Patent Application
20130328529
This invention relates generally to the battery management field, and more specifically to a new and useful apparatus and method of battery balancing in the battery management field.
Rechargeable, multi-cell systems have been used for decades to provide the desired voltage and power to a variety of applications. However, these systems all suffer from the problem of cell imbalance. Cell imbalance arises from the differences in individual cells' state of charge (SOC), self-discharge rate, capacity, impedance, and temperature characteristics, and occur even in battery cells of the same model from the same manufacturer, due to manufacturing variations. If unregulated, cell imbalances within a battery can result in cell overcharging and/or overheating, which, in turn, results in accelerated cell degradation and in some cases, fire or explosion.
Conventionally, battery balancing has been used to equalize voltage and SOC among the cells when they are at full charge. Battery balancing is typically categorized as two types: passive and active. Passive battery balancing discharges excess energy from high voltage cells through a dissipative bypass circuit. This bypass circuit is typically composed of one or more resistive components that bleed off the excess energy as heat. While this approach is substantially low-cost, it is undesirable for many applications as the generated heat results in thermal regulation issues, which, in turn, affects battery lifespan and operation. Active battery balancing typically employs inductive or capacitive elements to temporarily store and shuttle excess energy from cells with high voltage to cells with lower voltage. However, this approach costs substantially more than the passive battery balancing approach.
Thus, there is a need in the battery management field to create an new and useful apparatus and method of battery balancing.
The following description of the preferred embodiments of the invention is not intended to limit the invention to these preferred embodiments, but rather to enable any person skilled in the art to make and use this invention.
As shown in
This battery balancing system 100 affords several advantages over the prior art. First, by selectively dissipating the excess energy from high voltage cells as light instead of heat, the need for battery thermal management can be greatly reduced, and the lifespan of the battery cells can be increased. Second, this system is distinguished over other passive systems, such as U.S. Application 2005/0,162,130, as the present embodiment allows the system to selectively bleed energy from each module 200. This allows the system to dynamically determine the voltage to which each module 200 will be charged (e.g. by bleeding off as much or as little energy as needed through the balancing circuit 400), and also to shut off energy dissipation when desired (e.g. when charging has stopped and discharging is desired). This is in contrast with the '130 reference, which will always limit the module voltage to the voltage across the balancing circuit terminals, and will constantly bleed off energy as long as the module terminal voltage is greater than the balancing circuit 400 terminal voltage, even when battery conservation is desired (e.g. during battery discharge to power a load).
The battery pack of the battery balancing system 100 functions to store energy. As shown in
As shown in
As shown in
In one variation, the balancing circuit 400 includes an LED in series with one variable resistor 460 (e.g. potentiometer, shown in
The switch 440 of the balancing circuit 400 functions to connect and disconnect the light-emitting elements 420 to and from the module/cell, and is operable between a connected mode and a disconnected mode. The switch 440 is preferably coupled in series with the light-emitting elements 420. The switch 440 is preferably an electronic switch, but can alternatively be a physical switch. The switch 440 is preferably a transistor, and can be an NPN or PNP bipolar junction transistor (BJT, such as an IGBT, avalanche transistor, etc.), an N-channel or P-channel field effect transistor (FET, such as a MOSFET, JFET, etc.), a junctionless nanowire transistors (JNT), or any other suitable electronic switch.
As shown in
The controller 600 can additionally function to receive and process system parameter measurements 602. More specifically, the controller 600 preferably controls the switch position based on a comparison between a charge threshold and information indicative of the respective module charge. The charge threshold is preferably a preprogrammed voltage value, but can be determined from a chart (e.g. given the age, charge/discharge cycles, other module history, etc.), be determined empirically (e.g. determine the discharge rate when charged to a first voltage; adjust the desired voltage if the discharge rate is out of the desired range), or be determined in any suitable manner. Information indicative of module charge can be calculated from system parameter measurements, can be the system parameter measurement itself, or can be any other suitable information. Alternatively, the switch position can be controlled based on any suitable parameter. The information indicative of module charge is preferably the module voltage, but can alternatively be the amount of charge needed to achieve the desired voltage, the amount of time needed to achieve the desired voltage, or any other suitable information. The monitored system parameters preferably include module voltage and current, but can additionally/alternatively include time, temperature, resistivity, light, or any other suitable system parameter. The controller 600 preferably monitors the aforementioned system parameters for each module 200 within the battery pack, and controls each module 200 independently. Accordingly, the system, more preferably each module 200, preferably includes suitable sensors, such as timers, voltage, current, resistance, temperature, or light sensors, or any other suitable sensor.
Each module 200 can additionally include a sub-controller that functions to control the cells and/or cell balancing circuits 400 based on information indicative of the cell charge. Like the system controller, the module sub-controller can receive and process system parameter measurements (such as cell voltage, current, temperature, resistance, etc.) into information indicative of cell charge.
The battery balancing system 100 can additionally include a light pipe 700, which functions to remove the emitted light from the battery pack. A first end of the light pipe 700 is preferably positioned proximal the light emitting elements of the balancing circuit 400, while the second end of the light pipe 700 is preferably positioned substantially near the exterior of the battery pack. The battery balancing system 100 can additionally include light-concentrating elements, such as reflectors (mirrors), that assist in light dissipation and/or transport.
In a first variation, shown in
In a second variation, the light pipe 700 is an optical fiber that transmits the emitted light to a battery pack-battery charger interface, wherein the battery charger detects the transmitted light. The battery charger preferably ceases battery pack charging when the battery charger detects light emission from all the balancing circuits 400 of the battery pack (charging termination condition). The battery charger preferably includes one photodetector for each balancing circuit 400 of the battery pack, but can alternatively include a single photodetector, wherein the photodetector determines the charging termination condition based upon the lumens of emitted light.
As shown in
This method is preferably utilized with the battery balancing system 100 as described above, but can alternatively be utilized with any battery pack including multiple battery modules each coupled to a balancing circuit in parallel, wherein each battery module includes at least one battery cell and is coupled to the other modules in series. Each balancing circuit includes at least one light-emitting element (e.g. an LED) and a switch. A battery charger that includes an AC/DC or DC/DC conversion circuit preferably charges the battery modules, wherein the battery charger couples to the battery pack or battery module terminals.
Monitoring a system parameter indicative of module charge S200 functions to monitor the system for modules with excess charge. The system parameter is preferably monitored by a system controller coupled to a plurality of sensors. The system parameter is preferably monitored for each battery module, but can alternatively be monitored for a group of battery modules. The system parameter can be module voltage, current, temperature, resistance, charge duration, or any other suitable parameter. Accordingly, each module or group of modules being monitored preferably includes a voltage sensor, current sensor, temperature sensor, resistance sensor, timer, or any other suitable sensor.
Determining that a dissipation condition has been met S300 functions to identify modules with excess charge. The dissipation condition is preferably met when the substantially instantaneous module voltage and/or state of charge (SOC) exceeds the charge threshold and/or desired SOC (e.g. module overcharging), but can be met when the substantially instantaneous module voltage/SOC is substantially close to the charge threshold/SOC (e.g. is within 90-95% of the charge threshold). The substantially instantaneous module voltage/SOC is preferably determined from a measured system parameter. In a first variation, the instantaneous module voltage/SOC is the measured module voltage. In a second variation, the instantaneous module voltage/SOC is calculated from the module current and the module resistance. In a third variation, the instantaneous module voltage/SOC is calculated from the heat generated by the charging module. However, the dissipation condition can be met when a predetermined charging time is met (e.g. the system determines the initial module charge, the desired module charge, and the amount of time needed to charge the module to the desired module charge). The dissipation condition can alternatively be met when the module has been charged the predetermined charging amount. The predetermined charging amount is preferably calculated as the difference between the initial module charge and the desired module charge, and the amount the module has been charged is preferably determined from the amount of power adsorbed by the module (e.g. from the module resistance and current flow into the module). However, the dissipation condition can alternatively be any suitable condition indicative of a need for excess module energy dissipation. The charge threshold/SOC is preferably a predetermined (e.g. preprogrammed) voltage/SOC threshold, but can alternatively be selected from a chart (e.g. based on the number of module charge/discharge cycles, discharge rate, charge rate, or any other historical parameter), determined dynamically (e.g. wherein the charge threshold/SOC is lowered if the discharge or charge rate is too high for a given module), or determined in any suitable manner.
Connecting a balancing circuit to the module S400 functions to dissipate the excess module energy as light. This step is preferably performed by the switch of the balancing circuit, wherein a system controller preferably controls the switch position. This is preferably performed in response to the determination of a dissipation condition being met.
The method can additionally include disconnecting the module from the battery charger S500, which functions to cease energy input into the overcharged module. Charger disconnection is preferably performed substantially simultaneously with balancing circuit connection, particularly when the disconnection condition is module overcharging. However, charger disconnection after balancing circuit connection can be preferred when the disconnection condition is met when the module voltage/SOC nears the charge threshold/SOC. Charger disconnection preferably only includes cessation of power provision to the battery module, and does not include physical disconnection of the battery module from the battery charger. However, power provision to the battery module can otherwise be ceased (e.g. through physical disconnection of the battery module from the charger). Alternatively, the charger can not be disconnected until all the modules of the battery pack have reached the respective desired charge state, wherein the balancing circuits of the fully charged modules continuously dissipate the excess energy fed into the modules.
The method can additionally include monitoring the module while the balancing circuit is connected S600, which functions to determine when a module has been discharged to the desired charge. This step is preferably performed in substantially the same manner as S200, wherein system parameters are monitored to determine the module charge/discharge state. In a first variation, the amount of module charge is monitored by measuring the module voltage/SOC. In a second variation, the amount of module discharge is monitored by measuring an initial module voltage/SOC and measuring a second module voltage/SOC, wherein the module discharge amount is the difference between the two. In a third variation, the module discharge is monitored by a timer, wherein the system determines the amount that the module should be discharged (e.g. based off the initial module voltage/SOC and the charge threshold/SOC) and calculates the amount of time the balancing circuit needs to be connected (based off the balancing circuit discharge rate).
The method can additionally include determining that a disconnection condition has been met S700, which functions to identify modules that have reached the charge threshold. In a first variation, the disconnection condition is preferably met when the module voltage falls to the charge threshold/SOC, or can be met when the module voltage/SOC falls within a range of the charge threshold/SOC (e.g. between 95% and 105% of the charge threshold). In a second variation, the disconnection condition is met when the duration of module discharge (e.g. the amount of time the balancing circuit has been connected to the module) meets a predetermined time. This predetermined time is preferably calculated from the initial module voltage/SOC, the charge threshold/SOC, and the known dissipation rate of the balancing circuit. In a third variation, the disconnection condition is met when the amount of module discharge meets a predetermined discharge threshold. The threshold is preferably calculated as the difference between the initial module voltage/SOC and the charge threshold/SOC. The amount of module discharge can be calculated (e.g. from the known balancing circuit dissipation rate and the duration of balancing circuit connection), can be measured (e.g. by measuring and recording the current through the balancing circuit), or can be determined in any other suitable manner.
The method can additionally include disconnecting the balancing circuit from the module S800, which functions to end energy dissipation from the module. Balancing circuit disconnection is preferably performed after determination of a disconnection condition being met. Balancing circuit disconnection is preferably accomplished by placing the switch of the balancing circuit in an open configuration, and is preferably controlled by the system controller. Alternatively, balancing circuit disconnection can be accomplished by physical removal of the battery charger, wherein the balancing circuit is a portion of the battery charger.
The method can additionally include the step of determining a new charge threshold S900, which functions to adjust the desired state of charge to which a given module is charged. A new charge threshold is preferably determined when a trigger event is met. The trigger event is preferably indicative of a decrease in battery module performance. The trigger event can be dependent on module temperature, discharge rate, charge rate, module resistivity, or any other module parameter. For example, the trigger event can be an increased module temperature during or after charging (e.g. over a predetermined temperature threshold), an increased discharge rate (e.g. beyond a threshold discharge rate), a decreased charge rate (e.g. beyond a threshold charge rate), or higher resistivity. However, any other events indicative of the overall battery quality can serve as the trigger event. The charge threshold is preferably decreased in response to detection of a trigger event, such that the lifespan of the battery module can be extended (e.g. prevent overcharging). However, the charge threshold can be increased to compensate for the decrease in battery performance. The modified charge threshold is preferably determined from a chart (e.g. based on the module age, rating, discharge/charge rate, or any other suitable parameter), but can be predetermined (e.g. the charge threshold is adjusted a predetermined amount) or can be empirical, wherein the charge threshold is continuously adjusted until the trigger event is no longer detected. The modified charge threshold is preferably determined by the system controller, but can alternatively be determined by the module controller, the charger controller, an external controller, or any other suitable means.
As shown in
In a second example of the battery balancing method, the method includes: charging a plurality of battery modules, determining the amount of charge each module requires to meet the charge threshold (dissipation condition); monitoring the amount each module is charged (e.g. by monitoring the power transfer, current, etc.); and connecting the balancing circuit, wherein the balancing circuit is configured to dissipate power from the module until the charge threshold is met (e.g. if the charge threshold is 7V, then the balancing circuit includes the suitable number of LEDs).
As a person skilled in the art will recognize from the previous detailed description and from the figures and claims, modifications and changes can be made to the preferred embodiments of the invention without departing from the scope of this invention defined in the following claims.
