Patent Publication Number: US-2022216712-A1

Title: Battery charging cut-off circuit

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 17/097,611, filed Nov. 13, 2020, which is a divisional of U.S. application Ser. No. 15/871,999, filed Jan. 16, 2018. The entirety of each of these applications is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to the prevention of excess drainage from a rechargeable battery. 
     BACKGROUND 
     Internet of Things (IoT) devices such as, for example, data sensors, wireless routers/gateways, and switches, are typically deployed with rechargeable battery backup packs. Rechargeable battery backup packs are operative to use one or more rechargeable batteries to provide backup power when power is not provided by a main power source. In normal operation, the rechargeable batteries in a backup pack are charged by main AC (alternating current) or DC (direct current) power. If, for whatever reason, the main power source is interrupted, a device&#39;s system will switch to the backup pack to power the device until the main power recovers or until battery power is depleted. 
     Among currently available rechargeable battery technologies, due to its size, weight, capacity, price, supply/sourcing and other factors, Lithium-Ion (Li-Ion) is most commonly used in backup packs. However, due to their chemical characteristics, Li-Ion batteries should not be discharged below a predetermined voltage. There are safety concerns regarding subsequent recharging of Li-Ion batteries if the battery had previously been discharged to a voltage below the predetermined voltage level. In such a case, the recharged battery may overheat, catch fire, or even explode. For this reason, most devices and systems with rechargeable battery backup packs have low battery shut-down functions which enter a “shutdown mode”, i.e., shut down the battery backup pack when battery Status-Of-Charge (SOC) is lower than a preset system-dependent threshold level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of the disclosure will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: 
         FIG. 1  is a schematic illustration of an exemplary battery backup unit (BBU) cut-off and recharge circuit, constructed and operative in accordance with embodiments described herein; and 
         FIG. 2  is a flowchart of an exemplary BBU charging and discharging control process to be performed by a BBU cut-off microcontroller of the circuit in  FIG. 1 . 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     A battery backup unit (BBU) cut-off and recharge circuit includes: a first transistor, a power entry connection connected to a main power supply, where power from the power entry connection flows to application circuits for an electronic device, and the first transistor is positioned between a BBU and the power entry connection, and a microcontroller, where the microcontroller is operative to: detect a loss of power from the main power supply, turn on the first transistor to enable the BBU to discharge through the power entry connection to application circuits, detect a status of charge (SOC) for the BBU, and upon detecting that the SOC is under a predefined threshold, set the BBU cut-off and recharge circuit to a lockdown state by turning off the first transistor. 
     Detailed Description of Example Embodiments 
     It will be appreciated by one of ordinary skill in the art that entering a shutdown mode may not prevent the battery from continuing to discharge. For example, in shutdown mode, the battery may still be physically connected to the device/system to which it is configured to provide backup power. Very small amounts of current may therefore still flow through the battery which may continue to drain, albeit at a lower rate through a physical connection to the device/system. Shutting down the battery may therefore not necessarily prevent it from eventually discharging to a level at which recharging may be unsafe. Accordingly, if the battery remains in shutdown mode for an extended period of time, it may be still be necessary to replace, or at least disable, the battery when the main power is restored and the device/system returns to normal operations. 
     Furthermore, as a preventive measure designed to avoid permanent damage to the battery (which may be caused by leakage over time in shutdown mode), systems are often designed with an arbitrarily high charge level for triggering self shutdown. This preventive measure serves to effectively reduce the usable capacity of rechargeable batteries in the IoT systems. 
     Some commercially available battery backup units (BBUs) address these issues by adding lockout functionality in an attempt to reduce exposure to an excessively drained battery. For example, the battery power management circuit for a Cisco 1240 Connected Grid Router (also known as a “CGR1240”) is configured with lockdown functionality in addition to the shutdown functionality as described hereinabove. If main power is not restored within approximately five weeks after shutdown, the CGR1240 “locks down” the BBU, blocking BBU charging and discharging. The lockdown state prevents the BBU from recharging, and accordingly the BBU must be replaced and its internal battery discarded. 
     It will be appreciated that replacing BBUs in devices/systems in remote locations (where IoT systems are often deployed) may be very costly, and in fact may incur more expense than the replacement cost for the BBUs themselves. 
     In accordance with embodiments described herein, a battery backup unit (BBU) cut-off and recharge circuit may be employed to lock down a BBU by electronically cutting off the physical connection between a BBU and its associated device/system after a shutdown function is activated in response to a low battery condition. By cutting off the connection, the battery leakage from the BBU to the device/system may be reduced by multiple orders of magnitude, from a micro ampere range down to a nano-ampere range, or even zero. Once cut off from the device/system, battery drainage may cease other than via the intrinsic battery&#39;s internal current leakage (also known as “battery cell self-discharge”). The shutdown state, and by extension, the effective battery life, may therefore be extended from weeks to months, and possibly to over a year range, approaching the battery&#39;s typical shelf life when it would have to be replaced in any case. 
     Reference is now made to  FIG. 1 , which is a schematic illustration of an exemplary battery backup unit (BBU) cut-off and recharge circuit  100 , constructed and operative in accordance with embodiments described herein. Circuit  100  may be implemented as a component of an electronic device (e.g., an IoT device) and comprises BBU  110 , BBU cut-off microcontroller  120 , first transistor (T 1 )  130 , second transistor (T 2 )  140 , power entry connection  150 , first resistor (R 1 )  160 , second resistor (R 2 )  170 , and capacitor (C 1 )  180 . It will be appreciated that in normal operation, current may flow from connection  150  to application circuits associated with the device. It will also be appreciated that the depiction of the power entering at point  150  as “AC” is exemplary. The embodiments described herein may also support direct current (DC) power. 
     BBU  110  may be implemented as a 12 volt DC battery that is operative to provide 2-3 amperes of power in the event of an interruption of a main power source, e.g., AC power that flows into circuit  100  at connection  150 . T 1    130  and T 2    140  may be implemented as P-channel Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) which are characterized by high input gate resistance such that the current flowing through the P-channel between the source and drain is controlled by the gate voltage. For example, T 1    130  may be implemented as a DMG2302U MOSFET which has 100 nano-ampere of leakage current and is commercially available from Diodes Incorporated. In the exemplary embodiment of  FIG. 1 , the MOSFETs used to implement T 1    130  and T 2    140  may have drain source resistance R DS =8 mOhms, such that a power efficiency penalty for the circuit may be expressed as: 
       12⋅ R =(3 A ) 2 ×0.008Ω=0.072 W.   (equation 1)
 
     Or alternatively as: 
       0.072 W /(12 V⋅ 3 A )=0.2%.  (equation 2)
 
     Microcontroller  120  may be operative to monitor a SOC status for BBU  110  while main power is interrupted. Upon detecting a low SOC status, microcontroller  120  may first notify the device&#39;s system to enable the system&#39;s applications to be shut down in a controlled manner. A wait state, for example, ten milliseconds, may follow such a notification, in order to provide the system with sufficient time for the shutdown. Microcontroller  120  may then turn off T 2    140 , e.g. by using a low signal to raise T 2    140  to a high impedance state, such that the gate voltage of T 1    130  may increase through R 1    160 -C 1    180  charging over time, eventually turning off T 1    130  completely. 
     It will be appreciated that microcontroller  120  is powered by BBU  110  during main power interruption. Accordingly, microcontroller  120  will lose power after cut-off of T 1    130 . Microcontroller  120  may not be powered back on until the main power is restored, e.g., AC power entering through connection  150 . When the main power is restored, the current flows through connection  150  to power up the application circuits while also charging microcontroller  120  through T 2    140 . Once microcontroller  120  is powered up and restarts, it may turn on T 1    130  (by turning on T 2    140 ) to start charging BBU  110  with low series resistance. 
     It will be appreciated that the depiction of microcontroller  120  as a single integrated component may be exemplary; the embodiments described herein may also support the provision of the functionality of microcontroller  120  as a combination of discrete components. For example, the functionality of microcontroller  120  may be provided by individual components including memory, programmable input/output (I/O) peripherals, and processing circuitry. The processing circuitry may be implemented as a central processing unit (CPU), and/or one or more other integrated circuits such as application-specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), full-custom integrated circuits, etc., or a combination of such integrated circuits. 
     In accordance with embodiments described herein, circuit  100  may be operative to provide an additional safety measure layer of security by imposing a one minute interrupt service subroutine as part of the BBU charging and discharging control process. In this interrupt service subroutine, the battery SOC may be monitored. If per the monitoring, the battery SOC is lower than a preset threshold, BBU  110  may be cut off. In an exemplary implementation of the interrupt service subroutine, a “BBU cut-off reset control” pin may be driven by a general Input/Output (I/O) pin in microcontroller  120 . This I/O pin may be set as input and pulled-down by resistor R 2    170  when it is not driving the BBU cut-off reset control pin. Accordingly, if for whatever reason, microcontroller  120  malfunctions and does not come back into service as programmed, T 2    140  may not be turned on. Instead, C 1    180  will eventually be charged up by R 1    160 , thereby cutting off T 1    130 . Therefore, if the firmware in microcontroller  120  is corrupted or otherwise non-functional, T 1    130  may be turned off and therefore cut off BBU  110 . It will be appreciated that in such a situation, where there is an assumption that the firmware is corrupted, it may be preferable to cut off BBU  110  in order to prevent an unregulated discharge of battery power. 
     It will be appreciated that by default, T 1    130  may be turned off due to C 1    180  being charged up through R 1    160 . Accordingly, main power should be present from the start of operations in order for BBU  110  to be charged and available for use during a power interruption. Otherwise, even if BBU  110  is plugged in and available, T 1    130  will not turn on. However, in accordance with some embodiments described herein, a push button (called “start” button) may be implemented across C 1    180  to manually short C 1    180  momentarily to turn on T 1    130 . Afterwards, once the main power is available, the incoming current, e.g., from connection  150 , may power the device. 
     Accordingly, in operation, the system may boot with either the main power on, or a push of the start button. The functionality described herein may be implemented as a part of the device/system&#39;s initialization process before a main program (i.e., the program providing the functionality for which the device is configured). The initialization process may set up a timer (e.g., for one minute) and main power loss interrupt (e.g., an I/O input level change interrupt) before proceeding to the main system functions. 
     The interrupt service subroutine may start periodically according to the timer, e.g., once every minute. The subroutine&#39;s main function may be to check the SOC for BBU  110 , and based on the SOC, determine whether or not BBU  110  has enough power to continue powering the system. If not, it may send a signal to the system to shut down operations in a controlled manner. Otherwise, if either main power is on, and/or if BBU  110  has sufficient charge (e.g., at least 6%), the interrupt service subroutine may return to the main program. 
     It will be appreciated that the time set for the timer may be configurable, based on, for example, the amount of power the system uses, and/or the capacitance of BBU  110 ; the embodiments described herein may therefore support timers of lengths of other than one minute, either shorter or longer. In accordance with embodiments described herein, a base line for selecting a timer length may be that the SOC should not change more than 1% between two SOC status check intervals. 
     Similar logic may be employed for the main power loss interrupt. If the main power is lost, BBU  110  may be turned on, independent of whether BBU  110  had been turned on or off at a previous point. The assumption may be that BBU  110  has at least enough power to power the device&#39;s operation from the time that main power is lost until at least the next time the BBU SOC is checked in the next interrupt, e.g., for at least one minute as per the example provided hereinabove. For example, if the BBU SOC is close to, or even under 6%, it may be checked and turned off during the next run of the interrupt service subroutine. 
     Reference is now also made to  FIG. 2  which is a flowchart of an exemplary BBU charging and discharging control process  200  to be performed by BBU cut-off microcontroller  120  in accordance with embodiments described herein. It will be appreciated that the steps of process  200  may be described with respect to the elements of circuit  100  as described with respect to  FIG. 1 . 
     If BBU  110  is present in circuit  100  (step  210 ), microcontroller  120  may turn on (step  220 ) T 2    140  as discussed hereinabove, thereby effectively turning on T 1    130 . It will be appreciated that BBU  110  may receive power entering through connection  150  through a body diode in T 1    130 , even if T 1    130  is not yet turned on. However, by turning on T 1    130 , microcontroller  120  may enable BBU  110  to charge with low series resistance. In accordance with some embodiments described herein, at some point, e.g., when BBU  110  is fully charged or at least approaching a fully charged state, microcontroller  120  may turn off T 2    140  to effectively turn off T 1    130 . At that point, BBU  110  may continue to “top off” its charge through the T 1    130  body diode. 
     It will be appreciated that the device&#39;s application circuits may also receive current flowing through connection  150 . It will similarly be appreciated that the device uses power from the main power source to continue processing while process  200  continues to step  230  from either step  210  or  220 . 
     Microcontroller  120  may start (step  230 ) a timer for the interrupt service subroutine as described hereinabove, e.g., for one minute. It will be appreciated that it is possible that at this point T 1    130  may be turned off. For example, if BBU  110  was not present in step  210 , step  220  may not have been performed. Similarly, as described hereinabove, in some implementations, T 1    130  may be turned off after BBU  110  is fully charged. Accordingly, if T 1    130  is turned off and microcontroller  120  detects an I/O interrupt indicating an A/C power loss from the main power source (step  240 ), microcontroller  120  may turn on T 2    140  to effectively turn on (step  245 ) T 1    130 , thereby enabling current to discharge from BBU  110  across connection  150  to the device&#39;s application circuits, as well as microcontroller  120 . Otherwise, if T 1    130  is already on at the time of an A/C power loss from the main power source, BBU  110  may begin discharging through T 1    130  without active intervention by microcontroller  120 . 
     If there is a timer interrupt (step  250 ), i.e., the timer times out, microcontroller  120  may check to see if BBU  110  is present in circuit  100  in a similar manner as step  210 . If BBU  110  is not present in circuit  100 , process control may return to step  230  where another timer may be set for the next execution of the interrupt service subroutine. Otherwise, microcontroller  120  may check if there is A/C power present (step  260 ), i.e., if main power is flowing through connection  150 . It will be appreciated that microcontroller  120  may use methods known in the art to perform step  260 , for example, but not limited to, methods disclosed in U.S. patent application Ser. No. 15/261,860, entitled “ACTIVE AC POWER LOSS DETECTION” and filed on Sep. 9, 2016, which is incorporated herein by reference. If A/C power is present, there may be no need to use BBU  110  to power the device, and process control may return to step  230  where another timer may be set for the next execution of the interrupt service subroutine. 
     If there is no A/C power present, microcontroller  120  may read (step  264 ) the voltage from BBU  110 . Microcontroller  120  may then determine ( 268 ) the SOC for BBU  110  based on the voltage read in step  264 . It will be appreciated that in some embodiments, the accuracy of step  264  may be improved by microcontroller  120  first turning off (step  262 ) T 2    140  to effectively turn off T 1    130  prior to reading the voltage from BBU  110 . In such embodiments, microcontroller  120  may then turn on (step  266 ) T 2    140  to effectively turn T 1    130  back on. It will be appreciated that microcontroller  120  may receive power from C 1    180  during this brief period in which T 1    130  is turned off. It will also be appreciated that the application circuits may also be powered in a similar fashion during this period. 
     If the SOC for BBU  110  is less than a pre-defined threshold (step  270 ), e.g., 6% as per the above example, microcontroller  120  may signal (step  272 ) the device&#39;s system to shut down in anticipation of locking down BBU  110 . It will be appreciated that the pre-defined threshold to be used in step  270  may be determined, at least in part, as a function of battery type. For example, BBU  110  may be configured with different types of batteries with different battery chemistries. For example, the battery (or batteries) in BBU  110  may be lithium ion or lithium polymer, each of which may have different properties to be considered when determining an associated pre-defined threshold for shutting down BBU  110 . 
     Microcontroller  120  may then enter (step  274 ) a wait state to give the system time to shut down, e.g., ten milliseconds. Microcontroller  120  may then turn off (step  276 ) T 2    140  to effectively turn off T 1    130 . It will be appreciated that shortly after T 1    130  has been turned off, microcontroller  120  will run out of power and cease to operate. Process  200  may therefore be effectively suspended until main power is restored and process  200  is restarted from step  210 . 
     It will also be appreciated that the order of steps  266  and  268  may be exemplary; in some implementations step  268  may be performed prior to step  266 . In other implementations, the performance of step  266  may be contingent on the results of step  270 , e.g., it may be performed only if the SOC is greater than the pre-defined threshold. 
     It will also be appreciated that the embodiments described herein may support the insertion of BBU  110  as a plug-in module after the initial execution of process  200 . If BBU  110  is not present the first time steps  210  and/or  255  are run, T 2    140  may not be turned on. But if BBU  110  is plugged in after the system starts running, T 2    140  may be turned on in a subsequent iteration of step  255 , thereby turning on T 1    130  and enabling BBU  110  to being charged when main power is present and discharged when main power is lost. 
     In accordance with some embodiments described herein, some or all of BBU cut-off microcontroller  120 , first transistor (T 1 )  130 , second transistor (T 2 )  140 , first resistor (R 1 )  160 , second resistor (R 2 )  170 , and/or capacitor (C 1 )  180  may be provided in a separate, pluggable module that may be inserted into the device between connection  150  and BBU  110 , thereby facilitating a more flexible backup battery configuration depending, for example, on the likelihood of extended power outages and/or the accessibility of the device for battery replacement and maintenance. This embodiment may also facilitate the retrofitting of the protection capabilities described herein to existing systems that did not include them in their factory configuration. 
     It will be appreciated that the embodiments described herein may provide remedy for situations where battery backup is required but replacing the battery is either very difficult or very costly or both, such that longer battery life and/or longer replacement interval may be critical to the functioning of the device. For example, circuit  100  may be implemented in an IoT device in a remote/difficult to reach location such as a mountain-top, or in an arctic weather station. Circuit  100  may serve to preserve BBU  110  in a lockdown state even when AC or DC main power is absent for several months, or even a year, while enabling the associated device/system to shut down in a controlled manner. 
     It will be appreciated that the specific configuration of circuit  100  as presented in  FIG. 1  may be exemplary. The design principles described herein with respect to the embodiments of  FIGS. 1 and 2  may also be applied to other implementations of a BBU cut-off and recharge circuit. For example, in accordance with some embodiments described herein, T 1    130  may be replaced with a mechanical relay switch with self-latch capability. The mechanical relay switch may cut off the battery discharge path even more cleanly than T 1    130 , effectively extending the viability of the battery during a long lockdown state (as compared to using a MOSFET transistor) by reducing total leakage from BBU  110  to just battery cell self-discharge. However, there may be trade-offs to consider when using a mechanical relay. For example, a mechanical relay may be larger and/or may be more costly to implement than a MOSFET transistor such as T 1    130 . Accordingly, the use of a mechanical relay instead of T 1    130  may be a function of the relative importance of safety, reliability, size and/or cost of a given device. In accordance with some embodiments, both a mechanical relay and a MOSFET transistor may be implemented in series, where use of the mechanical relay is limited to locking down BBU  110  when the SOC is too low to continue powering the device from BBU  110 . 
     It will also be appreciated that the rate of BBU leakage in circuit  100  may be lower by one or two magnitudes than in non-MOSFET (and/or mechanical relay) cut-off implementations. Circuit  100  may reduce the leakage current from a tens of micro-amp level down to a nano amp range. It follows therefore, that by reducing the leakage from BBU  110  during low battery conditions, circuit  100  may effectively extend a standard interval for replacement of BBUs  110  in devices deployed in in the field, thereby serving to reduce the overall use of resources to operate the associated systems. It will, however, also be appreciated that the embodiments described herein may also support the use of n-channel MOSFET transistors instead of, or in addition to, p-channel MOSFET transistors. Similarly, the embodiments described herein may also support the use of other types of transistors that may be arrayed and manipulated to provide the same functionality as described herein with respect to MOSFET transistors. 
     In summary, the embodiments described herein may effectively detect an AC or DC main power loss condition and battery pack SOC, thereby enabling an associated device or system to turn on battery backup to compensate for main power loss and to ensure un-interrupted system operations. Upon a battery draining to a low SOC threshold level, the BBU discharge path is completely cut-off to prevent BBU  110  from over draining. Therefore, battery life may be extended and the corresponding BBU replacement cycle can be prolonged. 
     It is appreciated that software components of the embodiments of the disclosure may, if desired, be implemented in read only memory (ROM) form. The software functions may, generally, be implemented in hardware, if desired, using conventional analog or digital logic techniques. It is further appreciated that the software components may be instantiated, for example: as a computer program product or on a tangible medium. In some cases, it may be possible to instantiate the software components as a signal interpretable by an appropriate computer, although such an instantiation may be excluded in certain embodiments of the disclosure. 
     It is appreciated that various features of the embodiments of the disclosure which are, for clarity, described in the contexts of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features of the embodiments of the disclosure which are, for brevity, described in the context of a single embodiment may also be provided separately or in any suitable subcombination. 
     It will be appreciated by persons skilled in the art that the embodiments of the disclosure are not limited by what has been particularly shown and described hereinabove. Rather the scope of the embodiments of the disclosure is defined by the appended claims and equivalents thereof.