Abstract:
The invention relates to a power management system for supplying backup DC power to peak and/or high current demand battery applications, such as motor starting or an uninterruptible power supply (UPS) used to power a critical load, such as, a data bus or other critical load, after an event, such as loss of primary AC or DC input, during relatively cold ambient temperatures. Two or more heaters may be provided; for example, a low power heater and a high power heater. In a maintenance mode, the low power heater is used to maintain the batteries at a predetermined temperature. In this mode, the battery charger is used to power the low power heater. In a boost mode, after the primary AC or DC input is restored, and the battery temperature is too low to back up the critical load, the battery charger supplies power to one or both of the heaters. Since the capacity of the battery charger is normally insufficient to heat the batteries to an acceptable operating temperature in a relatively short period of time, a portion of the residual power from the batteries is used to boost the power to the heaters in order to speed up the time to get the battery to its rated operating temperature.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a power management system for enabling back-up batteries to supply peak and/or high current demand DC loads, such as DC motor starting or an uninterruptible power supply (UPS) used to power a critical load, such as, a data bus or other critical load after an event, such as a loss of primary AC or DC power, during relatively cold ambient temperatures. The power management system includes one or more heaters, for example, a low power heater and a high power heater. In a maintenance mode, when the data bus or other critical load is fed from a primary power source, the low power heater is used to maintain the battery packs at a predetermined temperature, such as the desired operating temperature, so that the batteries can provide their minimum required capability upon loss of the primary power source. In this mode, the battery charger powers the low power heater and also maintains the charge on the batteries. After an event, such as a loss of the primary power source, the battery temperature can become too low to enable the batteries to provide the required capacity to back up the critical load. During such a condition, the battery charger supplies power to one or both of the heaters as well as to the batteries. Since the capacity of the battery charger is normally insufficient to heat the batteries to an acceptable operating temperature in a relatively short period of time during such a condition, a portion of the residual power in the battery is used to boost the power to one or both of the heaters in order to speed up the time to get the battery to normal operating temperature. 
         [0003]    2. Description of the Prior Art 
         [0004]    Conventional systems which supply electrical power to critical loads, such as data buses or other critical loads, are known to be powered from an uninterruptible power system (UPS). Such UPS systems utilize a primary AC or DC power supply and a DC back-up power supply. Critical loads, such as data buses, are known to require DC power. As such, during normal operation, the AC power from the primary AC power supply is converted to DC by way of a converter and supplied to the critical load. Because of the criticality of the load, a back-up DC power supply is also selectively connectable to the critical load in the event of loss of the primary AC power supply. 
         [0005]    It is important that the DC back-up power supply be maintained at virtually full capability at all times. Unfortunately, some back-up battery systems may be exposed to relatively cold ambient conditions after a loss of the primary power source. Depending on the ambient temperature, the back-up battery system may not be able to deliver full capacity DC current to the critical load. The reason for this is that the internal resistance/impedance of the battery is inversely proportional to the temperature of the battery. Thus, the internal resistance of the battery will be relatively high at relatively low temperatures preventing the battery from delivering its full capacity. In addition, as batteries age, the internal resistance of the batteries tends to increase causing the battery to provide less output current and capacity. 
         [0006]    Because of the criticality of the load, different schemes have been provided to heat-up the back-up battery system when exposed to relatively cold ambient temperatures after a loss of the primary power source. Unfortunately, the primary power source can be unavailable for some period of time. During that time, the temperature of the battery can drop to a relatively low level. Once the primary power source becomes available, it is necessary that the battery capacity be sufficient to safely shut down the critical after a subsequent power loss, the required capacity. As such, the batteries are normally heated by the battery charger to a temperature that enables the batteries to deliver their rated capability. Unfortunately, the capacity of battery chargers in known systems do not have sufficient capacity to heat the batteries quickly. As such, the critical load must remain off-line until the batteries are heated to the desired operating temperature. Thus, there is a need to provide a system for heating up a battery to a temperature at which it can deliver its required current and capacity in a relatively short time in order to minimize the exposure of the critical load to a total loss of electrical power during a loss of the primary AC power system. There is also a need for compensating aging batteries so that such batteries provide a relatively constant performance over time, 
       SUMMARY OF THE INVENTION 
       [0007]    The invention relates to a power management system for enabling back-up batteries to supply peak and/or high current demand DC critical loads, for example, a data bus or other critical load after an event, such as a loss of primary AC or DC power, during relatively cold ambient temperatures. The power management system includes one or more heaters, for example, a low power heater and a high power heater. In a maintenance mode, when the data bus or other critical load is fed from a primary power source, the low power heater is used to maintain the battery at a predetermined, desired operating temperature so that the batteries can provide their required capacity upon loss of the primary power source. In this mode, the battery charger is used to power the low power heater as well as maintain the charge on the batteries. In a boost mode, it is assumed that the battery charger is available and the battery temperature is too low to back up the critical load. During this mode, the battery charger supplies power to the high power heater. Since the capacity of the battery charger is normally insufficient to heat the batteries to an desired operating temperature in a relatively short period of time, a portion of the residual power in the battery is used to boost the power to the high power heater in order to speed up the time to get the battery to normal operating temperature. 
     
    
     
       DESCRIPTION OF THE DRAWING 
         [0008]    These and other advantages of the present invention will be readily understood with reference to the following specification and attached drawing wherein: 
           [0009]      FIGS. 1A and 1B  is a simplified schematic diagram of the self-heating battery circuit in accordance with an aspect of the invention-shown connected to a battery charger and an external AC power input. 
           [0010]      FIGS. 1A and 1B  is an exploded view of the computing device illustrated in  FIGS. 1A and 1  BA, shown with all its pin designations. 
           [0011]      FIG. 2  is an alternative embodiment of the self-heating battery circuit illustrated in  FIGS. 1A and 1B . 
           [0012]      FIG. 3  is an exemplary exploded physical isometric diagram of a core pack which includes batteries and the self-heating battery circuit illustrated in  FIGS. 1A and 1B   
           [0013]      FIG. 4  is an isometric drawing of the core pack illustrated in  FIG. 3 , shown fully assembled. 
           [0014]      FIG. 5  illustrates several exemplary thermal curves superimposed on a time versus temperature graph illustrating the heater data showing the core pack temperature versus time for 30 W, 45 W, 200 W and 220 W heaters on a 0° C. ambient. 
           [0015]      FIG. 6  illustrates a table of exemplary data associated with the thermal curves illustrated in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The invention is part of an uninterruptible power system used to provide continuous power to a critical load, such as, a data bus during loss of AC or DC power. In particular, the invention relates to a power management system  20  which relies on residual current from the batteries in addition to power from a battery charger to heat batteries to their desired operating temperature, as well as charge the batteries  40 , after the batteries have been exposed to relatively low temperature conditions after an event, such as a loss of the primary AC or DC input  42 . In particular, the batteries  40  are used to back up a critical load  38  in the event of loss of the primary AC or DC. In order for the batteries  40  to provide enough power to safely shut down a critical load  38 , such as a data bus, after an event, such as a loss of the primary AC or DC input  42 , the batteries  40  must be fully charged and at their desired operating temperature. As such, the power management system  20  measures the battery state of charge and the battery temperature and allows the primary AC or DC input  38  to be reconnected to the load  38  when the batteries are charged to the required capacity and the batteries are at the desired operating temperature. 
         [0017]    The battery temperature is sensed by a temperature sensor  43 . The temperature sensor  43 , for example, a thermistor, thermocouple or other temperature sensor senses the ambient temperature in the vicinity of the batteries  40  and transmits a signal back to the computing device  22 . 
         [0018]    The state of charge of the battery may be determined by the charging current and the voltage. In particular, the charging current is sensed by a current sensing resistor  39  which measures the charge current supplied to the batteries  40 . The voltage of the batteries  40  is also sensed. This information is provided to the computing device  22  which can determine the state of charge of the batteries  40  from the charging current and voltage of the batteries  40 . As such, the computing device  22  can determine when the batteries  40  are charged to the required capacity and when the batteries are up to their desired operating temperature in order to reconnect the primary AC or DC input to the load  38 . 
         [0019]    Assuming a condition when the batteries are exposed to extremely cold ambient temperatures, for example, 0° C., after an event, such as a loss of the primary AC or DC input  42 , it is necessary that the batteries  40  be fully charged and heated to the desired operating temperature, after the primary power is restored but before the primary AC or DC input can be reconnected to the critical load. Otherwise, the batteries  40  would not be able to supply sufficient power to the critical load during a subsequent loss of the primary AC or DC input  42 . 
         [0020]    As such, when the primary AC or DC power is restored, it is important to get the batteries  40  to their desired operating temperature so that they can provide sufficient power to the critical load so that the battery can be available to reconnect to the critical load  38  as soon as possible. In addition, the batteries  40  need to be maintained at their desired operating temperature during all operating conditions. 
         [0021]    In order to deal with temperature changes of the batteries  40  due to fluctuating ambient conditions, the power management system includes heaters, for example, a high power heater  24  and a low power heater  26 . For example, in a maintenance mode of operation, i.e. while the primary AC or DC input  42  is connected to the load  38 , the battery charger  41  provides power to the low power heater  26  as a function of the battery temperature in order to maintain the temperature of the batteries  40  at a predetermined, desired operating temperature, for example 25° C. After an event, such as loss of the primary AC or DC input  42 , the temperature of the batteries can drop to a relatively low level, for example, 0° C., depending on the ambient temperature and the length of time the primary AC or DC input  42  is not available. Once the primary AC or DC input  42  is restored, the battery charge and temperature must be returned to their desired operating values. As such, during such a condition when the batteries  40  are exposed to relative low ambient temperatures, the power management system  20  enters a boost mode. In this boost mode, the batteries  40  are charged by the battery charger  41  and heated by one or both of the low power heater  26  and the high power heater  24 . In particular, upon initial loss of the primary AC or DC input  42 , the batteries  40  are used to safely shut down the critical load  38  resulting in the batteries  40  being partially discharged. Once the primary AC or DC input  38  is restored, the batteries  40  are charged by the battery charger  41 . If the batteries  40  are also at a relatively low temperature after such an event, the battery charger  41  multitasks. In this condition battery charger  41  supplies power to the batteries  40  and one or both of the low power heaters  26  and the high power heater  24  at the same time. During this condition, the switches  32 ,  34  and  36  are closed to enable the battery charger  41  to power one or both of the heaters  24  and  26 . In accordance with an important aspect of the invention, the power management system  20  enables residual current in the batteries  40  to boost the current being supplied to one or both of the heaters  24  and  26 . In particular, in a boost mode, the switch  30  is closed. This allows power from the battery charger  41  and the batteries  40  at the same time to provide power to one or both of the heaters  24  and  26 . In this configuration the switch  28  is open to isolate the batteries  40  from the load  38 . However, with the switch  30  closed, the residual current of the batteries  40  back feeds the closed switch  30  in order to add to or boost the current being supplied to one or both of the heaters  24  and  26  in order to speed up the time to get the batteries  40  up to a predetermined operating temperature, for example 25° C., while charging the batteries  40 . 
         [0022]    Referring to  FIGS. 1A and 1B , the power management circuit  20  includes a computing device  22  and a number of switches which may be implemented by FETs, BJTs, relays or virtually any switching device. In the exemplary schematic illustrated in  FIGS. 1A and 1B , five (5) switches  28 ,  30 ,  32 ,  34  and  36  are shown. These switches  28 ,  30 ,  32 ,  34  and  36  are controlled by the computing device  22 , for example, a digital signal processor (DSP) or other computing device. 
         [0023]    All of the various switches are controlled by the computing device  22 . The operation of each of the switches is discussed below. 
         [0024]    The switch  28  may be used to selectively connect the load, illustrated by the reference numeral  38 , to the batteries  40 , during a loss of the primary AC or DC input  42 . 
         [0025]    The switch  30  connects the batteries  40  to the battery charger  41 . Each of the heaters  24  and  26  has its own switch  34  and  36 , serially connected to the heaters  24  and  26 , respectively. The switches  34  and  36  are connected to a main heater switch  32 . The switch  32  is an optional safety switch, which is normally closed. In the event of failure of the switches  34  or  36  in a closed position, the switch  32  can be opened to disconnect the heaters  24  and  26  from the battery charger  41  or the batteries  40 . 
         [0026]    During a normal condition, when the primary AC or DC input  42  is available, the switch  28  is open and the load  38  is fed from an external primary source of AC or DC power (not shown). While the primary AC or DC power source is available, the computing device  22  monitors the charge of the batteries  40  by way of the Ports PV 1 , PV 2 , PV 3  and PV 4 . Over time, the charge on the batteries  40  will dissipate. In order to keep the batteries  40  fully charged, the battery charger  41  is maintains the batteries  40  at required charge so that they are available at required capability in the event of a loss of the primary AC or DC power supply. Various battery chargers are suitable for this application. Exemplary battery charging techniques are disclosed in U.S. Pat. Nos. 8,436,583; 7,898,220; 7,683,574; 7,626,362; 7,394,225; and 7,227,337, hereby incorporated by reference. 
         [0027]    In addition, during a maintenance mode, the temperature of the batteries  40  is monitored in order to maintain a relatively constant battery temperature, for example, 25° C. In this mode, the battery charger  41  powers the low power heater  26  to maintain the temperature of the batteries  40  to be constant. During this mode, the switch  34  is selectively closed. During a normal mode of operation, when the load  38  is being fed from the primary AC or DC power supply (not shown), the switch  28  is open which isolates the batteries  40  from the load  38 . The switch  30  which selectively connects the battery charger  41  to the batteries  40  to allow the batteries  40  to be charged by the battery charger  41 , as a function of the state of charge of the batteries  40 . 
         [0028]    The switches  34  and  36  are used to selectively connect the heaters  24  and  26  to the battery charger  41 . The switch  32  is an optional safety switch that is normally closed and can be used in the event of failure of the switches  34  and  36  in order to disconnect the heaters  24  and  26  during such a condition. The switches  34  and  36  are selectively controlled as a function of the ambient temperature to which the batteries  40  are exposed. During a maintenance mode, as discussed above, the switch  36  is selectively controlled to allow the low power heater  24  to be powered by the battery charger  41  in order to maintain the temperature of batteries  40  to remain fairly constant at the desired operating temperature. Depending on the ambient temperature, the computing device  22  will signal the switch  34  to enable the battery charger  41  to power the low power heater  26  to maintain the batteries  40  at a desired operating temperature, for example, 25° C. Once the batteries  40  are at desired operating temperature, the switch  34  may be turned on and off under the control of the computing device  22  by way of closed loop temperature control. 
         [0029]    In another operating mode, namely a boost mode, after a loss of the primary AC power supply and if the batteries  40  are exposed to a relatively cold ambient temperature, after a restoration of the primary AC or DC input  42 , the battery charger  41  is multi-tasked. In this mode of operation, the battery charger  41  provides power to the high power heater  24  by way of the closed switches  30 ,  32  and  34 . In accordance with the invention, in order to speed up battery heating, the residual charge in the batteries  40  is used to boost the power to the heater  26  to speed up the heating time of the batteries  40 . During this boost mode, the switch  28  remains opened to isolate the batteries  40  from the load  38  until the batteries are at a predetermined, required capacity. 
         [0030]    An exemplary battery pack  40  is shown which includes eight batteries configured as four (4) pairs of lithium ion batteries in series. Each of the four (4) exemplary pairs includes two (2) batteries in parallel. Exemplary batteries may include battery cells of the type high-rate 18650 3.6-3.7V nominal with 20-40 Ampere rate capability. Such batteries are used to back-up a nominal 12 volt DC load  38  and can collectively provide about 1-5 KW of power. Other batteries are contemplated, such as lead acid and other batteries. 
         [0031]    In alternate embodiments, multiple heaters may be used in concert with multiple sensors located throughout the battery pack to heat different regions of the battery pack at different rates depending on the temperature. This could provide a uniform battery temperature that would help to maintain the internal charge balance, provide more uniform impedance across the pack, and improve the accuracy of pack capacity measurements. As such, multiple heater units would be controlled individually to provide localized heating to create a uniform temperature across one or multiple cells in the battery pack in the case that thermal differences or thermal gradients exist across the battery. 
         [0032]    A block diagram of an alternative embodiment of the self-heating control circuit  20  is shown in  FIG. 2 . The block diagram may include a battery management system (BMS) for providing safety limits and measuring various battery parameters including temperature. The BMS may include the computing device  22  ( FIGS. 1A and 1B ) and be programmed to provide battery management functions, for example, as disclosed in U.S. Pat. No. 6,456,046, hereby incorporated by reference. As well as control the various switches, as discussed above. 
         [0033]      FIGS. 3 and 4  illustrate an exemplary physical embodiment of other aspects of the invention.  FIG. 3  is an exploded perspective of an exemplary embodiment of the invention.  FIG. 4  is a side elevation view of an assembled version of the device illustrated in  FIG. 3 . 
         [0034]    Referring first to  FIG. 3 . In particular, an important aspect of the invention relates to the use of a heat sink or heat spreader, generally identified with the reference numerals  50  and  52 . The heat sinks  50  and  52  are disposed between the heaters  54  and  56  and the battery packs, generally identified with the reference numeral  58 . The heat spreaders  50  and  52  have two (2) functions. During a condition when the battery packs  58  are being heated, the heat spreaders  50  and  52  spread the heat among the battery cells to provide uniform heating of the battery cells from the heaters  54 ,  56  to the battery packs  58 . 
         [0035]    During a discharge condition, when the battery packs  40  are typically rapidly discharged, 200-300 amps, for example. During such a discharge, the battery packs  58  will heat up significantly. During this condition, the heat sinks  50  and  52  function to channel the heat away from the battery packs  58 . 
         [0036]    The heat spreader/sinks  50  and  52  may be made from aluminum, for example up to ¼ inch thick or more or other heat conductive materials such as a copper or graphite or various phase change materials. The heat spreader/sinks  50  and  52  may be formed to match the undulating surface of the battery packs  58 , as shown in  FIG. 3 . 
         [0037]    The embodiment shown in  FIG. 3  is merely exemplary. Multiple battery packs  58  with eight (8) cells each are shown. As shown, each 8 cell pack is in parallel and provides a nominal 3.7 volts DC. The four (4) eight cell packs are connected in series to provide a nominal 14.8 volts DC. Other configurations are contemplated. 
         [0038]    In addition, the embodiment in  FIG. 3  illustrates two (2) heaters  50  and  54 . The invention also contemplates a single heater as illustrated in  FIG. 2 . Moreover in multi-heater embodiments, the heaters  54  and  56  can have the same or different capacities. For example, one heater may be a high capacity 200 watt or 220 watt heater and the other heater may be a low capacity heater, such as a 30 watt or 45 watt heater. The heaters may be flexible polyimide heaters as manufactured by Watlow, model no. K030050C3-0009B. 
         [0039]    In order to provide electrical isolation between the battery packs  58  and the heaters  54  and  56 , electrical insulation tape  60  and  62  is provided between the heaters  56  and  54  and the battery packs. Thermally conductive pads  64  and  66  may be provided between the heat sinks  50  and  52  and the insulation tape  60  and  62 . Electrical insulation paper  68  and  70  is also provided between the heaters  56  and  54  and an external housing (not shown). 
         [0040]    Referring to  FIG. 4 , one or more rod heaters (not shown) may be disposed in the interstices  72 ,  74  and  76  of the battery packs  58 . For the battery pack  58  shown in  FIG. 4 , three (3) rod heaters (not shown) may be disposed in the interstices  72 ,  74  and  76 . These rod heaters may be used in lieu of or in addition to the  54  and  56 . The rod heaters may be Cal Rod type heaters or Watlow “Firerod” cartridge heaters. 
         [0041]      FIGS. 5 and 6  illustrate exemplary data regarding the heating of the batteries in a 0° C. ambient. Ideally, the batteries should be brought up to a desired operating temperature of about 25-50° C. As shown and illustrated in  FIGS. 5 and 6 , thermal curves for the low power 30 watt and 45 watt heaters as well as the high power 200 watt and 220 watt heaters. 
         [0042]    In accordance with another aspect of the invention, it is also considered to use the heaters to offset impedance increase of the battery due to aging of the batteries by increasing the desired operating temperature according to in-situ DC resistance measurements taken during use. It is also considered that the DC-resistance measurements be taken using the two different heaters in the invention as the loads necessary to discharge the battery at two different currents for this DC resistance measurement. More specifically, in order to determine the change in the DC-resistance (also referred to as DC impedance) of the battery pack, measurements are taken using the two different heaters of different wattage requirements at the discharge loads necessary to determine the DC resistance. The delta between the voltage points at two different discharge loads are used to determine DC-resistance according to the equation dV/dI=R, where R is the DC resistance. 
         [0043]    Obviously, many modifications and variations of the invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above. 
         [0044]    What is claimed and desired to be secured by a Letters Patent of the United States is: