG. 
1. The vehicle reserve battery RES. is connected to some resistive loads 
15 (trunk light, for example), even when the ON switch is open (the "Off" 
position). Resistive load 16 represents the vehicle loads when the 
ignition keyswitch is operated to close the ON switch, which stays closed, 
during and after starting. The START switch is closed during starting, to 
supply electrical power from the alternator 14 through resistance 17, and 
is then opened. 
Essential vehicle loads may include such loads as the vehicle engine/power 
train controller, lighting systems, braking system controller, safety 
system controller and the like which are powered during vehicle operation. 
Other vehicle loads may include entertainment systems, convenience 
features and others which are not essential or required for vehicle 
operation. 
When the ignition keyswitch is moved to the "Start" position ("START" 
switch and "ON" switch closed), an ignition coil L1 is energized to close 
the starting contacts 12, and energy is supplied from the starting battery 
HPB to the starting motor 13. 
During times when the vehicle engine is off, and at times after the vehicle 
is started, it is desired to charge the starting battery HPB. When the 
vehicle is being operated, such recharging of the battery is provided by 
the alternator 14. When the vehicle is off, such recharging can now be 
provided by the reserve battery RES. 
In order to control the voltage and rate of charging of the starting 
battery HPB, it is coupled through a charging circuit 20, including a 
voltage boost circuit 21 to be described below. The charging circuit 20 is 
electrically connected to the starting battery HPB through a relay 18 
having a normally closed (NC) position and an open (O) position 
represented by respective contacts "NC" and "O". With the relay 18 in the 
normally closed condition, the charging circuit 20 is connected to one 
side to the starting battery HPB. In order to be operable, however, the 
charging circuit 20 must also be connected to the reserve battery RES. to 
receive power, and the charging circuit 20 must be enabled by a signal on 
an ENABLE line running from an electronic controller 30. With the relay 18 
energized, the charging circuit 20 is bypassed through an alternative 
circuit path including an optional PTC (positive temperature coefficient) 
resistance 19. This PTC resistance 19 acts as a time-dependent fuse and 
provides what is effectively an open circuit at certain thresholds of 
current flowing for corresponding periods of time, and then resets after 
current decreases sufficiently. A 9-amp polyswitch available from Raychem 
is one example of a suitable component for use for this purpose. The 
controller 30 is also connected to relay coil R1 to operate the relay 18. 
The controller 30 is further connected to sense the state of the keyswitch 
at one input. The controller 30 is also connected through diodes D1 and D2 
to receive power from the batteries HPB, RES. A power supply capacitor 22 
connects to the cathode of diode D1 in the circuit path coming from the 
starting battery HPB. 
Controller 30 operates in accordance with a set of logic states or 
conditions, the logic and programming of which will be further explained 
below. 
Referring next to FIG. 2, the vehicle electrical system 10 is shown in 
further detail concerning the charging circuit 20 and electronic 
controller 30. A detailed schematic of the voltage boost circuit 21 is 
illustrated in FIG. 3. 
The reserve battery RES. connects through diode D3 to a voltage regulator 
23 for providing a +5-volt voltage supply for powering electronics in 
circuitry shown in FIG. 2. A DIP switch 24 and pull-up resistors (not 
shown) are provided to provide a plurality of inputs to a programmable 
array logic () circuit 25 which provides the control logic for the 
overall circuitry shown in FIGS. 1 and 2. The circuit is preferably a 
CE22V10Z-25IP integrated circuit, available from Vantis, a successor of 
Advanced Micro Devices. A time base generator circuit 26 provides timing 
signals to the circuit 25. The circuit 26 includes an ICM 7555 
integrated timer circuit available from Harris Semiconductor. This circuit 
provides timing signals, such as a pulse train with a 5-second periodic 
waveform that is counted by the circuit 25 to time longer delay 
periods. 
The reserve battery RES. has its output connected to a divider circuit 27 
for scaling the output to a +2.5-volt nominal level. The divider circuit 
27 is provided by four pairs of two resistors in series, which forms a 
precision voltage divider with four ratios determined by the desired trip 
point of comparators 28. The comparators 28 are TCL2252IP dual op amps 
available from Texas Instruments. The comparators 28 compare the output of 
the reserve battery RES. to various thresholds including 12.2 DC volts, 
13.1 DC volts and 13.4 DC volts, the significance of which will be 
explained below. The outputs of the comparators 28 are connected to inputs 
on the circuit 25 to sense the voltage of the reserve battery RES. 
The starting battery HPB in FIG. 2 is connected to the HPB switch input of 
the relay 18 seen in FIG. 2. The normally closed (NC) terminal in FIG. 2 
is connected to a voltage boost circuit 21, which is a more detailed 
embodiment of the charging circuit 20 in FIG. 1. The voltage boost circuit 
21 translates or converts the voltage of the reserve battery from its 
level at 13.1 to 13.4 volts up to a level such as 14.0 volts, representing 
the fully charged condition of the starting battery HPB. The voltage of 
the starting battery is sensed through the voltage boost circuit 
connection and through another set of comparators 29, which detect 
thresholds such as 14.0 DC volts, 13.8 DC volts and 12.75 DC volts for the 
starting battery HPB, the significance of these thresholds being explained 
below. The same type of commercial circuits can be used for comparators 
29, as were described above for comparators 28. A second voltage divider 
circuit 33 is connected between the output of the voltage boost converter 
21 and the inputs to the comparators 29. The voltage divider circuit 33 is 
formed of resistor pairs to scale the comparator trip points to a nominal 
+2.5 volts. 
FIG. 2 shows that the circuit 25 controls operation of the relay 18 
through a TI ON signal which is applied to a base of a transistor T1 to 
switch on the transistor T1 and allow current flow through the relay coil 
R1. 
FIG. 3 shows the voltage boost circuit 21, which is constructed around a 
pulse width modulator (PWM) circuit 31, which is provided by a UC3844N 
integrated circuit available from Unitrol and by a 20 .mu.H inductor 32. 
The inductor 32 is connected on one side to the Vc pin on the PWM circuit 
31 and on its other side to a collector of a transistor Q1. An output pin 
on the PWM circuit controls the base of transistor Q1 for switching the 
transistor Q1. The output of the inductor 32 is connected through diode D5 
to the NC contact of the relay 18 (FIG. 2). The PWM circuit 31 is enabled 
through an ENABLE line from the controller 30 which switches on a 
transistor Q2 to provide a ground path for PWM circuit 31. The PWM circuit 
31 has appropriate biasing networks to control the signal from the OUTPUT 
terminal which controls the output of the inductor 32. The inductor 32 has 
the voltage of the reserve battery RES. fed to an input side, and the 
output is controlled by the PWM circuit 31. The switching on and off of 
the PWM circuit causes the voltage at the output of the inductor to 
increase to a level higher than the 13.1 to 13.4 volts seen at the reserve 
battery. The switching produces an output voltage of about 14.0 DC volts. 
The output of the inductor 32 is coupled to the comparators seen in FIG. 2 
for sensing the voltage level of the starting battery. The output of the 
inductor 32 is also connected to the normally closed (NC) contact of the 
relay 18 which, when the relay in the normally closed state, is connected 
to the starting battery HPB. 
OPERATION 
The vehicle engine is defined as having three states: 1) rest, 2) starting 
and 3) running. The controller 30 senses these states by sensing the state 
of the ignition switch represented by the ON and START switches in FIG. 1. 
When the vehicle is at rest and the voltage of the starting battery 
decreases to a preselected level, such as 12.75 DC volts, this signifies 
that the vehicle has been off for an extended period of time such as one 
month. Then, the charging circuit 20 is enabled to allow reserve battery 
RES. to charge the starting battery HPB, provided the reserve battery is 
at a voltage of at least 12.2 DC volts signifying its charge level. One of 
the comparators 28 senses the 12.2-volt threshold for the reserve battery 
RES. One of the comparators 29 senses the 12.75-volt threshold for the 
starting battery HPB. The charging circuit 20 is enabled for a period of 
time determined by logic in the 25 and voltage sensing of the HPB 
battery. 
When the vehicle engine is running, this is sensed by the controller 30, 
and provided that the reserve battery RES. voltage exceeds the +13.4-volt 
upper threshold, the relay 18 will be energized to cause the relay 
moveable contact to contact the open position contact. During this twenty 
minute period, charge will flow from the alternator 14 to both the reserve 
battery RES. and the starting battery HPB through the PTC resistor 19. 
When the timer times out or when the reserve battery falls below the 
13.1-volt threshold, this indicates the reserve battery is too low to be 
used to recharge the starting battery. The relay 18 will be in the 
normally closed (NC) position, but the ENABLE signal to the charging 
circuit 20 will be disabled to disconnect the starting battery 14 from 
receiving charge from the reserve battery RES. This will allow the 
alternator 14 to charge the reserve battery RES. and it will protect 
against discharge through the starting battery HPB. Conversely, if the 
starting battery voltage rises above the 14.0-volt threshold for greater 
than a predetermined period of time, this represents a fully charged 
condition for the starting battery HPB and it will no longer accept charge 
from the reserve battery RES., so the relay 18 will be de-energized and 
the charging circuit 20 is disabled to protect the charge condition of the 
reserve battery RES. and to prevent overcharging of the starting battery. 
When the charge on the starting battery HPB falls below 13.8 volts, and 
assuming the vehicle engine is running, the circuit 25 will energize 
the relay 18 to allow the charging of the starting battery HPB from the 
alternator 14 through PTC resistor 19. 
A third function is provided when the car is starting. If the controller 30 
senses that the reserve battery is below some threshold such that the 
reserve battery is dead, it will not start on the first operation of the 
ignition switch, but will start on the second try. 
It should be appreciated that in an alternate embodiment, the components 
shown within the dashed lines in FIG. 2 as the controller 30 can be 
replaced with an equivalent microelectronic CPU, suitably programmed to 
provide the functions described herein. Some external timing circuit would 
be required to provide timing signals to such a CPU circuit. Such a 
circuit may have analog-to-digital inputs for sensing battery voltage 
levels. 
And, while in the illustrated embodiments, a relay 18 is the switchable 
device controlling the connection of the starting battery HPB to various 
charging sources, it should be understood that various types and numbers 
of semiconductor devices could be substituted to perform this function 
without departing from the scope of the invention. 
It will be appreciated that threshold voltages are representative of 
preferred voltages and that various vehicle systems may require different 
voltage thresholds. Likewise, the various timer values are representative 
of preferred values although other values may be chosen without departing 
from the scope of the present invention. Furthermore, it will be 
understood that other details of the preferred embodiment and the 
alternate embodiment may be varied without departing from the scope of the 
invention as defined by the following claims.