Battery thermal control system and method

A battery thermal control system and method is provided which selectively heats or cools a storage battery in accordance with the battery temperature and state of charge. The thermal control system includes a battery housing having an internal airspace between the battery and the housing which extends between two openings in the battery housing, a reversible fan configured to generate an airstream along the airspace, a temperature sensor mounted to detect the temperature of the battery, and a control circuit for monitoring battery temperature and voltage and for controlling the operation of the fan. Battery state of charge is approximated by measuring battery voltage and monitored to prevent excessive battery discharge by the thermal control system. When the battery voltage exceeds a predetermined voltage and the battery temperature is outside of a predetermined range of suitable operating and storage temperatures, the thermal control system operates the fan to heat or cool the battery, as needed.

FIELD OF THE INVENTION 
This invention relates to a system and method for controlling the 
temperature of a battery and, more particularly, to a system and method 
for selectively heating or cooling a rechargeable storage battery to 
thereby extend the life of the battery. 
BACKGROUND OF THE INVENTION 
Rechargeable storage batteries are used in a wide variety of applications. 
Lead-acid batteries are one such type of rechargeable storage battery and 
are commonly used in automobiles, other vehicles, and in numerous back-up 
power systems. As such, these batteries experience a broad range of 
operating and storage temperatures. In northern U.S. climates, ambient 
temperatures often fall well below -10.degree. C. and in southern U.S. 
climates, ambient temperatures may exceed 50.degree. C. 
Two other factors influence automobile battery temperature. Because 
automotive batteries are typically located in the automobile engine 
compartment, they are exposed to engine heat. When engine cooling is at a 
minimum, such as during city stop and go driving or while the engine is 
idling, heat generated by the engine may elevate the underhood air 
temperature above 100.degree. C. Moreover, the heat generated within the 
battery during use further elevates the battery temperature. Recent trends 
in vehicle design have resulted in even greater underhood temperatures. 
This is due to hotter engine designs built to improve engine efficiency 
and due to automobile downsizing, improvements in automobile aerodynamics, 
and an increased number of underhood components which restrict underhood 
air flow. 
These battery, temperature extremes are of concern to automobile and 
battery manufacturers because the storage capability and usable life of 
these batteries is largely dependent upon battery temperature. At colder 
temperatures, the ability of lead-acid batteries to deliver current is 
decreased. Furthermore, the average running state-of-charge is typically 
lower at colder temperatures, due to decreased charge acceptance rates and 
the increased loads placed on the automotive charging systems. As a 
result, lead-acid batteries provide relatively poor cold starting 
performance, especially when the battery is used multiple times to restart 
the engine within a short period of time. At higher temperatures, the 
battery grids are subject to accelerated corrosion and battery water loss 
due to electrolysis caused by an excessive voltage and, in some instances, 
evaporation of the water from the acid. Such corrosion may create a 
protective layer on the battery grids, thereby reducing the amount of 
exposed plate material available for storing energy. Additionally, boiling 
and electrolysis of the battery acid loosens and separates plate material 
from the battery grids, thereby reducing battery storage capacity. 
By controlling battery temperature within desirable limits, useable battery 
life can be more than doubled in some instances. It would therefore be 
useful to have a battery thermal control system that maintains battery 
temperature to within desirable limits. However, such a system preferably 
has several features. First, the system should be capable of both heating 
or cooling the battery, as necessary. Secondly, the system should operate 
independent of vehicle operation. This is especially true where the 
automotive battery is subject to engine heat, since the engine acts as a 
large thermal mass which can maintain the underhood temperature at an 
elevated level up to several hours after the engine is turned off. 
Thirdly, the system should exhibit very little power drain and should be 
sensitive to battery state of charge. The system should prevent 
discharging the battery beyond the point where the benefit of additional 
temperature adjustment is less than the deficit of a deeper discharge. 
Deep discharging of batteries is another contributor to decreased battery 
life. 
Although many systems for changing battery temperature have been developed, 
none provide all the above-mentioned features required for proper battery 
thermal protection. For instance, U.S. Pat. Nos. 2,104,769, 2,104,773, 
3,977,490, 4,976,327, and 5,015,545 disclose various methods for cooling 
an automobile battery. None of the apparatuses disclosed in these patents 
provide adequate thermal protection, because, among other things, they are 
not able to heat the battery to a temperature which provides a 
satisfactory battery charge acceptance. U.S. Pat. Nos. 2,717,045, 
4,840,855, and 5,031,712 each disclose systems for selectively heating or 
cooling the automobile battery. Although some of these systems provide for 
heating or cooling of the battery when the engine is not running, none of 
these systems are, among other things, sensitive to battery state of 
charge to prevent undesirable discharging of the battery. 
SUMMARY OF THE INVENTION 
The present invention provides a battery thermal control system which is 
responsive to battery temperature and state of charge to selectively heat 
or cool the battery, as needed. The invention includes a battery housing 
having first and second openings and an air flow path extending between 
the openings when the battery is placed in the housing, air flow means 
such as a fan for directing the air along the air flow path, sensor means 
for detecting the battery temperature, and control means responsive to the 
sensor means and the battery voltage for selectively operating the air 
flow means in accordance with the battery temperature detected by the 
sensor means and in response to the battery voltage. The system senses 
battery voltage to estimate battery state of charge. The operation of the 
air flow means is enabled only when battery voltage exceeds a 
predetermined voltage to prevent the system from discharging the battery 
below a predetermined state of charge. 
The control means can be configured to operate the air flow means when 
voltage exceeds a first predetermined level and then continue to operate 
until the voltage falls below a second, lower predetermined voltage. 
Hysteresis is thereby introduced which prevents the controller from 
cycling on and off about a voltage set point. Similarly, the maximum and 
minimum temperature set points can each be separated into a control means 
on and off threshold so that, for instance, the system begins cooling the 
battery at one temperature and continues to cool the battery until it 
falls below a second, lower temperature. 
In another embodiment, the system provides both heating and cooling by 
utilizing a reversible fan disposed to provide heated air into the first 
opening or unheated air into the second opening. Additionally, conduits 
can be used to route heated and unheated air to the battery openings. 
The present invention further provides a method for changing the 
temperature of a battery which includes the steps of sensing the voltage 
and temperature of the battery and generating an airstream of unheated air 
that impinges upon the battery when the temperature of the battery is 
above a predetermined maximum temperature and the voltage of the battery 
is above a predetermined voltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIGS. 1 and 2, the thermal control system of the present 
invention, designated generally as 10, includes a battery housing 12 
having openings 13 and 15 in opposite sides thereof for transmitting air 
to and from housing 12, a fan 14 attached to opening 15 of housing 12, a 
conduit 16 attached to fan 14 and a conduit 18 attached to opening 13 to 
transmit air to and from housing 12, a temperature sensor 20, and a 
control circuit 22 connected to fan 14, sensor 20, and the terminals 24 of 
a battery 26. Fan 14, sensor 20, and terminals 24 are connected to control 
circuit 22 via pairs of wires 28, 30, and 32, respectively. Control 
circuit 22 monitors the voltage of battery 26 via wires 32 and the 
temperature of battery 26 via sensor 20. When the voltage of battery 26 
exceeds a predetermined maximum and the temperature of battery 26 falls 
outside of a predetermined range, control circuit 22 operates fan 14 in 
either a forward or reverse mode to control the temperature of battery 26. 
In its forward mode, fan 14 operates to force air from conduit 16, into 
housing 12, and out through conduit 18. In its reversed mode, fan 14 
operates to draw air through conduit 18, into housing 12, and out through 
conduit 16. 
Battery housing 12 may suitably be any housing configured to hold battery 
26 and adapted to have an airspace 25 between the first and second 
openings 13 and 15 when battery 26 is placed in housing 12. For instance, 
housing 12 can be configured either to enclose only a portion of battery 
26 (as shown in FIG. 1) or to completely enclose battery 26. If housing 12 
is configured to enclose only a portion of battery 26, a top portion 34 
can be used to provide a generally air tight seal between housing 12 and 
battery 26. However, as will be understood by those skilled in the art, no 
airtight seal is needed and, in the broader aspects of the invention, 
housing 12 is not required so long as fan 14 is configured to generate an 
airstream that impinges upon battery 26 in a manner sufficient to alter 
the temperature of battery 26. If used, housing 12 can be made from any of 
a variety of either insulating or non-insulating materials. Preferably, 
battery 26 is configured in housing 12 such that the airspace 25 
established between the first and second openings 13 and 15 of housing 12 
covers a substantial portion of battery 26. To accomplish this, the length 
and width of housing 12 can be made larger than the corresponding length 
and width of battery 26 and may include pads (not shown) on the bottom of 
housing 12, to thereby allow air to flow around and underneath battery 26. 
Alternatively, the first and second openings 13 and 15 can be located in a 
common side of housing 12 and housing 12 can be configured to have an 
internal barrier located between the first and second openings and 
extending between battery 26 and the common side of housing 12, to thereby 
establish an airspace that extends substantially around the perimeter of 
battery 26. Other configurations of housing 12 will be apparent to those 
skilled in the art. 
Preferably, fan 14 is mounted on housing 12 to provide air flow into or out 
of either the first or second opening in housing 12. Fan 14 can be located 
anywhere along the airflow path defined by housing 12, adapter 38 conduit 
16, and conduit 18. For instance, fan 14 Alternatively, fan 14 could be 
located within housing 12. Furthermore, other means for providing an air 
flow into housing 12 can suitably be employed. For example, thermal 
control system 10 can be configured such that air flows through housing 
12, except as limited by an electrically, pneumatically, or mechanically 
controlled damper or air valve that is responsive to control circuit 22. 
Preferably, conduits 16 and 18 are employed to provide housing 12 with 
heated and unheated air. To that end, conduit 16 may be routed to a 
location proximate the engine exhaust manifold to provide housing 12 with 
a source of heated air and conduit 18 may be routed underneath the front 
bumper of the automobile to provide housing 12 with a source of unheated 
air. It will be understood by those skilled in the art that conduits 16 
and 18 are needed only if the first and second openings 13 and 15 of 
housing 12 cannot be located proximate a source of heated or unheated air, 
as desired. If conduits 16 and 18 are not employed, then the airflow path 
utilized by thermal control system 10 is simply the airspace 25 existing 
between the first and second openings 13 and 15 of housing 12 when battery 
26 is placed therein. As shown in FIG. 1, an adapter 38 may be employed to 
interface conduit 16 and/or conduit 18 to fan 14 and/or housing 12, as 
will be appreciated by those skilled in the art. 
Sensor 20 can be any means for detecting the temperature of battery 26. 
Preferably, sensor 20 is thermally coupled to battery 26 and can be 
mounted on housing 12, within housing 12, on battery 26, within battery 26 
as shown, or at any other location suitable for detecting a temperature 
representative of the temperature of battery 26. Alternatively, sensor 20 
could comprise an apparatus for estimating battery temperature, such as 
that disclosed in U.S. Pat. No. 5,079,716, and entitled "Method and 
Apparatus for Estimating a Battery Temperature," which is hereby 
incorporated by reference. 
One feature of the present invention is its ability to change battery 
temperature during periods when the automobile engine is not running. 
However, battery 26 is only recharged when the engine is operating. To 
prevent excessive battery discharge, control circuit 22 is therefore 
configured to operate fan 14 only if the voltage of battery 26 is above a 
predetermined voltage. Since battery voltage while discharging roughly 
approximates battery state of charge, configuring control circuit 22 in 
this way permits thermal control system 10 to operate fan 14 only when 
battery 26 is at a high state of charge. Moreover, battery 26 will only 
have an elevated voltage for a period of time beginning when the engine is 
running and there is excess alternator capacity until sometime after the 
engine is turned off, depending upon the amount of battery loads that 
exist once the engine has been turned off. Thus, by choosing an 
appropriate first predetermined threshold voltage V1, thermal control 
system 10 can be designed to operate fan 14 during the period of time that 
battery 26 typically requires the most cooling, i.e., during engine 
operation and for a period of time after the engine has been turned off, 
during which the underhood temperature remains elevated. It should be 
noted that the actual voltages used for the first threshold voltage V1 and 
second threshold voltage V2, to be referred to hereinafter, are highly 
battery and application specific, as will be appreciated by those skilled 
in the art. 
Control circuit 22 is responsive to battery voltage and temperature (as 
measured by sensor 20) to control the operation of fan 14. More 
particularly, control circuit 22 is configured to operate fan 14 in either 
a forward or reverse mode to heat or cool, respectively, battery 26 when 
the voltage of battery 26 (V.sub.bat) is above the first predetermined 
voltage (V1) and the temperature of battery 26 (T.sub.bat) is outside 
predetermined limits (T1, T2) which define a range of desirable battery 
storage and operating temperatures. Preferably, control circuit 22 is 
configured to include both a voltage and temperature hysteresis 
characteristic. This can be accomplished as follows. Control circuit 22 
operates fan 14 when V.sub.bat is above V1 and T.sub.bat is outside of the 
range of temperatures defined by minimum temperature T1 and maximum 
temperature T2. Control circuit 22 thereafter continues to operate fan 14 
until either V.sub.bat falls below the second and lower predetermined 
voltage (V2) or T.sub.bat falls between a second, predetermined minimum 
temperature (T3) and a second, predetermined maximum temperature (T4), 
both of which fall within the T1, T2 temperature range. Introducing 
hysteresis in this manner prevents thermal control system 10 from cycling 
about the temperature and voltage set points. 
Referring now to FIG. 3, it depicts a state diagram for the function of 
control circuit 22. Without hysteresis, there are only three possible 
states, denoted by reference numerals 40, 42, and 44. Thermal control 
system 10 will be in what can be considered an initial state 40 whenever 
either V.sub.bat is less than V1 or T.sub.bat is within the predetermined 
limits (i.e., T1&lt;T.sub.bat &lt;T2). Therefore, if V.sub.bat exceeds V1 and 
T.sub.bat falls below T1, thermal control system 10 drives fan 14 in the 
forward mode to heat battery 26, as indicated at state 42. If, however, 
V.sub.bat exceeds V1 and T.sub.bat rises above T2, thermal control system 
10 drives fan 14 in the reverse mode to cool battery 26, as indicated at 
state 44. Thus, thermal control system 10 will merely switch between 
states 40 and 42 and states 40 and 44, depending upon battery voltage and 
temperature. 
By introducing voltage and temperature hysteresis, four more states are 
defined in which fan 14 is off, but from which the forward and reverse 
operating states (42 and 44, respectively) can be entered under different 
conditions than are required when entering those states from initial state 
40. Thermal control system 10 either remains in state 40 or moves into 
forward mode (state 42) or reverse mode (state 44). Once in state 42, 
thermal control system 10 will remain there as long as V.sub.bat is 
greater than the second, lower predetermined voltage V2 and T.sub.bat is 
less than the second, predetermined minimum temperature T3. If V.sub.bat 
falls below V2, thermal control system 10 enters a state 46. It will 
remain in state 46 until either V.sub.bat exceeds V1, for which it will 
return to state 42, or T.sub.bat exceeds T3, for which it will return to 
initial state 40. Once in state 42, if T.sub.bat exceeds T3, thermal 
control system 10 will move to a state 48, where it will remain until 
either T.sub.bat falls below T1, for which it will return to state 42, or 
V.sub.bat falls below V2, for which it will return to initial state 40. 
Operation of thermal control system 10 is similar when the temperature of 
battery 26 is near the upper end of the T1, T2 temperature range. 
Specifically, when V.sub.bat is greater than V1 and T.sub.bat is greater 
than T2, thermal control system 10 will move from initial state 40 to 
state 44, in which fan 14 is operating in the reverse mode to cool battery 
26. From there, thermal control system 10 will move into a state 50 if 
V.sub.bat falls below V2 or will move into a state 52 if T.sub.bat falls 
below the second, predetermined maximum temperature T4. Once in state 50, 
thermal control system 10 will return to initial state 40 if T.sub.bat 
falls below T4 or will return to state 44 if V.sub.bat returns above V1. 
From state 52, thermal control system 10 will return to initial state 40 
if V.sub.bat falls below V2 or will return to state 44 if T.sub.bat 
exceeds T2. 
By way of example, and not as a limitation, typical values for the 
predetermined voltages and predetermined temperatures are as follows: 
V1=13.0 Volts 
V2=12.7 Volts 
T1=10.0.degree. Celsius 
T2=40.0.degree. Celsius 
T3=20.0.degree. Celsius 
T4=35.0.degree. Celsius 
These predetermined values represent the preferred values for a lead-acid 
wet cell battery selectively heated or cooled with a reversible 60 mA fan 
within an automotive application. The selection of the predetermined 
values is entirely battery, application, and air-flow device dependent and 
determining the preferred values will be known by those skilled in the 
art. 
A circuit suitable for providing the functions performed by control circuit 
22 is depicted in FIG. 4. Control circuit 22 includes a power supply 54, a 
voltage sensing circuit 56, a temperature sensing circuit 58, and a fan 
controller 60. Power supply 54 uses the power available from battery 26 to 
generate supply and reference voltages for the components of control 
circuit 22. Voltage sensing circuit 56 generates a logic signal indicative 
of V.sub.bat which includes the voltage hysteresis characteristic. 
Temperature sensing circuit 58 generates a pair of logic signals 
indicative of whether T.sub.bat is less than T1, greater than T2, or 
between T1 and T2 and which includes the temperature hysteresis 
characteristic. Fan controller 60 is responsive to the logic signals 
provided by voltage sensing circuit 56 and temperature sensing circuit 58 
to operate fan 14 accordingly. 
Power supply 54 includes resistors 62, 64, 66, and 68, capacitors 70 and 
72, and voltage references 74 and 76. Resistor 62 is a variable resistor 
connected between the positive terminal 27 of battery 26 and one end of 
resistor 64. The other end of resistor 64 is connected to a node denoted 
VCC. Voltage reference 74 is a three terminal regulator having its cathode 
connected to VCC, its anode connected to the negative terminal 29 of 
battery 26, which is used throughout the control circuit 22 as a common 
ground, and its output control pin connected to the wiper arm of variable 
resistor 66. One end of resistor 66 is connected to VCC and the other end 
is connected to ground. Resistor 68 is connected between VCC and a node 
denoted VREF. Capacitor 70 is connected between VCC and ground. Capacitor 
72 is connected between VREF and ground. Voltage reference 76 is a two 
terminal voltage reference having its cathode connected to VREF and its 
anode connected to ground. 
Voltage sensing circuit 56 includes resistors 78, 80, 82, 84, 86, and 88, 
capacitor 90, and operational amplifier 92. Resistor 78 is connected 
between the positive terminal 27 of battery 26 and a first end of resistor 
80 and capacitor 90. The second end of capacitor 90 is connected to 
ground. Resistor 82 is connected between the second end of resistor 80 and 
ground. Resistor 80 is a three terminal variable resistor having its wiper 
arm connected to the inverting input of op-amp 92. Resistor 84 is 
connected between VREF and the non-inverting input of op-amp 92. Resistor 
86 is connected between the output and non-inverting input of op-amp 92. 
Resistor 88 is connected between the output of op-amp 92 and a node 
denoted VOLTAGE HIGH. 
Temperature sensing circuit 58 includes thermistor 94, resistors 96, 98, 
100, 102, 104, 106, 108, 110, 112, 114, and 116, capacitors 118, 120, 122, 
and operational amplifiers 124, 126, and 128. The resistance of thermistor 
94 is strongly temperature dependent and is used in thermal control system 
10 as temperature sensor 20. Thermistor 94 is connected between VCC and 
one end of resistor 96 and capacitor 118. The other end of capacitor 118 
is connected to ground. Resistor 98 is a three terminal variable resistor 
connected between the other end of resistor 96 and one end of resistor 
100. The wiper arm of resistor 98 is connected to the non-inverting input 
of op-amp 124. Resistor 102 is a three terminal variable resistor 
connected between the other end of resistor 100 and one end of resistor 
104. The wiper arm of resistor 102 is connected to the inverting input of 
op-amp 128 The other end of resistor 104 is connected to ground. The 
output of op-amp 124 is connected to its inverting input and one end of 
resistor 106. The other end of resistor 106 is connected to the 
non-inverting input of op-amp 126 and one end of resistor 108 and 
capacitor 120. The other end of resistor 108 is connected to the other end 
of capacitor 120 and to the output of op-amp 126. The inverting input of 
op-amp 126 is connected to VREF. Resistor 110 is connected between the 
output of op-amp 126 and a node denoted TEMP LOW. Resistor 112 is 
connected between VREF and the non-inverting input of op-amp 128. Resistor 
114 and capacitor 122 are connected between the output and non-inverting 
input of op-amp 128. Resistor 116 is connected between the output of 
op-amp 128 and a node denoted TEMP HIGH. 
Fan controller 60 includes resistors 130, 132, 133, 134, 136, 138, and 140, 
capacitor 142, NOR gates 144 and 146, and transistors 148, 150, 152, 154, 
156, and 158. Resistor 130 is connected between the positive terminal 27 
of battery 26 and one end of resistor 132, one end of capacitor 142, one 
end of resistor 133, and the source of transistors 150 and 156. The other 
end of capacitor 142 is connected to ground. The other end of resistor 132 
is connected to the drain of transistor 148 and the gate of transistor 
150. The other end of resistor 133 is connected to the drain of transistor 
154 and the gate of transistor 156. The drain of transistor 150 is 
connected to the drain of transistor 152 and a first power lead 28a of fan 
14. The source of transistors 148 and 152 are connected to ground. The 
drain of transistor 156 is connected to the drain of transistor 158 and to 
a second power lead 28b of fan 14. The source of transistors 154 and 158 
are connected to ground. NOR gates 144 and 146 are two-input NOR gates, 
each having one input connected to VOLTAGE HIGH. The other input of NOR 
gate 144 is connected to TEMP LOW. The other input of NOR gate 146 is 
connected to TEMP HIGH. The output of NOR gate 144 is connected to one end 
of resistors 134 and 140. The other end of resistor 134 is connected to 
the gate of transistor 148. The other end of resistor 140 is connected to 
the gate of transistor 158. The output of NOR gate 146 is connected to one 
end of resistors 136 and 138. The other end of resistor 136 is connected 
to the gate of transistor 152. The other end of resistor 138 is connected 
to the gate of transistor 154. 
Voltage reference 74 can comprise an LM336-5.0 and voltage reference 76 can 
comprise an LM385-1.2, both manufactured by National Semiconductor. 
Thermistor 94 can be a 100K thermistor, as manufactured by Keystone Carbon 
Co. Op-amps 92, 124, 126, and 128 may each suitably comprise one-fourth of 
an TLC27L4, manufactured by Texas Instruments. NOR gates 144 and 146 may 
each be one-quarter of an MC14001UB, manufactured by Motorola. Transistors 
148, 152, 154, and 158 can each be an IRFD110, manufactured by 
International Rectifier. Transistors 150 and 156 can each be an IRFD9110, 
manufactured by International Rectifier. By way of example, and not as a 
limitation, the following tables list preferred values for the resistors 
and capacitors of control circuit 22 when control circuit 22 is used for 
automotive applications. 
______________________________________ 
Resistor 
Value Resistor Value Resistor 
Value 
______________________________________ 
62 20 K 88 10 K 112 27 K 
64 1 K 96 27 K 114 909 K 
66 100 K 98 20 K 116 10 K 
68 1.3 K 100 10 K 130 3.6 .OMEGA. 
78 1.24 M 102 20 K 132 10 K 
80 20 K 104 24 K 134 220 .OMEGA. 
82 121 K 106 100 K 136 220 .OMEGA. 
84 5.1 K 108 909 K 138 220 .OMEGA. 
86 2.2 M 110 10 K 140 220 .OMEGA. 
Capacitor 
Value 
______________________________________ 
70 .1 .mu.F 
72 .1 .mu.F 
90 1.0 .mu.F 
118 1.0 .mu.F 
120 .01 .mu.F 
122 .01 .mu.F 
142 1.0 .mu.F 
______________________________________ 
Power supply 54 is used to derive the regulated supply voltage (VCC) and 
the reference voltage (VREF). VCC is used to power various active 
components used in control circuit 22 and to derive voltages indicative of 
T.sub.bat. VREF is used to provide the voltage and temperature set points 
(V1, V2, and T1-T4). 
Voltage sensing circuit 56 is responsive to V.sub.bat to generate VOLTAGE 
HIGH. VOLTAGE HIGH is active low, i.e., it is a logical 0 whenever 
V.sub.bat exceeds the predetermined minimum voltage V1. As shown in FIG. 
4, voltage hysteresis is introduced into voltage sensing circuit 56 using 
resistors 84 and 86. In this configuration, VOLTAGE HIGH is a logical 0 
when V.sub.bat &gt;V1 and thereafter remains a logical 0 until V.sub.bat &lt;V2, 
after which V.sub.bat must exceed V1 for VOLTAGE HIGH to again become a 
logical 0. The amount of hysteresis is determined by the relative values 
of resistors 84 and 86. Where no hysteresis is desired, resistor 86 can be 
connected to ground rather than the output of op-amp 92, as will be 
appreciated by those skilled in the art. 
The operation of voltage sensing circuit 56 is as follows. The inverting 
input of op-amp 92 receives a voltage indicative of V.sub.bat. The voltage 
at the non-inverting input of op-amp 92 is set by resistors 84 and 86 such 
that when the output of op-amp 92 is at its maximum voltage, the inputs to 
op-amp 92 will be equal when V.sub.bat equals V1. When V.sub.bat &gt;V1, the 
output of op-amp 92 will go to its lowest value, which corresponds to a 
logical 0. When the output of op-amp 92 is at this lowest voltage, the 
voltage at the non-inverting input of op-amp 92 will be less than when the 
output of op-amp 92 is at its maximum value. Thus, once the output of 
op-amp 92 falls to its lowest voltage, V.sub.bat must fall below a voltage 
somewhat lower than V1 (i.e., V2) before the output of op-amp 92 will be 
driven to its maximum value. Op-amp 92 therefore operates as a comparator 
with hysteresis. 
Temperature sensing circuit 58 operates in a manner similar to voltage 
sensing circuit 56 to determine when T.sub.bat is outside of the range of 
temperatures defined by T1 and T2. As can be seen in FIG. 4, temperature 
sensing circuit 58 has two channels, one each for determining when 
T.sub.bat crosses the temperature range limits T1 and T2. The upper 
channel, which utilizes op-amps 124 and 126, corresponds to the lower 
limit T1 of the temperature range and the lower channel, which utilizes 
op-amp 128, corresponds to the upper limit T2 of the temperature range. 
Thermistor 94 is thermally coupled to battery 26 and has a negative 
temperature co-efficient such that the voltages provided to op-amps 124 
and 128 by the voltage divider defined by thermistor 94 and resistors 96, 
98, 100, 102, and 104 have a positive temperature co-efficient. 
The operation of the lower channel of temperature sensing circuit 58 (which 
corresponds to T2), is the same as that of voltage sensing circuit 56. 
Op-amp 128 generates TEMP HIGH, which is a logical 0 whenever T.sub.bat 
&gt;T2. By connecting resistor 114 to the output of op-amp 128 rather than 
ground, temperature hysteresis about the upper limit T2 can be introduced 
in the same way that voltage hysteresis is introduced in voltage sensing 
circuit 56. The operation of the upper channel of temperature sensing 
circuit 58 (which corresponds to T1) is similar. However, the inputs to 
op-amp 126 are reversed such that its output will be a logical 1, rather 
than a logical 0, when T.sub.bat &gt;T1. These connections are reversed 
because the upper channel signals fan controller 60 to operate fan 14 to 
heat battery 26 when T.sub.bat falls below T1, rather than when it exceeds 
some value, as is done for V.sub.bat in voltage sensing circuit 56 and for 
the upper temperature limit T2 in the lower channel. Op-amp 124 operates 
as a unity gain amplifier of the voltage appearing on the wiper arm of 
variable resistor 98. As in the lower channel, resistors 106 and 108 
provide the hysteresis characteristic about the lower temperature limit 
T1. 
Fan controller 60 is responsive to VOLTAGE HIGH, TEMP LOW, and TEMP HIGH 
provided by voltage sensing circuit 56 and temperature sensing circuit 58 
to selectively operate fan 14 in either the forward or reverse mode. More 
particularly, NOR gate 144 and transistors 148, 150, and 158 are 
responsive to VOLTAGE HIGH and TEMP LOW to operate fan 14 in the forward 
mode. NOR gate 146 and transistors 152, 154, and 156 are responsive to 
VOLTAGE HIGH and TEMP HIGH to operate fan 14 in the reverse mode. 
Transistors 148, 150, and 158 are switched on when the output of NOR gate 
144 is at a high logic level. This output will only be a logical 1 when 
VOLTAGE HIGH is a logical 0 (i.e., V.sub.bat &gt;V1) and TEMP LOW is a 
logical 0 (i.e., T.sub.bat &lt;T1). Similarly, NOR gate 146 turns on 
transistors 152, 154 and 156 to operate fan 14 in the reverse mode when 
V.sub.bat &gt;V1 and T.sub.bat &gt;T2. The outputs of NOR gates 144 and 146 will 
never both be a logical 1 since T1&lt;T2. Additionally, as previously 
discussed, the voltage and temperature hysteresis introduced by voltage 
sensing circuit 56 and temperature sensing circuit 58, respectively, 
provides the four additional states 46, 48, 50, and 52 shown in FIG. 3 to 
continue operating fan 14 in either the forward or reverse mode until the 
voltage hysteresis (V1-V2) or temperature hysteresis (T3-T1 or T2-T4) is 
overcome. Note that T1 should be less than T4 and T2 should be greater 
than T3 so that control circuit 22 does not attempt to operate fan 14 
simultaneously in both the forward and reverse modes. 
Referring now to FIG. 5, it illustrates a second embodiment of the present 
invention, designated generally as 200, which includes a battery housing 
202 having an air inlet 204 connected via a conduit 206 to a fan 208. A 
second conduit 210 connects fan 208 to the outlet of an air valve 212. 
Valve 212 has two air inlets, one of which is connected to a third conduit 
214 and the other of which is connected to a fourth conduit 216. A 
temperature sensor 218 is mounted in housing 202 to sense the temperature 
of an automotive battery placed therein (not shown). Fan 208, valve 212, 
sensor 218, and the battery are all electrically connected to a control 
circuit 220. 
Control circuit 220 monitors battery voltage and temperature as discussed 
in connection with the first embodiment. However, rather than reversing 
the air flow through housing 202 to selectively heat or cool the 
automotive battery, control circuit 220 operates fan 208 in one direction 
only and controls air valve 212 to select either heated or unheated air 
via conduits 214 and 216, respectively. Preferably, conduit 214 is routed 
such that it intakes heated air proximate an exhaust manifold 222 of an 
automobile engine 224, as depicted in FIG. 5. Conduit 216 is preferably 
routed underneath the front bumper (not shown) of the automobile to 
receive unheated air. 
Control circuit 220 can be implemented using the circuitry shown in FIG. 4 
for control circuit 22, except that fan controller 60 would be modified to 
operate fan 208 and valve 212 in accordance with the functions described 
above. Such modifications will be known by those skilled in the art. 
In addition to air intake 204, housing 202 should have an air outlet for 
permitting air forced into housing 202 via fan 208 to escape. This can be 
accomplished in various ways, as will be appreciated by those skilled in 
the art. For instance, housing 202 can include a removable top portion 226 
which has openings for access to the terminals of the battery contained 
within housing 202 and which operate as air outlets. Such a housing is 
shown and described in U.S. Pat. No. 5,031,712, which is hereby 
incorporated by reference. 
While the preferred embodiment disclosed herein is specifically directed to 
automotive application, the battery thermal control system and method of 
the present invention may also be employed equally as effectively in other 
rechargeable battery applications, as can be recognized by those skilled 
in the art. One example of such an application is Uninterruptible Power 
Systems (UPS) where batteries and battery packs are used to power devices 
when the primary power source fails. These systems are found commonly in 
the telecommunication, computer, cable/TV, etc. industries. Some of the 
smaller systems involve one or two batteries being enclosed in a 
protective container exposed to the environment. During the hot summer 
days this enclosure can get excessively hot, and the present invention may 
be employed in conjunction with a fan or damper to cool the batteries 
within the enclosure. During the colder winter nights, the present 
invention may be employed in conjunction with a heater or damper to warm 
the batteries within the enclosure. 
Thus, there has been provided in accordance with the present invention a 
battery thermal control system that fully satisfies the aims and 
advantages described herein. Although the invention has been described in 
conjunction with specific embodiments thereof, it is evident that many 
alternatives, modifications, and variations will be apparent to those 
skilled in the art. For instance, the functions performed by control 
circuits 22 and 220 could be carried out in a general purpose computer. 
Accordingly, it is intended that all such alternatives, modifications, and 
variations be embraced within the spirit and scope of the appended claims.