Abstract:
A system and method for identifying and responding to exceptional charge events of series-connected energy storage elements. An exceptional charge event detector for an energy storage system having a plurality of series-connected battery elements, comprising: a data acquisition system determining, for each battery element of the plurality of series-connected battery elements, a series of charge dependent parameters during an operational period of the energy storage system; a controller, coupled to said data acquisition system, establishing a characterization for each battery element of the plurality of series-connected battery elements from said series of charge dependent parameters; wherein said characterization includes either a normal characteristic pattern or an exceptional characteristic pattern for each time segment of a plurality of time segments spanning said operational period; and wherein said controller detects an exceptional charge event for a selected one battery element whenever said characterization for said selected one battery element includes an exceptional characteristic pattern in any particular time segment where a normal characteristic pattern is expected.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to chargeable battery packs, and more specifically, but not exclusively, to detection and remediation of an exceptional charge state of a series element of a chargeable battery packs. 
     BACKGROUND OF THE INVENTION 
     The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions. 
     Battery packs, for purposes of this disclosure, are series-connected battery elements. These elements may, in turn, include a parallel, series, or combination of both, collection of chargeable energy storage cells, usually rechargeable cells. Collectively all these cells store energy for the battery pack. The series-connected battery elements may, in turn be subdivided into collections of modules, each module including one or more series-connected battery element. 
     In many instances, the battery pack may be treated as a monolithic unit, providing energy for operation. However, to enable such treatment, individual cells, series-elements, and modules are processed in order to achieve a desired average monolithic effect. One particular concern is that at the individual level, the series elements are not the same and store differing amounts of energy and charge/discharge at different rates. These variations are natural and expected. In certain situations, the variations can lead to an exceptional variation, defined herein as an exceptional charge state, in which an individual series element is excessively overcharged or overdischarged (as compared to some threshold). 
     Overcharge of a lithium-ion battery can lead to thermal runaway, either directly or via increased susceptibility to abuse due to decreased chemical stability. When charging a battery pack which includes more than one series element, an initial imbalance in the state of charge of the series elements can result in overcharge of one or more of the series elements, even when the voltage of the battery pack does not indicate overcharge. There are conventional solutions to mitigate this potential hazard that monitor series element voltages and are in place to ensure balance prior to charging. However, due to the potential severity of exceptional charge events, particularly for overcharge events, additional systems for identifying series element overcharge in a battery pack before the event becomes severe are desired in the event that the voltage monitoring and balancing system malfunctions or may otherwise insufficiently address the exceptional charge event. 
     What is needed is a system and method for identifying exceptional charge events of series-connected energy storage elements, and responding appropriately to detected exceptional charge events. 
     BRIEF SUMMARY OF THE INVENTION 
     Disclosed is a system and method for identifying and responding to exceptional charge events of series-connected energy storage elements. The following summary of the invention is provided to facilitate an understanding of some of technical features related to detecting to exceptional charge events of series-connected elements and responses thereto, and is not intended to be a full description of the present invention. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, drawings, and abstract as a whole. The present invention is applicable to other implementations in addition to electric vehicles such as stored energy cases providing energy time shifting of renewable energy generation (e.g., solar and wind generators), to other arrangements of series-connected energy storage elements, and may be applied to other cell chemistries. 
     An exceptional charge event detector for an energy storage system having a plurality of series-connected battery elements, including a data acquisition system determining, for each battery element of the plurality of series-connected battery elements, a series of charge dependent parameters during an operational period of the energy storage system; a controller, coupled to the data acquisition system, establishing a characterization for each battery element of the plurality of series-connected battery elements from the series of charge dependent parameters; wherein the characterization includes one of a normal characteristic pattern and an exceptional characteristic pattern for each time segment of a plurality of time segments spanning the operational period; and wherein the controller detects an exceptional charge event for a selected one battery element whenever the characterization for the selected one battery element includes an exceptional characteristic pattern in any particular time segment where a normal characteristic pattern is expected. 
     A rechargeable energy storage system having a plurality of series-connected lithium-ion battery elements, including a voltage measurement and balancing system detecting and reducing voltage imbalances between voltage levels of the plurality of battery elements of the energy storage system wherein the voltage measurement and balancing system uses a first detection modality to determine an existence of a voltage imbalance; and an exceptional charge event detector monitoring the plurality of battery elements for an exceptional charge event using a second detection modality different from the first detection modality. 
     A computer-implemented method for detecting an exceptional charge event for an energy storage system having a plurality of series-connected battery elements, including a) determining, for each battery element of the plurality of series-connected battery elements, a series of charge dependent parameters during an operational period of the energy storage system; b) establishing a characterization for each battery-element of the plurality of series-connected battery elements from the series of charge dependent parameters; wherein the characterization includes one of a normal characteristic pattern and an exceptional characteristic pattern for each time segment of a plurality of time segments spanning the operational period; and c) detecting an exceptional charge event for a selected one battery element whenever the characterization for the selected one battery element includes an exceptional characteristic pattern in any particular time segment where a normal characteristic pattern is expected. 
     Any of the embodiments described herein may be used alone or together with one another in any combination. Inventions encompassed within this specification may also include embodiments that are only partially mentioned or alluded to or are not mentioned or alluded to at all in this brief summary or in the abstract. Although various embodiments of the invention may have been motivated by various deficiencies with the prior art, which may be discussed or alluded to in one or more places in the specification, the embodiments of the invention do not necessarily address any of these deficiencies. In other words, different embodiments of the invention may address different deficiencies that may be discussed in the specification. Some embodiments may only partially address some deficiencies or just one deficiency that may be discussed in the specification, and some embodiments may not address any of these deficiencies. 
     Other features, benefits, and advantages of the present invention will be apparent upon a review of the present disclosure, including the specification, drawings, and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form a part of the specification, further illustrate the present invention and, together with the detailed description of the invention, serve to explain the principles of the present invention. 
         FIG. 1  illustrates an energy storage system; 
         FIG. 2  illustrates a chart of loaded voltage versus State-of-Charge curve for a single representative series element; 
         FIG. 3  illustrates a chart of a derivative of the curve of  FIG. 2 ; 
         FIG. 4  illustrates a graph of a set of series element voltages during charging; 
         FIG. 5  illustrates a system including exceptional charge event detection and response; 
         FIG. 6  illustrates a flowchart for a detection process; 
         FIG. 7  illustrates a graph of a transient discharge event for a set of series battery elements; and 
         FIG. 8  illustrates a graph of a transient charge event for a set of series battery elements. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention provide a system and method for identifying exceptional charge events of series-connected energy storage elements. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. 
     Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     It is important that a rechargeable energy storage system having multiple series-connected energy storage elements, particularly for those storing high energy levels and/or charging at high energy transfer rates, to detect exceptional charge events, more particularly in identified individual series elements. The following disclosure includes detection of exceptional charge events and possible responses to detected exceptional charge events. The detection addresses detection in two different types of cases: a “steady state” case and a “transient” case. Steady state is characterized as those scenarios where energy change events tend to be predictable, consistent, longer time scales (several minutes, to hours, to longer periods), and relatively lower energy and “transient” cases tend to be on an opposite side of this spectrum: unpredictable, variable, shorter time scales (˜1 second to several minutes), and relatively higher energy. There are not bright lines categorizing these cases. There are two representative uses that highlight these cases: 1) an electric vehicle (EV) case and 2) a stationary storage case. In the EV case, external charging is representative of the steady state case. In the stationary storage case, withdrawal or external grid charging of stored energy is representative of the steady state case. For the EV, internal regenerative events are representative of the transient case, as well as energy use that responds to the user&#39;s driving pattern and traffic/road conditions. For the stationary storage case, charging from a wind or solar generator is representative of the transient case. 
     Whether categorized as transient or steady state operation, during charging a system advantageously monitors for and is able to detect an exceptional charge event as an overcharge event. In contrast, during discharge the system monitors for and is able to detect any exceptional charge event as an overdischarge event. Different information and methods are used for the detection based upon which type of case. Broadly speaking, detection takes advantage of anything that affects loaded voltage vs. SOC. The system and methods of the preferred embodiments detect differences between an actual SOC and a measured SOC as energy storage in one or more battery cells changes (e.g., during charging and discharging). The preferred embodiments are able to take advantage of anything measurably different between battery cells that arise because of SOC differences. 
     Responding to a detected exceptional charge event does not depend upon the use, but on the nature of the exceptional charge event. That is, an appropriate response will depend upon whether the detected exceptional charge event is an overcharge event or an overdischarge event. 
     For purposes of this application, “transient” means a period of about one second, a few minutes, or a shorter period. For purposes of this application, “steady state” means a period of several minutes, several hours, or longer period. 
     This application is one of four related applications that address various aspects of The detection and response to exceptional charge events. In addition to this application, those other three applications are: U.S. patent application Ser. No. 13/689,787 entitled “TRANSIENT DETECTION OF AN EXCEPTIONAL CHARGE EVENT IN A SERIES CONNECTED BATTERY ELEMENT” and filed concurrently herewith, U.S. patent application Ser. No. 13/690,203 entitled “RESPONSE TO DETECTION OF AN OVERCHARGE EVENT IN A SERIES CONNECTED BATTERY ELEMENT” and filed concurrently herewith, and U.S. patent application Ser. No. 13/690,341 entitled “RESPONSE TO DETECTION OF AN OVERDISCHARGE EVENT IN A SERIES CONNECTED BATTERY ELEMENT” and filed concurrently herewith. All these applications are hereby expressly incorporated by reference thereto in their entireties for all purposes. 
       FIG. 1  illustrates an energy storage system  100  (ESS). ESS  100  includes a set of battery elements  105   i , i=1 to N where N can be tens, to hundreds, to thousands of series-coupled elements. A battery element  105   x  in the present context includes individual battery cells, but also includes aggregations of parallel and/or series connected elements that are, in turn, connected in series with another battery element (e.g., battery elements  105   x− 1 and  105   x+ 1). The embodiment disclosed herein is focused primarily on battery elements  105  with lithium-ion chemistry but may be adapted to work with other cell chemistries. 
     A battery management system (BMS)  110  is electrically communicated to the components of ESS  100  for monitoring, data acquisition, and control. As such it includes sensors and control elements (e.g., combinatorial and arithmetic logic elements and in some cases stored-program processing units executing instructions from a memory and the like) that perform the functions and processes described herein. 
     Individual cells are packaged and there may be additional packaging of collections of the battery cells of each particular battery element  105   i . Individual cells may be combined in desired parallel and series arrangements and packaged together into modules that may be further electrically connected together. There are logical sets of elements (e.g., battery element  105 ) that each include a positive terminal and a negative terminal reflecting a relative voltage level. In general for the series-connected battery element, a positive terminal of a particular battery element  105   i  is electrically communicated to a negative terminal of an “upstream” battery element  105   i− 1. A negative terminal of battery element  105   i  is electrically communicated to a “downstream” battery element  105   i+ 1. This is done for i=1 to N number of battery elements, with the positive terminal of battery element  1051  coupled to an ESS positive terminal  115  and the negative terminal of battery element  105 N coupled to an ESS negative terminal  120 . 
     A relative terminal voltage between ESS positive terminal  115  and ESS negative terminal  120  provides the collective net contributions of the N number of battery elements  105 N. ESS  100  is charged and discharged through these terminals. ESS  100  stores energy for many different applications, including energy for an electric propulsion motor of an electric vehicle (EV) and energy storage supporting time-shifted energy production from wind and solar generators and the like. While the terminal voltage presents a statistical effect of the aggregated series-connected battery elements  105 , the voltages of the individual battery elements  105   i  vary between each other. There are two different types of variations that are specifically mentioned here, though other variations may also come into play, sometimes with lesser or greater contribution. An individual battery element  105   i  will have a voltage offset variation and a voltage gain measurement error. (Note that this is a measurement error, not an actual variation in the voltage of the battery element. The voltage of the battery element will include natural variations, but this is due to normal charge/discharge cycling of the element. The same charge/discharge cycle may not change SOC of two battery elements identically.) BMS  110  includes a voltage measurement and balancing system (VMBS)  125  that acquires individual voltage measurements, including the individual voltages of battery elements  105 . 
     It is the case that a voltage of ESS  100  could indicate a charge level within design specification while an actual voltage level of a battery element  105   x  could be different than other elements. When the difference is excessive, the different voltage levels of the battery elements produce a potentially dangerous charge imbalance. Without VMBS  125 , the charge imbalance could produce a potential hazard of overcharging some battery elements  105  because a charger for ESS  100  would see the terminal voltages as being within specification. Also, without VMBS  125 , the charge imbalance could produce a potential hazard of overdischarging some battery elements  105  during energy withdrawal from ESS  100  which would be based on the “average” charge and any undercharged cells could have too much energy withdrawn. 
     VMBS  125  mitigates this potential hazard by monitoring the individual voltages of battery elements  105  and ensures balance and/or non-hazardous charging prior to charging of ESS  100 . As noted above, due to potential severity of overcharge events, BMS  110  includes further structures and processes for identification of series element exceptional charge events in ESS  100  before such event(s) become severe. This can be considered a backup to VMBS  125  in case VMBS malfunctions or otherwise does not detect a particular condition of battery elements  105  that could lead to dangerous imbalance. 
     VMBS  125  can malfunction in different ways, one of which is that it can experience a measurement error. Without such a measurement error, VMBS  125  could respond to an imbalance by asserting a status signal to BMS  110  that results in some type of response. That response could include inhibition of an initiation of charging of ESS  100 , or termination of charging after it has begun. VMBS  125  with a measurement error could contribute to a dangerous imbalance by allowing charging to start/continue or by improperly balancing voltage levels based upon incorrect measurement values, the improper balancing creating the very overcharge or undercharge event to be protected against. 
     BMS  110  further includes an exceptional charge event detector  130  that monitors battery elements  105  during operation of ESS  100 . One or more measureable parameters of battery elements are monitored and compared against a reference to determine whether an exceptional charge event is occurring, or has occurred. As discussed in more detail below, the reference may be determined from a statistical characterization of the collection of battery elements over time, with a “sufficient” deviation of any measured value from the reference an indication of a possible exceptional charge event, or a real-time characterization of relevant parameters from ESS  100 . 
       FIG. 2  illustrates a chart of loaded voltage versus State of Charge (SOC) curve  200  for a single representative series element  105 . Specifically in this case, battery element  105  includes eight parallel Lithium-ion cylindrical format cells being charged in a steady-state implementation. (Other implementations and arrangements will likely produce a different absolute curve; however the effect will be similar to that shown.) SOC curve  200  provides the voltage of the battery element as it is charged to different SOC levels. SOC curve  200  is effectively linear in the range of 10% to 100% SOC during “normal” charging. 
       FIG. 3  illustrates a chart of a derivative curve  300  of SOC curve  200  of  FIG. 2 . Derivative curve  300  is a change of voltage per change of charge (dV/dQ) and confirms the linearity of SOC curve  200  in the normal charge range. Above the normal range (e.g., greater than 100% SOC) where overcharge begins, a slope of SOC curve  200  begins to become non-linear. Initially the slope increases and then it decreases, with the change significant. These changes in the slope are confirmed in derivative curve  300  at charge levels greater than 100%. 
     Not shown in  FIG. 2  and  FIG. 3  is that portion of SOC curve  200  (and corresponding portion of derivative curve  300 ) relating to overdischarge (charge levels below the “normal” range). Overdischarge in that portion of the charging range “below” the normal range will also experience marked changes in slope. The general concepts described herein with respect to overcharge may be applied to the overdischarge situation as well. The present discussion highlights the overcharge case in the representative discussion because it is generally associated with the more significant potential short-term risk. 
     The particular charge levels will vary among different cells, cell aggregations/arrangements, and cell chemistries used by ESS  100 , therefore the specific shape of SOC curve  200  applicable to any particular ESS  100  varies. In these various implements of the present invention, there will be a corresponding normal range for an SOC curve where the operation is considered linear and end zones that are above and below the normal range where overcharge and overdischarge, respectively, occur resulting in the SOC curve becoming non-linear. Measuring and detecting these linear and non-linear regions by BMS  110  is used to detect exceptional charge events. Once detected, BMS  110  may assert control and/or status signals to other components for a desired response. 
       FIG. 4  illustrates a chart of a set  400  of series element voltages during steady state charging. A subset of voltages are identified by a set  405  of traces that include a large number (e.g., ˜100) series elements voltages. Battery elements  105   i  of set  405  represent the bulk of battery elements that charge normally and are balanced within a predetermined threshold (e.g., +/−1%) prior to charging. The traces of set  405  are linear in the normal range and indicate charging within design specification. 
     Set  400  includes additional traces that could represent problematic exceptional charge events. These additional traces include a first trace  410  having a small (e.g., a 0.2 V offset or less) and a second trace  415  having a small (e.g., 5% gain error or less) that are representative of a 20% imbalance. Set  400  also includes traces with a larger imbalance, for example, a 30% imbalance. A third trace  420  includes a 0.3 V offset and a fourth trace  425  includes a 7.5% gain error. For the 20% imbalance, overcharge starts at about 80% SOC while for the 30% imbalance, overcharge starts at about 70% SOC. As noted herein, both the offset and the gain errors are broadly described as instances of measurement errors. Any measurement error producing a significant deviation of a CCV—SOC curve relative to an expected curve may be encompassed by embodiments of the present invention. 
     Detection 
     BMS  110  registers a possible series element overcharge event in response to a sustained increase or decrease in voltage of one or more series elements beyond some predetermined reference (the value of the reference itself may vary). For steady state for example, the reference may be a statistically established average. For transient operation, the reference may be a value obtained from a lookup table. This predetermined reference may be dependent upon a total number of series elements and/or other factors. For example this threshold may be a deviation that is more than 1.5×, 2×, or 2.5× relative to a predetermined parameter and exceeding this threshold results in the detection of the exceptional charge event. The parameter may be a statistical assessment (e.g. a standard deviation), or some measurement or calculation or the like (e.g., an average) that is derived from a distribution (preferably a real-time distribution) of series element voltages during charge. For  FIG. 4 , overcharge is indicated by a variation in a measured value that exceeds 1% of the values of set  405 . 
     For some embodiments, there are variations in charging behavior that are responsive to physical conditions of ESS  100  (e.g., temperature, cycle life, SOC, and the like that in turn are dependent on specific cell type and chemistry). These variations can be predictable and determined in advance to be known quite accurately. In such situations, BMS  110  may use a look up table for comparison of individual voltages in place of the evaluation of the instantaneous distribution of series element voltages and subsequent comparison. 
     The measurements, tests, and comparisons of battery element voltages against the SOC curve (e.g.,  FIG. 2 ) is also pertinent to a measurement, test, and comparison of the derivation curve (e.g.,  FIG. 3 ). For example, BMS  110  compares first derivatives of the CCV versus SOC curves for battery elements  105  of ESS  100  against a predetermined reference (the predetermined value). This predetermined reference may be dependent upon a total number of series elements and/or other factors. For example this threshold may be a deviation that is more than 1.5×, 2×, or 2.5× relative to a predetermined parameter and exceeding this threshold results in the detection of the exceptional charge event. The parameter may be a statistical assessment (e.g. a standard deviation), or some measurement or calculation or the like) that is derived from a distribution (preferably a real-time distribution) of series element voltages during charge. For  FIG. 4 , overcharge is indicated by a variation in a measured value that exceeds 1% of the values of set  405 . 
     Some embodiments include BMS  110  using variations of other characterizations of ESS  100  as a function of time. The characterizations may also include elapsed time (assuming a constant current charging rate), integrated charge, integrated charging energy, or estimated open-circuit voltage (OCV) that may be used in place of SOC as a dependent variable. In other embodiments, a measure of time, SOC, or integrated charge, or integrated energy, or estimated OCV versus series element voltage could be used by BMS  110  to detect exceptional charge events. 
     While the description above includes embodiments with BMS  110  configured for data acquisition of individual voltages of all the battery elements, not all embodiments need be configured in this way. Some embodiments provide a composite assessment in which battery elements  105  are combined into measurement units. Such a composite assessment may be in addition to, or in lieu of, individual measurement. For example, a pair of battery elements  105  may be collectively measured and that collective measurement compared against a predetermined reference. Other embodiments may use triplets instead of pairs, or other number of battery elements measured together. Practically any collective assessment may be used provided that a precision of the measurement enables variations due to an exceptional charge event of any individual series battery element. Such collective measurements can be compared with the sum of the measurements of the included individual elements, with a discrepancy indicating a possible exceptional charge event. Possible advantages of such collections include the situation that embodiments employing such collective assessment may have cost-advantages when the collective measurement simplifies and reduces measurement and data acquisition components of BMS  110 . 
     As noted elsewhere, the SOC curve includes a sharp drop in voltage at the low SOC end of charging which BMS  110  is able to use when identifying possible overdischarge of a series element during discharge cycles by similar comparisons previously described. Though overdischarge is not nearly as severe an event as overcharge, detection of a potential overdischarge can be used to trigger an appropriate response that may prevent possible overcharge on subsequent charging cycles. 
     The disclosed embodiments include several different ways of detecting an exceptional charge event associated with one or more battery elements. Embodiments of the present invention provide ESS  100  and/or BMS  110  with a range of possible responses to a detected exceptional charge event. Overcharge of one or more individual battery elements  105  during charging is generally viewed as the more potentially serious and urgent situation. 
     As noted herein, the detection modalities are categorized based upon a type of use of ESS  100 . When the use, during charging or discharging, provides for rapid changes in energy transfer to or from ESS  100 , then the principles described in the context of transient detection are employed. When the use, during charging or discharging, provides for steady energy transfer that is provided at a rate fairly constant over several measurement cycles, then the principles described in the context of steady state detection are employed. 
     A primary difference is the type of data that is collected within measurement and analysis cycles. In the transient case, energy flows relative to ESS  100  change fairly rapidly. It may take time to gather enough information for statistical evaluation of means, averages, deviations, and the like and in a transient operation, those values are not as helpful to quickly and unambiguously detect an exceptional charge event. As a point of generality, the transient case is often associated with potentially larger peak energy transfer rates. Even when it may be possible to begin to gather data over enough time, there can be increased risk to waiting and it is desirable to employ other faster-responding detection modalities. 
     In steady state operation (e.g., charging an EV, or withdrawing current from the stationary storage), the current to or from ESS  100  is regular and more predictable. When in steady state operation, the current flow relative to ESS  100  will for the most part be constant, or at least slowly changing, and a relatively long time history can be taken and used in the controller. This includes collecting enough data from all the battery elements and developing the requisite reference(s), and then performing the tests of battery element values versus the reference(s). 
     In transient operation (e.g., withdrawing charge from the EV ESS or charging the stationary storage device using wind turbines or solar panels and the like), the current flow relative to ESS  100  will be irregular and unpredictable, and is determined by the operational pattern (driving, wind, and sun). 
     In steady state operation, statistical information can be determined from large enough collections of data to be statistically significant. During operational modes which experience predominately transient patterns, there are periods in which operation is steady enough to include steady state tests. These steady state tests (standard deviation of absolute voltage measurement or derivative over a period of time) can be made when current is relatively stable (cruising on the highway for example), but transient detection takes advantage of different characteristics of the cell during transient events. In particular, during a transient discharge event such as gunning the EV for several seconds, if a severe imbalance is present that is masked by gain/offset errors, the imbalanced battery cell will deviate from the rest significantly due to much different cell impedance. This deviation can be detected in a few seconds, versus the slower deviation that would occur during charging over several minutes. A lookup table of impedance versus SOC can be used to bound the degree of deviation that triggers a response (changes in temperature throughout the pack can also cause changes in cell impedance which would cause deviation in a transient current event like flooring the car, so the controller monitors for an extreme deviation). 
     In this transient operation, a lookup table stores impedance values for a range of SOC values for individual battery elements (or aggregations of battery elements). During transient operation, impedance values are established for each loaded battery element. The lookup table is accessed with the loaded impedance value and produces the corresponding SOC value. If the SOC falls outside a specified range (either representing an overcharged or overdischarged battery element) then the controller has detected the exceptional charge event (e.g., overcharge or overdischarge). Because changes of impedance versus SOC are not linear, and can vary by temperature, impedance values are not compared or used directly in the preferred embodiments. The impedance is converted to a corresponding SOC and then the SOC is evaluated for an exceptional charge event. 
     Some implementations may rely on some other charge-related parameter besides, or in addition to, impedance. When the relationship to the measured parameter and the charge-related parameter can be calculated in real-time without resources that are too expensive for the particular application, the controller may determine the numbers without reference to a lookup table. What is too expensive is dependent upon the implementation, with those resources including one or more of calculation time, parts cost, parts weight, parts reliability, and the like. 
     Responses 
     In the disclosed embodiments, responses are based upon a type of exceptional charge event that was detected. One set of responses relate to overcharge events and another set of responses relate to overdischarge events. 
     Overcharge 
     Responses to a detection of a possible overcharge event (or response to a determination of a sufficient likelihood of such an event) of a battery element  105  include one or more of the following. 
     A simple first type of response is for BMS  110  to assert a signal that stops any charging of ESS  100 . In this way any risk of an adverse effect of overcharge may be greatly reduced. 
     A second type of response includes assertion by BMS  110  of a diagnostic trouble code (DTC) or malfunction indicator light (MIL). A management system coupled to ESS  100  is able to detect and respond to the DTC or MIL and take further corrective action, including suspending any charging. 
     A third type of response locks out future charging until ESS  100  has been serviced. In some embodiments charging is not completely locked, but rather is limited to reduce overcharge risks while enabling some function. For an EV, the limited function is sufficient to enable the EV to be driven to a service center. Two or three of these first types of response can be used together to provide a simple response modality to detected overcharge events. 
     A fourth type of response includes use of VMBS  125  to bleed overcharged battery elements, or those with a significant enough chance to be overcharged, to a reduced charge level. When event detector  130  is triggered, VMBS  125  is by definition in a compromised operational mode and it therefore cannot be used to properly manage and regulate bleeding of the overcharged battery element(s). BMS  110  triggers a bleed circuit, which could be part of VMBS  125 , to bleed a desired amount of charge capacity from the subject battery elements. The amount of reduction or bleed can be implementation specific to reduce risks of overcharge to a desired level. Some embodiments bleed about 50% capacity. BMS  110  uses the bleed circuit (e.g., a selectively engageable circuit having a known impedance) along with an assumed (non-measured) SOC (e.g., assume 110% SOC irrespective of measured value) to determine an amount of time to bleed energy from the subject battery element to reach the desired reduced capacity. In some cases BMS  110  may manage the bleed and set the desired level by monitoring bleed current directly. 
     A fifth type of response includes adjustment of SOC or voltage targets based on true SOC at the time of detection. True SOC may be determined by BMS  110  in multiple different ways including integrated charge, total voltage of ESS  100 , and the like. The adjustments of SOC or voltage targets could be performed incrementally as information is collected with each charge cycle. In this response, the SOC target for charging is reduced by some value when the exceptional charge event is detected. The reduction may be some set amount or be responsive to the degree of mismatch between corresponding attributes (e.g., measured voltage) an exceptional battery element (battery element  105  having the exceptional charge event) and battery elements within set  405 . 
     A sixth type of response, similar to the fifth type, reduces the max SOC or target voltage for all battery elements  105 . BMS  110  causes ESS  100  to run at a reduced max SOC or voltage until ESS  100  is serviced. 
     A seventh type of response includes use of the BMS  110  measurement and data acquisition components to log the various events. When the detected overcharge event is evaluated and poses no significant risk to ESS  100  or operators, then logging is an appropriate response as it does not impair operation or use which could, in some situations, pose more risk or inconvenience than the overcharge event. The logging event includes determination and recording of one or more of the following: temperature, SOC of all bricks, voltage measurements of all bricks, dV/dQ of all bricks at the time of the event, and charge/discharge rate. The log may be stored in BMS  110  or it may be sent to a data center for processing. In some embodiments, BMS  110  includes a transmitter (e.g., a wireless device that uses an available network connection (such as at home or other location frequented by a mobile implementation of ESS  100 )) that sends the log information to a datacenter for monitoring and processing. 
     An eighth type of response includes adjustment of element parameters (e.g., voltage gain and/or voltage offset factors) for affected battery elements  105 . In this type of response to a detected exceptional charge event, each battery element  105  having an exceptional charging profile that tags the battery element as experiencing a possible exceptional charge event is flagged. BMS  110  adjusts those element parameters to reduce the variation between the exceptional charging profile and the charging profiles of the battery elements of set  405 . Preferably the adjustments move the exceptional charging profile into set  405 . 
     A ninth type of response addresses thermal control of ESS  100 . As discussed herein, some of the embodiments are implemented to address potential risks of a potential thermal runaway for any battery element with an imbalance that could, responsive to charging, lead to overcharge and excess heating. This response relates to reducing the temperature of ESS  100  and/or its components to reduce overheating which can lead to the thermal runaway. ESS  100  typically includes some type of thermal control system (e.g., a cooling system that circulates a coolant through ESS  100  to extract heat and some type of heat exchanger to transfer heat from the coolant and the like). This response includes BMS  110  initiating increased cooling from the thermal control system. The thermal control system is either operated at increased cooling capacity or maximum depending upon what is needed or desired to sufficiently reduce any risk of thermal runaway. The increased cooling reduces any chance that the battery element experiencing an overcharge event has to self-heat to a point of thermal runaway. 
     In some embodiments, the thermal control system is engaged in the heightened cooling mode immediately upon detecting the overcharge event. As discussed above, it is also the case that the detected overcharge event could initiate another additional response as well. That additional response may correct or reduce the overcharge event, in which case the heightened cooling mode may be reduced or terminated. For example, after initiating the heightened cooling mode, the exceptional battery element may be bled by VMBS  125  so it is no longer overcharged and further chances of overcharging are reduced. Thereafter the thermal control system may be turned off (as is typical for charging). 
     In other implementations, a version of this tenth type of response includes adjustments to one or more thermal setpoints of the thermal control system. BMS  110  is able to reduce the thermal setpoints (which initiate actuation of the thermal control system) at a lower temperature to provide an extra margin of safety in response to a detected overcharge event. 
     A tenth type of response includes a backup response that addresses situations in which the parameter (e.g., voltage) measurement system becomes inoperative. This backup response may, in some implementations, take a more primary role in the event a voltage measurement facility becomes fully inoperative (e.g. a voltage sense wire becomes disconnected), even when there isn&#39;t an exceptional charge event detected. Many systems respond to an inoperative voltage measurement by immediately disallowing drive. Some embodiments of the present operate on voltage measurements of a pair of non-contiguous battery elements, where the missing voltage measurement is of a battery element between the pair of battery elements. This response allows for some protection from exceptional charge events (both overcharge and overdischarge), even though a parameter measurement is missing for the particular battery element. This is a special operational mode that is not designed for sustained drive. However, including such an optional response could allow a user to drive the car into service, rather than becoming immediately stranded and requiring towing in the event a voltage measurement went dead. 
     Overdischarge 
     In the event of a detected exceptional charge event that includes an overdischarge of one or more battery elements, BMS  110  provides one or more appropriate responses. The following seven responses are counterparts to some of the overcharge responses noted herein. 
     A first type of response to an overdischarge includes prevention of future charging of the vehicle until the vehicle is serviced. It is somewhat counter-intuitive to address overdischarge by controlling, limiting, or preventing charging but it has been observed that some problems which lead to overdischarge may lead to overcharge on subsequent charging cycles. 
     A second type of response includes adjusting charge SOC or voltage targets (possibly incrementally as information is collected with each charge cycle) based on true SOC at the time of detection. In this case, in contrast to overcharge responses, the charge SOC or voltage targets may be selectively increased. 
     A third type of response includes BMS  110  initiating ESS  100  to operate using increased minimum SOC or voltage targets. 
     A fourth type of response includes logging the event, which may include one or more of temperature, SOC of all bricks, voltage measurements of all bricks, dV/dQ of all bricks at the time of the event, and charge/discharge rate. As noted above, the log may be stored in an internal memory associated with ESS  100  or communicated to a data center. The internal memory does not have to physically be part of ESS  100  but may be included in another control or data acquisition system electrically communicated with ESS  100 . 
     A fifth type of response includes adjustments to parameters (e.g., gain/offset factor) for the affected battery element. In some cases the adjustment may also affect other battery elements in addition to the targeted battery elements. That is, some parameter adjustments may affect several elements and BMS  110  may need to use a secondary adjustment modality to compensate for having adjusted non-targeted battery elements. 
     A sixth type of response includes a backup response that addresses situations in which the parameter (e.g., voltage) measurement system becomes inoperative. This backup response may, in some implementations, take a more primary role in the event a voltage measurement facility becomes fully inoperative (e.g. a voltage sense wire becomes disconnected), even when there isn&#39;t an exceptional charge event detected. Many systems respond to an inoperative voltage measurement by immediately disallowing drive. Some embodiments of the present operate on voltage measurements of a pair of non-contiguous battery elements, where the missing voltage measurement is of a battery element between the pair of battery elements. This response allows for some protection from exceptional charge events (both overcharge and overdischarge), even though a parameter measurement is missing for the particular battery element. This is a special operational mode that is not designed for sustained drive. However, including such an optional response could allow a user to drive the car into service, rather than becoming immediately stranded and requiring towing in the event a voltage measurement went dead. 
     A seventh type of response includes BMS  110  limiting a maximum current draw from ESS  100 . This response artificially throttles back the power available to the EV which prevents rapid discharging and possible overheating during the exceptional charge event. This response isn&#39;t generally applicable to overcharge because the charging current would be set by the charging parameters and the currents are generally much lower than during discharge. 
       FIG. 5  illustrates a system  500  implementing exceptional charge event detection and response. System  500  includes an energy environment  505  that encompasses ESS  100 , a control system  510 , an energy converter  515 , and a support system. Energy environment  505  may be an electric vehicle or other context in which electrical energy is stored and converted. Control system  510  monitors state and status of the various components and includes I/O (input/output) features appropriate to the context, such as receiving input from a user regarding velocity. Control system  510  typically includes a stored program computing system that has memory and a processing unit configured to implement control mechanisms of the components of system  500 . Control system  510  may be integrated wholly or partially with another component of system  500 , such as with BMS  110  of ESS  100 . In other cases, some or all of the functions of BMS  110  may be implemented by control system  510 . Energy converter  515  represents one or more elements that use energy from ESS  100 . These elements may include electric motors (e.g., an electric propulsion motor) and other devices that use energy in energy environment  505 . Support  520  represents thermal control systems and other components of energy environment  505  that support its use and operation. 
     System  500  further includes a charger  525  that is typically a stationary energy charging station and provides a source of power (typically AC power) that is used to charge the battery elements of ESS  100 . One or more components of energy environment  505  may implement any of the detection and response modalities describe herein. 
       FIG. 6  illustrates a flowchart for a detection process  600 . Process  600  includes steps  605 - 630  to establish an existence, or likelihood of existence, of an exception charge event (ECE). Step  605  first assesses the reference. In a simple case, ESS  100  includes monitoring and data acquisition equipment (e.g., voltage and current sensors and logic devices for measurement and evaluation) of the reference that will be used. During charging, the ECE of most concern is the overcharge event, while overdischarge could be relevant during charging of ESS  100  as well as during operation of energy environment  505 . 
     The reference of step  605  is preferably a statistical representation of samples of the collection of voltages of battery elements  105  of ESS  100 . Other references may be employed, such as outputs of look-up tables or other data organization. 
     Next in process  600  after step  605 , step  610  determines desired parameter values for individual battery elements  105 . The discussion includes several ways to do this, including measuring individual voltage levels or measuring voltage levels of aggregations (e.g., pairs of battery elements  105 ). 
     Next step  615  performs a comparison of the determined individual voltage levels against the reference. In a preferred embodiment, a determination of a charge event as normal or exceptional is based upon an evaluation of parameter value performance, which in turn can be dependent upon transient or steady state operation. Specifically for steady state operation, upon the SOC curve and the derivation curve. In this sense, this type of detection implemented by the embodiments of the present invention are not instantaneous determinations as this process is responsive to trend of the parameter values of the battery elements, preferably as a function of state of charge, and most preferably as loaded battery element voltage as a function of SOC. Transient operation, as noted above, is more instantaneous and can provide quicker responses. 
     After, or during, the comparison of parameter values against the reference, process  600  at step  620  tests the results of the comparison against a threshold value. Depending upon how the test is implemented (e.g., whether exceeding a threshold is true or not exceeding the threshold is true), step  620  establishes whether a particular comparison represents, or has a sufficient likelihood to represent, an exceptional charge event. Further differentiation is based upon whether system  500  is charging at the time of the test. Exceptional charge events during charging typically represent overcharge events and exceptional charge events at other times typically represent overdischarge events. 
     When process  600  does not determine that an ECE exists at the test of step  620 , process  600  continues with the detection process. What that means is dependent upon specifics of how steps  605 - 620  were implemented. For an interrupt-driven test, step  625  returns to the operation it was performing when interrupted. In some cases step  615  and step  620  are combined, so step  626  returns to step  615  for another comparison of another parameter against the reference. In some cases process  600  updates the reference so step  625  returns to step  605  to determine whether the reference needs to be updated. Step  625  can simply return to step  610  to continue to monitor parameters of battery elements  105 . 
     When the test at step  620  is true, process  600  uses the comparison to identify and flag the exceptional battery element as having an exceptional charge event (which may be further identified as an overcharge or an overdischarge charge event based upon use/implementation and a status of charging from charger  525 . Most implementations further include one or more responses to any detected ECE at step  630 . The response may be dependent upon the type of ECE as well. Embodiments of the present invention described herein include representative types of responses. 
       FIG. 7  illustrates a graph of a transient discharge event  700  for a set of series battery elements; and  FIG. 8  illustrates a graph of a transient charge event  800  for a set of series battery elements. Event  700  and event  800  make use of impedance for transient detection of exceptional charge events. In each graph, the set of curves that are generally matched represent series elements that are not exceptionally charged. A series element  705  in the transient discharge event  700  and a series element  805  in the transient charge event represent exceptionally charged elements. Outside of the transient event, each exceptional charge element appears like a non-exceptionally charged element because of measurement offset/gain error during no/low current draw. During a high current draw, such as event  700 , the voltage of all series elements drop as current is pulled from the elements (discharge), but the exceptionally charged element drops significantly more than the rest ( FIG. 7 ). During a high current charge, such as event  800 , the voltage of all series elements rise as current is transferred to the elements (charge), but the exceptionally charged element rises significantly more than the rest ( FIG. 8 ). The detection methods outlined would detect these exceptional charge events. The impedance approach is useful because the very high current pulse magnifies the effects of the impedance difference. During a steady state event, currents are typically much (˜10×) lower, so the impedance approach wouldn&#39;t be as useful as the steady observation of voltage drift with time. 
     The system and methods above has been described in general terms as an aid to understanding details of preferred embodiments of the present invention. In the description herein, numerous specific details are provided, such as examples of components and/or methods, to provide a thorough understanding of embodiments of the present invention. Some features and benefits of the present invention are realized in such modes and are not required in every case. One skilled in the relevant art will recognize, however, that an embodiment of the invention can be practiced without one or more of the specific details, or with other apparatus, systems, assemblies, methods, components, materials, parts, and/or the like. In other instances, well-known structures, materials, or operations are not specifically shown or described in detail to avoid obscuring aspects of embodiments of the present invention. 
     Reference throughout this specification to “one embodiment”, “an embodiment”, or “a specific embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention and not necessarily in all embodiments. Thus, respective appearances of the phrases “in one embodiment”, “in an embodiment”, or “in a specific embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics of any specific embodiment of the present invention may be combined in any suitable manner with one or more other embodiments. It is to be understood that other variations and modifications of the embodiments of the present invention described and illustrated herein are possible in light of the teachings herein and are to be considered as part of the spirit and scope of the present invention. 
     It will also be appreciated that one or more of the elements depicted in the drawings/figures can also be implemented in a more separated or integrated manner, or even removed or rendered as inoperable in certain cases, as is useful in accordance with a particular application. 
     Additionally, any signal arrows in the drawings/Figures should be considered only as exemplary, and not limiting, unless otherwise specifically noted. Furthermore, the term “or” as used herein is generally intended to mean “and/or” unless otherwise indicated. Combinations of components or steps will also be considered as being noted, where terminology is foreseen as rendering the ability to separate or combine is unclear. 
     As used in the description herein and throughout the claims that follow, “a”, “an”, and “the” includes plural references unless the context clearly dictates otherwise. Also, as used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. 
     The foregoing description of illustrated embodiments of the present invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed herein. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes only, various equivalent modifications are possible within the spirit and scope of the present invention, as those skilled in the relevant art will recognize and appreciate. As indicated, these modifications may be made to the present invention in light of the foregoing description of illustrated embodiments of the present invention and are to be included within the spirit and scope of the present invention. 
     Thus, while the present invention has been described herein with reference to particular embodiments thereof, a latitude of modification, various changes and substitutions are intended in the foregoing disclosures, and it will be appreciated that in some instances some features of embodiments of the invention will be employed without a corresponding use of other features without departing from the scope and spirit of the invention as set forth. Therefore, many modifications may be made to adapt a particular situation or material to the essential scope and spirit of the present invention. It is intended that the invention not be limited to the particular terms used in following claims and/or to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include any and all embodiments and equivalents falling within the scope of the appended claims. Thus, the scope of the invention is to be determined solely by the appended claims.