Patent Publication Number: US-11656294-B2

Title: Battery diagnostics system and method using second path redundant measurement approach

Description:
This application is a continuation of U.S. patent application Ser. No. 15/923,635, filed on Mar. 16, 2018, which application is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to an electronic system and method, and, in particular embodiments, to a battery diagnostics system and method using second path redundant measurement approach. 
     BACKGROUND 
     Electric vehicles are vehicles that use an electric motor for propulsion. Typically, an electric vehicle uses a battery pack to power the electric motor. A battery pack typically includes a stack of battery cells in series to achieve voltages such as 400 V, or higher. For example, a battery pack may include a stack of 96 lithium-ion battery cells connected in series. Voltages lower than 400 V may also be used. 
     Since electric vehicles are typically powered by a battery pack, the health of the battery pack is a major safety concern. In some cases, failure of a single battery cell of the battery pack may be catastrophic. For example, due to manufacturing or usage-related variations, some battery cells may have slightly less capacity than other battery cells in the battery pack. Without battery cell balancing, one or more battery cells may fail after multiple charge/discharge cycles. 
     Vehicles, therefore, typically monitor and periodically rebalance each individual battery cell. The monitoring and balancing of the battery cells is typically done by an external integrated circuit (IC), often called battery stack monitor, battery monitor IC or sensing IC, which is connected to the battery pack via a balancing network. In some implementations, the external IC monitors the voltage across each battery cell of the battery pack and then discharges some of the battery cells based on the monitored voltage to ensure that each battery cell is balanced with respect to the other battery cells in the battery pack. Since the balancing network used to connect the external IC to the battery pack may fail, the external IC may be capable to detect open circuits of the balancing network as another diagnostic feature. 
     SUMMARY 
     In accordance with an embodiment, a method for providing battery diagnostics includes: measuring a first voltage across a first battery cell of a rechargeable battery via a first measurement path of a network using a first measurement circuit, measuring the first voltage including taking at least one first voltage sample during a first time period using the first measurement circuit; measuring a second voltage across the first battery cell via a second measurement path of the network using a second measurement circuit, measuring the second voltage including taking at least one second voltage sample during the first time period using the second measurement circuit, where the second measurement path of the network is different from the first measurement path of the network; comparing the measured first voltage with the measured second voltage; and generating a diagnostic output signal based on the comparison. 
     In an embodiment, a circuit includes: a first measurement circuit configured to be coupled to a first battery cell via a first path of a network; a second measurement circuit configured to be coupled to the first battery cell via a second path of the network, the second path being different than the first path; and a controller configured to: cause the first measurement circuit to measure a first plurality of voltage samples across first and second terminals of the first battery cell during a first time period, cause the second measurement circuit to measure a second plurality of voltage samples across the first and second terminals of the first battery cell during the first time period, compare an output of the first measurement circuit with an output of the second measurement circuit, and generate a diagnostic output signal based on the comparison. 
     In an embodiment, a battery management system includes: a rechargeable battery including N battery cells coupled in series, where N is a positive integer greater than zero; a balancing network coupled to the rechargeable battery; and a battery monitoring circuit coupled to the balancing network, the battery monitoring circuit including: a sigma-delta analog-to-digital converter (ADC) having an input configured to be coupled to a first battery cell of the N battery cells via a first path of the balancing network, the sigma-delta ADC coupled to a first reference voltage generator; a measurement circuit having an input configured to be coupled to the first battery cell via a second path of the balancing network, the second path being different from the first path, the measurement circuit coupled to a second reference voltage generator different from the first reference voltage generator, the measurement circuit having a different architecture than the sigma-delta ADC; and a controller configured to: control the sigma-delta ADC to measure a first plurality of voltage samples across first and second terminals of the first battery cell during a first time period, control the measurement circuit to measure a second plurality of voltage samples across the first and second terminals of the first battery cell during the first time period, compare an output of the sigma-delta ADC with an output of the measurement circuit, and generate a diagnostic output signal based on the comparison. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    shows a schematic diagram of a portion of a battery management system, according to an embodiment of the present invention; 
         FIG.  2    shows a flow chart of an embodiment method of detecting a failure of a battery management system, according to an embodiment of the present invention; 
         FIG.  3    shows a schematic diagram of a portion of a battery management system, according to another embodiment of the present invention; 
         FIG.  4    shows a schematic diagram of a portion of a battery management system, according to an embodiment of the present invention; 
         FIG.  5    shows a schematic diagram of a portion of a battery management system, according to another embodiment of the present invention; 
         FIG.  6    shows a schematic diagram of a portion of a battery management system, according to yet another embodiment of the present invention; 
         FIG.  7    shows a schematic diagram of a portion of a battery management system, according to an embodiment of the present invention; 
         FIG.  8    shows a schematic diagram that illustrates a possible measurement system for achieving simultaneous measurements, according to an embodiment of the present invention; 
         FIG.  9    illustrates timing diagrams for measuring voltage across each of the battery cells of the battery management system of  FIG.  7   , according to an embodiment of the present invention; 
         FIG.  10    shows a schematic diagram of a portion of a battery management system, according to another embodiment of the present invention; and 
         FIG.  11    shows a flow chart of an embodiment method of detecting a failure of a battery management system, according to an embodiment of the present invention. 
     
    
    
     Corresponding numerals and symbols in different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily drawn to scale. To more clearly illustrate certain embodiments, a letter indicating variations of the same structure, material, or process step may follow a figure number. 
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     The description below illustrates the various specific details to provide an in-depth understanding of several example embodiments according to the description. The embodiments may be obtained without one or more of the specific details, or with other methods, components, materials and the like. In other cases, known structures, materials or operations are not shown or described in detail so as not to obscure the different aspects of the embodiments. References to “an embodiment” in this description indicate that a particular configuration, structure or feature described in relation to the embodiment is included in at least one embodiment. Consequently, phrases such as “in one embodiment” that may appear at different points of the present description do not necessarily refer exactly to the same embodiment. Furthermore, specific formations, structures or features may be combined in any appropriate manner in one or more embodiments. 
     The present invention will be described with respect to embodiments in a specific context, a diagnostic circuit coupled to a battery pack of an electric vehicle and configured to detect faults in the battery pack, balancing network, and other components internal and external to the diagnostic circuit by using a redundant voltage measurement circuits implemented with different technologies. Some embodiments may be used in systems other than an electric vehicle, such as other systems that measure voltage. Technologies to measure voltage other than the ones described herein may also be used. 
     System redundancy may be used in safety critical systems, such as battery management systems, to decrease the failure probability of the system. The decrease in the probability of failure is generally most effective when the redundant system and the primary system are uncorrelated (i.e., mutually exclusive). For example, typically, the less the correlated the redundant measurement system is with the primary system, the lower the probability that the primary system and the redundant system fail as a result of a single specific event or root cause (also known as common cause failure). 
     In an embodiment of the present invention, a battery management system uses a battery monitor IC that uses redundancy to monitor the voltage across each battery cell of a battery pack. The voltage across a battery cell is measured with a primary voltage measurement circuit and with a secondary voltage measurement circuit. The primary and secondary voltage measurement circuits are implemented with different technologies, use different reference voltage generators, and measure the voltage across the battery cell simultaneously via different paths. Faults internal to the battery monitor IC or external to the battery monitor IC, such as faulty components and leaky paths, may be detected by comparing a difference between the voltages measured by the primary and secondary voltage measurement circuits with a voltage threshold. 
       FIG.  1    shows a schematic diagram of battery management system  100 , according to an embodiment of the present invention. Some components and details of battery management system  100  have been omitted for clarity purposes. Battery management system  100  includes battery pack  101  coupled to battery monitor IC  102  via analog filter and balancing network  104 . Battery monitor IC  102  includes electrostatic discharge (ESD) protection circuit  106 , open-loop diagnostics and balancing circuit  108 , and voltage measurement circuit  109 . Battery pack  101  includes a plurality of battery cells coupled in series. However,  FIG.  1    illustrates only battery cell cell i  of battery pack  101  for clarity purposes. 
     During normal operation, the voltage across battery cell cell i  is monitored by voltage measurement circuit  109 . The voltage measured by voltage measurement circuit  109  is used, for example, to determine whether battery cell cell i  should be discharged to be balanced with respect to other battery cells in battery pack  101 . The voltage across battery cell cell i  may also be used to determine whether battery cell cell i  is discharged, or over-charged. The voltage across battery cell cell i  may also be used for other purposes, such as to recalibrate a system-on-chip (SoC) in open circuit voltage (OCV). 
     Voltage measurement circuit  109  includes redundancy for measuring the voltage across battery cell cell i . For example, voltage measurement circuit  109  includes voltage measurement circuit  110  and voltage measurement circuit  112 . Voltage measurement circuits  110  and  112  may be implemented with different measurement schemes. Different measurement schemes involve a different architecture (e.g., SAR ADC or Σ-Δ ADC) and/or a different measurement principle. For instance, voltage measurement circuit  110  could be implemented as a 13-bit Σ-Δ ADC with a specific digital filtering and voltage measurement circuit  112  could be implemented as a 16-bit Σ-Δ ADC using a different digital filtering. Each of the voltage measurement circuits  110  and  112  uses a different reference voltage. Reference generator  111  and  113  generate the reference voltages for voltage measurement circuits  110  and  112 , respectively. 
     As shown in  FIG.  1   , voltage measurement circuit  110  measures the voltage across battery cell cell i  using path  122  and voltage measurement circuit  112  measures the voltage across battery cell cell i  using path  124 . Paths  112  and  124  use different pins of battery monitor IC  102  and use different components of analog filter and balancing network  104 . Paths  112  and  124  may also be exposed to different ESD structures, since each pin of battery monitor IC  102  has its own ESD protection circuit. The ESD structures of the pins of IC  102  are collectively shown in FIG.  1  as ESD protection circuit  106  for clarity purposes. Paths  112  and  124  may also be exposed to different circuits of open-loop diagnostics and balancing circuit  108  (depending on the implementation of open-loop diagnostics and balancing circuit  108 ). 
     Both voltage measurement circuits  110  and  112  measure the voltage across battery cell cell i  at the same time. The filtering characteristics of paths  112  and  124  are dimensioned to allow for a comparable result when measuring the same signal at the same time by both measurement circuits  110  and  112 . 
     Reference generator circuits  111  and  113  may be implemented in any way known in the art. For example, reference generator circuits  111  and  113  may be implemented by using independent band-gap circuits that supply independent voltages to be used as references for the voltage measurement units  112  and  110 . In some embodiments, the architecture of the respective band-gap circuits and voltage regulators may be different. In other embodiments, the architecture of reference generator circuits  111  and  113  may be identical. Other implementations are also possible. 
     Open-loop diagnostics and balancing circuit  108  detects whether an open circuit condition exists in analog filter and balancing network  104  by performing an open-loop test. Open-loop diagnostics and balancing circuit  108  also discharges battery cell cell i  to a desired voltage to balance battery cell cell i  with respect to other battery cells in battery pack  101 . Open-loop diagnostics and balancing circuit  108  may be implemented in any way known in the art. For example, typical implementations include current sources, comparators, and transistors. 
     Analog filter and balancing network  104  includes a balancing network that includes resistors and an analog filter that includes capacitors that provide filtering in combination with the resistors of the balancing network. As a non-limiting example, in some embodiments, resistors R 1  have a resistance of 5Ω, resistor R 2  has a resistance of 20Ω, and capacitors C 1  have a capacitance of 330 nF. Other resistance and capacitance values may be used. For example, the resistances of resistors R 1  and R 2  and the capacitances of capacitors C 1  may be selected based on the filtering characteristics desired to allow for the measurements and of  112  and  110  to be comparable when measuring the same signal at the same time. In some embodiments, the filtering characteristics of balancing network may be supplemented by subsequent filtering after sampling the voltage to obtain a matching total effective filtering characteristic. Analog filter and balancing network  104  may be implemented with other arrangements, such as shown, for example, in  FIGS.  3  and  4   . 
     ESD protection circuit  106  provides a path for ESD discharge to some or all pins of battery monitor IC  102 . ESD protection circuit  106  may be implemented in any way known in the art. For example, ESD diodes may be coupled in a reverse biasing configuration between a pin (e.g., U i ) and a ground node and/or a battery supply node and/or another pin of battery monitor IC  102 . Other implementations are also possible. 
     Battery monitor IC  102  is implemented in a monolithic semiconductor substrate. In some embodiments, battery monitor IC  102  may be implemented in a multi-chip architecture, where, for example, voltage measurement circuit  110  is disposed in a first monolithic semiconductor substrate together with reference generator in, and voltage measurement circuit  112  is disposed together with reference generator  113  in a second monolithic semiconductor substrate different than the first monolithic semiconductor substrate and packaged in the same package. Other implementations are also possible. 
     Battery pack  101  includes a plurality of battery cells. For example, battery pack  101  may include 12 rechargeable lithium ion battery cells stacked in series. A different number of battery cells stacked in series may also be used. In some embodiments, stacks of battery cells coupled in series may be coupled to other stacks of battery cells in parallel and/or in series. For example, a battery pack may include 4 stacks of 8 series connected battery cell stacks, where the 4 stacks are connected in parallel, and where each of the 8 series connected battery cell stacks includes 12 lithium ion battery cells connected in series. Some battery packs may use battery cells with a different chemistry. For example, some battery packs may use other lithium based chemistries. Other chemistries may be used. 
     Advantages of some embodiments include the increase of the common cause robustness of the battery management system by having redundant voltage measurement circuits that rely on independent measurement circuits implemented with different measurement technologies, and using different reference voltages and different measurement paths. Some embodiments further improve the diagnostics capabilities by performing statistical analysis between the voltage measurements of both voltage measurement circuits, such as, for example, checking for correlation between the measurements measured by both voltage measurement circuits. 
       FIG.  2    shows a flow chart of embodiment method  200  of detecting a failure of a battery management system, according to an embodiment of the present invention. Method  200  may be implemented using battery management system  100 . Alternatively, method  200  may be implemented in other battery management system implementations. The discussion that follows assumes that battery management system  100 , as shown in  FIG.  1   , implements method  200 . 
     During step  202 , a voltage across a battery cell, such as battery cell cell i  of battery pack  101 , is measured by a first voltage measurement circuit, such as, for example, voltage measurement circuit  110 . The first voltage measurement circuit is coupled to the battery cell via a network, such as analog filter and balancing network  104 . The first voltage measurement circuit measures the voltage across the battery cell via a first path during a first time. 
     During step  204 , the voltage across the battery cell is measured by a second voltage measurement circuit, such as, for example, voltage measurement circuit  112 , which is different than the first voltage measurement circuit. The second voltage measurement circuit measures the voltage across the battery cell via a second path during the first time. In other words, the first voltage measurement circuit and the second voltage measurement circuit simultaneously measure the voltage across the battery cell via different paths. 
     During step  206 , the voltage measured by the first voltage measurement circuit is compared with the voltage measured by the second voltage measurement circuit. If the voltages measured by the first and second voltage measurement circuits are substantially equal (e.g., the difference between the measurements is lower than or equal to a voltage threshold V th ), the battery management system is operating normally, and the measured voltage may be used for other purposes, such as, for example, deciding whether to rebalance the battery cell or stop charging. If the voltages measured by the first and second voltage measurement circuits are different (e.g., the difference between the measurements is higher than the voltage threshold V th ), a fault is detected. 
     Since the first and second voltage measurement circuits are measuring the same signal at the same time with comparable filters, sudden changes in the voltage of the battery cell will be equally captured by both voltage measurement circuits. Such common mode rejection allows for the setting of a low voltage threshold V th  for determining whether a fault exists. Using a low voltage threshold V th  may allow for the detection of faults such as leakage of the capacitor of analog filter and balancing network  104 , leakage in pins U i  and/or G i  of the battery monitor IC  102 , leakage in the ESD structures, and leakage in transistors  116  and/or  118   
     In some embodiments, for example using lithium-ion battery cells, it is determined that the first and second voltages are substantially equal if the absolute value of the difference between the first and second voltages is lower than a voltage threshold V th  of, e.g., 10 mV. Lower threshold values, e.g., 5 mV or lower, or higher threshold values, e.g., 20 mV, 50 mV or higher, may also be used. 
     Advantages of some embodiment include the detection of faults beyond open loop detection. For example, leakage in various components internal to the battery monitor IC and external to the battery monitor IC may be detected. 
       FIG.  3    shows a schematic diagram of a portion of battery management system  300 , according to another embodiment of the present invention. Battery management system  300  operates in a similar manner than battery management system  100  and may implement method  200  of detecting a failure of the battery management system. However, battery management system  300  includes two different pins along the different paths for monitoring each node across battery cell cell i . For example, node BC i  may be accessed using path  310  via pin U i  or via path  312  using pin G i-1 , and node BC i-1  may be accessed using path  310  via pin or via path  312  using pin G i-2 . 
     In some embodiments, voltage measurement circuit  110  is used as a primary measurement circuit to accurately measure the voltage across battery cell cell i  using pins U i  and while voltage measurement circuit  112  is used as a secondary measurement circuit for safety reasons to verify the voltage measured by measurement circuit  110  using pins G i-1  and G i-2 . By using two independent pins for the secondary measurement circuit as shown in  FIG.  3   , the voltage measured by the secondary measurement circuit is not influenced, e.g., by balancing currents. For example, balancing currents may flow into pin G i-1  and out through pin G i-2 . 
     Advantages of some embodiments include the inclusion of a redundant voltage measurement path by reusing existing structures of the balancing network, as shown, for example, in  FIGS.  1  and  3   . 
       FIG.  4    shows a schematic diagram of a portion of battery management system  400 , according to an embodiment of the present invention. Some components and details of battery management system  400 , such as ESD structures and open-loop diagnostics and balancing circuits, have been omitted for clarity purposes. Battery management system  400  includes n battery cells arranged in series coupled to battery monitor IC  402  via analog filter and balancing network  404 . Battery monitor IC  402  includes n voltage measurement circuits  410  and n voltage measurement circuits  412 . Each battery cell is coupled to a respective voltage measurement circuit  410  and voltage measurement circuit  412  via analog filter and balancing network  404 . 
     In some embodiments, n may be 12. In such embodiments, voltage at pin VS may be, for example, about 60 V during normal operation. Other embodiments may be implemented with n smaller than 12, such as for example, 6 or lower. Other embodiments may be implemented with values higher than 12, such as 15, 24, or higher. 
     As shown in  FIG.  4   , each of the redundant measurement voltage circuits is coupled to the respective battery cell via 3 pins, similar to the configuration shown in  FIG.  1   . Some embodiments may implement battery management system  400  using a 4 pin configuration, such as shown in  FIG.  3   . 
     By having n voltage measurement circuits  410  and n voltage measurement circuits  412 , battery management system  400  can simultaneously measure the voltage across each of the n battery cells. In other words, all the battery cells of battery pack  101  may have their associated voltages measured simultaneously and redundantly by the respective voltage measurement circuits  410  and  412 . 
     Some embodiments may use one or more multiplexers (MUXs) to share a voltage measurement circuit between two or more battery cells. For example,  FIG.  5    shows a schematic diagram of a portion of battery management system  500 , according to an embodiment of the present invention. Battery management system  500  operates in a similar manner than battery management system  400 . Battery management system  500 , however, shares voltage measurement circuit  510  over the n battery cells of battery pack  101  using MUX  505  instead of having n voltage measurement circuits  510 . 
     Battery management system  500  simultaneously measures the voltage across each of the n battery cells of battery pack  101  using voltage measurement circuit  510  and the respective voltage measurement circuit  512 . For example, the voltage across battery cell cell 1  is measured simultaneously by measurement circuit  512   0  and by voltage measurement circuit  510 , where MUX  505  is configured to select the channels associated with battery cell cell 1 . After the voltage across battery cell cell 1  is measured, the voltage across battery cell cell 2  may be measured simultaneously by measurement circuit  512 , and by voltage measurement circuit  510 , where MUX  505  is configured to select the channels associated with battery cell cell 2 . The sequence is repeated for each of the battery cells in battery pack  101 , although not necessarily in that order. 
     As shown in  FIG.  5   , voltage measurement circuit  510  is shared over the n battery cells of battery pack  101 . Some embodiments, may share a voltage measurement circuit  510  over k battery cells, where k is smaller than or equal to n. 
     Some embodiments may also share voltage measurement circuit  512  over more than one battery cells. For example,  FIG.  6    shows a schematic diagram of a portion of battery management system  600 , according to an embodiment of the present invention. Battery management system  600  operates in a similar manner than battery management system  500 . Battery management system  600 , however, shares voltage measurement circuit  512  over the n battery cells of battery pack  101  using MUX  604  instead of having n voltage measurement circuits  512 . 
     Battery management system  600  simultaneously measures the voltage across each of the n battery cells of battery pack  101  using voltage measurement circuits  510  and  612 . For example, the voltage across battery cell cell 1  is measured simultaneously by measurement circuit  510  and  612 , where MUXes  505  and  604  are respectively configured to select the channels associated with battery cell cell 1 . After the voltage across battery cell cell 1  is measured, the voltage across battery cell cell 2  may be measured simultaneously by measurement circuits  510  and  612 , where MUXes  505  and  604  are respectively configured to select the channels associated with battery cell cell 2 . The sequence is repeated for each of the battery cells in battery pack  101 , although not necessarily in that order. 
     As shown in  FIG.  6   , voltage measurement circuit  612  is shared over the n battery cells of battery pack  101 . Some embodiments, may share a voltage measurement circuit  612  over j battery cells, where j is smaller than n. In some embodiments, j may be equal to k. In some embodiments, j, k and n may be equal to each other. 
       FIG.  7    shows a schematic diagram of a portion of battery management system  700 , according to an embodiment of the present invention. Battery management system  700  includes battery pack  701  (which has for example 12 lithium ion battery cells), and battery monitor IC  702  coupled to battery pack  701  via analog filter and balancing network  703 . Battery management system  700  operates in a similar manner than battery management system  600 . Battery management system  700 , however, implements voltage measurement circuit  612  with SAR ADC  706  and implements 12 Σ-Δ ADC  704 , each coupled to the respective battery cell of battery pack  701  via analog filter and balancing network  703  instead of a single voltage measurement circuit  510  coupled to the battery pack via MUX  505 . Reference generator  111  provides reference voltage V ref1  to all 12 Σ-Δ ADC  704  while reference generator  113  provides reference voltage V ref2  to SAR ADC  706 . 
     During normal operation, controller  702  configures MUX  604  to select a channel (e.g., CH 0 ) associated with a particular battery cell (e.g., cell 1 ) and configures SAR ADC  707  and the respective Σ-Δ ADC  704  (e.g., Σ-Δ ADC  704   0 ) to measure the voltage across the particular battery cell simultaneously. The process is repeated for each of the battery cells in battery pack  701 . 
     As known, although Σ-Δ ADC  704  and SAR ADC  706  each produce a digital value associated with the sampled analog voltage at the input, Σ-Δ ADC  704  and SAR ADC  706  produces their respective digital value by using a different architecture and operating principle. For example, a SAR ADC typically samples and holds an analog voltage sample at the ADC input, and then generates a voltage with an m-bit DAC and compares the voltage generated from the m-bit DAC with the voltage sampled at the input using a comparator. The digital output produced by the SAR ADC is the digital code that when used to configure the m-bit DAC generates the voltage that is closest to that of the sampled input. Typically, a SAR ADC uses binary search to find such m-bit DAC code. The m-bit DAC typically has 8, 10, 12, or more bits. 
     A Σ-Δ ADC uses a combination of oversampling and noise shaping techniques to convert the analog input into a digital output. Typically, a 1-bit DAC is used in combination with a differentiator, an integrator and digital filtering to produce the digital code. In contrast to the SAR ADC, the Σ-Δ ADC is mostly implemented using digital logic rather than analog components. SAR ADCs and Σ-Δ ADCs are well known in the art and will not be discussed further. 
     Since Σ-Δ ADCs and SAR ADCs typically have different sampling rates, the filtering characteristics of analog filter and balancing network  703  may be designed to prevent aliasing for both types of ADCs. In some embodiments, anti-aliasing is achieved by reducing noise to values lower than e.g., 1 mV, at a frequency fs/2, where fs is the sampling frequency of the slowest ADC (e.g., SAR ADC). 
     Battery monitor IC  702  is implemented in a monolithic semiconductor substrate. In some embodiments, battery monitor IC  702  may be implemented in a multi-chip architecture, where, for example, SAR ADC  706  and MUX  604  are disposed in a first monolithic semiconductor substrate together with reference generator in, and Σ-Δ ADC  704  are disposed together with reference generator  113  in a second monolithic semiconductor substrate different than the first monolithic semiconductor substrate and packaged in the same package. Other implementations are also possible. 
       FIG.  8    shows a schematic diagram of measurement system  800  for achieving simultaneous measurements, according to an embodiment of the present invention. Measurement system  800  simultaneously measures the voltage across cell cell i  for the same duration of time with primary measurement circuit  804  and secondary measurement circuit  810  and generates a primary measurement result and a secondary measurement result based on the respective measured voltage. In some embodiments, voltage measurement circuit  110  may be implemented as primary measurement circuit  804  and voltage measurement circuit  112  may be implemented as secondary measurement circuit  810 . 
     Measurement system  800  includes primary measurement circuit  804 , secondary measurement circuit  810 , and anti-aliasing filters  802  and  808 . In some embodiments, antialiasing filters  802  and  808  correspond to analog filters and balancing networks (e.g., such as  104 ,  404  and  703 , previously described).  FIG.  8    shows a primary measurement result and a secondary measurement result each having 16 bits. It is understood that other values, such as 8 bits, 12 bits, 14 bits, 24 bits, 32 bits, may be used. 
     Primary measurement circuit  804  has a total effective filtering characteristic that includes the filtering characteristics of anti-aliasing filter  802  and sampling and averaging block  806 . Secondary measurement circuit  810  has a total effective filtering characteristic that includes the filtering characteristics of anti-aliasing filter  808 , and sampling block  812  and digital averaging block  814 . The total effective filtering characteristics of primary measurement circuit  804  and secondary measurement circuit  810  are dimensioned to allow for measuring the same signal at the same time by both measurement circuits  804  and  810 . 
     In some embodiments, the total effective filtering characteristics of measurement circuits  804  and  810  is matched by having similar dominant poles and similar step responses. As a non-limiting example, in an embodiment where primary measurement circuit  804  is implemented with a Σ-Δ ADC, and secondary measurement circuit  810  is implemented with a SAR ADC, anti-aliasing filter  802  may have a pole of about 500 kHz, the Σ-Δ ADC has a sampling and averaging block  806  that may sample the respective input signal at about 20 MHz and have an averaging N-order filter with a pole at about 100 Hz, where n may be greater or equal to 1, anti-aliasing filter  808  may have a pole of about 10 kHz, the SAR ADC has a sampling block  812  that may sample the input signal at about 400 Hz, and an N-order digital averaging block  814  may perform digital averaging with a pole at about 100 Hz. In this example, the dominant pole of each of measurements circuits  804  and  810  is about 100 Hz with the same frequency slope (e.g., 20 dB per decade when n is equal to 1). Other values for the pole frequencies, sampling frequencies and order of the filters may be used. 
     Some embodiments may achieve similar (matched) total effective filtering characteristics without having equal dominant poles that decay at the same rate. For example, in some embodiments, primary measurement circuit  804  may have a first order pole at 100 Hz and a first order pole at 105 Hz while secondary measurement circuit  810  may have a second order pole at 99 Hz. In such embodiment, the total effective filtering characteristic of primary measurement circuit  804  is considered as matching the total effective filtering characteristic of secondary measurement circuit  81   o . Other filtering characteristics and other values for the dominant poles and frequency slopes may be used. 
     In some embodiments, the Σ-Δ ADC is implemented with a first order cascaded integrator-comb (CIC) filter. The CIC filter may be implemented in any way known in the art. 
     In some embodiments, measurements circuits  804  and  810  may be implemented in the same manner. For example, in some embodiments, measurement circuits  804  and  810  may both be implemented with Σ-Δ ADCs. In some embodiments, measurement circuits  804  and  810  may both be implemented with SAR ADCs. In yet other embodiments, primary measurement circuit  804  is implemented with a Σ-Δ ADC while secondary measurement circuit  810  is implemented with comparators. Other implementations are also possible. 
       FIG.  9    illustrates timing diagrams for measuring voltage across each of the 12 battery cells of battery pack  701 , according to an embodiment of the present invention. Waveform  904  corresponds to conversion timing of Σ-Δ ADC  704 . Waveform  906  corresponds to conversion timing of SAR ADC  706  and associated MUX  604 . Waveform  905  is a zoomed-in version of a portion of waveforms  904  and  906 . 
     As shown in  FIG.  9   , during diagnostic time t CH_Diagnose  the voltage across each of the 12 battery cells of battery pack  701  are measured. For example, during time t cho , Σ-Δ ADC  704   0  measures the voltage across battery cell cell 1  while SAR ADC  706  measures the voltage across channel CH 0  of MUX  604 , which is associated with the voltage across battery cell cell 1 . During time t ch1 , Σ-Δ ADC  7041  measures the voltage across battery cell cell 2  while SAR ADC  706  measures the voltage across channel CH 1  of MUX  604 , which is associated with the voltage across battery cell cell 2 . The sequence is repeated for all the 12 battery cells of battery pack  701 . 
     As shown in waveform  905 , SAR ADC  706  and the respective Σ-Δ ADC  704  measures the voltage across the respective battery cell at the same time and for the same duration of time. In other words, even though SAR ADC  706  typically collects a smaller number of samples than Σ-Δ ADC  704  during the measurement time, the time during which the samples are collected should be the same. The samples collected by each ADC during the measurement time are then respectively averaged (e.g., by using a low pass filter) to obtain a respective final value. For example, the frequency J of Σ-Δ ADC  704  and the number of Q input samples taken by Σ-Δ ADC  704  are selected to take the same time as the time taken by SAR ADC  706  to take L samples at a frequency P. As a non-limiting example, if the duration of time t cho  is 75 μs, SAR ADC  706 , which operates at 666 kHz, collects 50 input samples while Σ-Δ ADC  704   0  takes 1024 samples of the input signal while operating at an over-sampling rate of 13.65 MHz. In some embodiments, time t 1  between conversions is minimized, for example, to 1 μs. 
       FIG.  10    shows a schematic diagram of a portion of battery management system  1000 , according to an embodiment of the present invention. Battery management system  1000  includes battery pack  701  and battery monitor IC  1002  coupled to battery pack  701  via analog filter and balancing network  703 . Battery management system  1000  operates in a similar manner than battery management system  400 . Battery management system  1000 , however, implements each voltage measurement circuit  410  with a respective Σ-Δ ADC  704  and each of the voltage measurement circuits  512  with a respective comparator circuit  1008 . Reference generator  111  provides reference voltage V ref1  to all 12 Σ-Δ ADC  704  while reference generator  113  provides reference voltage V ref2  to DAC  1016 , which provides a reference to all comparator circuits  1008 . 
     During normal operation, each of the comparator circuits  1008  operates as a window comparator. The high threshold and low threshold of each comparator circuit  1008  is provided by DAC  1016 . 
     During the measurement time, the respective Σ-Δ ADC  704  collects samples and generates a digital value associated with the measured voltage while the respective comparator circuit  1008  generates a plurality of comparison results and generates as a final value the most frequent comparison result. For example, assuming that battery cell 1  has voltage V 3  (e.g., 3.6 V) across its terminals, Σ-Δ ADC  704   0  collects a plurality of samples during a first measurement time and generates as a result a digital value that corresponds to 3.6 V. During the same first measurement time, comparator circuit  1008   0  compares the value at its inputs with the values provided by DAC  1016  and, for each comparison, generates a value (e.g., 0) representative of the input being outside the window if the voltage is outside the window and a value (e.g., 1) representative of the input being inside the window if the voltage is inside the window. If comparator circuit  1008   0  generates Z samples (e.g., 100 samples), of which more samples (e.g., 51 or more) are inside the window (e.g., 1) and less samples (e.g., 49 or less samples) are outside the window (e.g., 0), the final result is that the input is inside the window (e.g., 1), which means that the sampled voltage is within the limits provided by DAC  1016 . If instead, out of the Z samples, more samples are outside the window (e.g., 0) and less samples are inside the window (e.g., 1), the final result is that the input is outside the window (e.g., 0), which means that the sampled voltage is outside the window provided by DAC  1016 . The same measurement is performed for each battery cell of battery pack  701 . In this way, comparator circuits  1008  may be used to verify that the voltage measured by the respective Σ-Δ ADC  704  is inside the window specified by DAC  1016 . 
     In some embodiments, comparator circuit  1008   0  generates a 0 when the input is outside the window and a 1 when the input is inside the window. In other embodiments, comparator circuit  1008   0  generates a 1 when the input is outside the window and a 0 when the input is inside the window. In other embodiments, comparator circuit  1008   0  generates a negative value when the input is outside the window and a positive value when the input is inside the window. In other embodiments, comparator circuit  1008   0  generates a positive value when the input is outside the window and a negative value when the input is inside the window. Other implementations are possible. 
     In some embodiments, DAC  1016  may set the high threshold to the same level as the maximum recommended operating voltage for the battery cell (e.g., 4.5 V for a lithium ion cell) and the low threshold to the minimum operating voltage for the battery cell (e.g., 2.7 V for a lithium ion cell). Other embodiments may implement a tighter window, such as, for example, a 50 mV window, or lower. 
     The voltage curve of a battery cell, such as a lithium battery cell, across different charge levels is not linear. For example, when a lithium battery cell is fully charged, the voltage across the battery cell may be as high as 4.2 V or higher, and when the lithium battery cell is discharged, the voltage across the battery cell may be 3 V or lower. During most of the time (e.g., from 80% charge level to 20% charge level) the voltage may be about 3.6 V. To achieve a tight window (of e.g., 5 mV) of comparison for comparator circuits  1008  during different points in the charge curve, some embodiments dynamically generate the values of the window. For example, some embodiments may make a first measurement with the respective Σ-Δ ADC  704  during a first time, then configure DAC  1016  to generate a window centered at the measured value, and then simultaneously measure during a second time the voltage across the battery cell with the respective Σ-Δ ADC  704  and the respective comparator circuit  1008 . 
       FIG.  11    shows a flow chart of embodiment method  1100  of detecting a failure of a battery management system, according to an embodiment of the present invention. Method  1100  may be implemented using battery management system  1000 . Alternatively, method  1100  may be implemented in other battery management system implementations. The discussion that follows assumes that battery management system  1000 , as shown in  FIG.  10   , implements method  1100 . 
     During step  1102 , a first voltage Volt, across a battery cell, such as battery cell cell 1  of battery pack  701 , is measured by a first voltage measurement circuit, such as, for example, Σ-Δ ADC  704   0  via a first path during a first time. 
     During step  1104 , a high voltage threshold V th_high  and a low voltage threshold V th_low  are set based on the first voltage. For example, if the first voltage measured is 3.6 V, the high voltage threshold V th_high  may be set to 3.65 V and the low voltage threshold V th_low  may be set to 3.55 V. In some embodiments, the measured voltage may be centered between the high voltage threshold V th_high  and a low voltage threshold V th_low  selected. In other embodiments, the measured voltage may not be centered between the high voltage threshold V th_high  and a low voltage threshold V th_low  selected. The voltage thresholds are applied to a window comparator, such as comparator circuit  1008   0  by using, for example a DAC, such as DAC  1016 . 
     During steps  1106 , a second voltage Volt 2  is measured using the first voltage measurement circuit during a second time. During the same second time, a third voltage Volt 3  is sampled with the comparator circuit to determine whether the voltage Volt 3  is inside the window specified by the high voltage threshold V th_high  and the low voltage threshold V th_low . 
     If the second voltage Volt 2  is outside the window, step  1102  is executed again, as shown by step  1110 . Voltage Volt 2  may be outside the window, for example, because of a sudden spike in current. If voltage Volt 2  is inside the window and the comparator circuit indicates that the third voltage Volt 3  is inside the window, then the battery management system is operating normally, and no errors are affecting the measurements. Else, if voltage Volt 2  is inside the window and the comparator circuit indicates that the third voltage Volt 3  is outside the window, a fault is detected, as shown by steps  1112  and  1116 . 
     In some embodiments, method  1100  is executed sequentially in each battery cells. In other embodiments, multiple memory cells (such as 2, 3, 4, or more, including all memory cells of battery pack  701 ) execute method  1100  simultaneously. 
     Example embodiments of the present invention are summarized here. Other embodiments can also be understood from the entirety of the specification and the claims filed herein. 
     Example 1. A method for providing battery diagnostics, the method including: measuring a first voltage across a first battery cell of a rechargeable battery via a first measurement path of a network using a first measurement circuit, measuring the first voltage including taking at least one first voltage sample during a first time period using the first measurement circuit; measuring a second voltage across the first battery cell via a second measurement path of the network using a second measurement circuit, measuring the second voltage including taking at least one second voltage sample during the first time period using the second measurement circuit, where the second measurement path of the network is different from the first measurement path of the network; comparing the measured first voltage with the measured second voltage; and generating a diagnostic output signal based on the comparison. 
     Example 2. The method of example 1, where the at least one first voltage sample is taken using a first measurement scheme and the at least one second voltage sample is taken using a second measurement scheme, where the second measurement scheme is different from the first measurement scheme. 
     Example 3. The method of one of examples 1 or 2, where measuring the first voltage further includes taking a first plurality of voltage samples during the first time period using the first measurement circuit; and where measuring the second voltage further includes taking a second plurality of voltage samples during the first time period using the second measurement circuit. 
     Example 4. The method of one of examples 1 to 3, where measuring the second voltage further includes averaging an output of the second measurement circuit using an averaging circuit coupled to the second measurement circuit. 
     Example 5. The method of one of examples 1 to 4, where the diagnostic output signal is asserted when a difference between the measured first voltage and the measured second voltage is higher than a first voltage threshold. 
     Example 6. The method of one of examples 1 to 5, where the first measurement circuit has a first dominant pole and the second measurement circuit has a second dominant pole, where the method further includes matching the first dominant pole and the second dominant pole. 
     Example 7. The method of one of examples 1 to 6, where a first total effective filtering characteristic of the first measurement path together with the first measurement circuit is substantially similar to a second total effective filtering characteristic of the second measurement path together with the second measurement circuit. 
     Example 8. The method of one of examples 1 to 7, further including: providing a first reference voltage to the first measurement circuit with a first reference voltage generator; and providing a second reference voltage to the second measurement circuit with a second reference voltage generator different from the first reference voltage generator. 
     Example 9. The method of one of examples 1 to 8, further including: providing the first reference voltage to a third measurement circuit with the first reference voltage generator, where the third measurement circuit and the first measurement circuit are based on a same measurement scheme; measuring a third voltage across a second battery cell of the rechargeable battery via a third measurement path of the network using the third measurement circuit, measuring the third voltage including taking a third plurality of third voltage samples during a second time period using the third measurement circuit, the second time period occurring after the first time period, the second battery cell coupled in series with the first battery cell; measuring a fourth voltage across the second battery cell via a fourth measurement path of the network using the second measurement circuit, measuring the fourth voltage including taking a fourth plurality of fourth voltage samples during the second time period using the second measurement circuit, the fourth measurement path being different than the third measurement path; comparing the measured third voltage with the measured fourth voltage; and asserting the diagnostic output signal when a difference between the third voltage and the fourth voltage is higher than a first voltage threshold. 
     Example 10. A circuit including: a first measurement circuit configured to be coupled to a first battery cell via a first path of a network; a second measurement circuit configured to be coupled to the first battery cell via a second path of the network, the second path being different than the first path; and a controller configured to: cause the first measurement circuit to measure a first plurality of voltage samples across first and second terminals of the first battery cell during a first time period, cause the second measurement circuit to measure a second plurality of voltage samples across the first and second terminals of the first battery cell during the first time period, compare an output of the first measurement circuit with an output of the second measurement circuit, and generate a diagnostic output signal based on the comparison. 
     Example 11. The circuit of example 10, where the first measurement circuit has a different architecture than the second measurement circuit. 
     Example 12. The circuit of one of examples 10 or 11, where the first measurement circuit has a first dominant pole, the second measurement circuit has a second dominant pole, and the first dominant pole is substantially equal to the second dominant pole. 
     Example 13. The circuit of one of examples 10 to 12, further including: a first reference voltage generator coupled to the first measurement circuit; and a second reference voltage generator coupled to the second measurement circuit. 
     Example 14. The circuit of one of examples 10 to 13, where the first measurement circuit includes a sigma-delta analog-to-digital converter (ADC) and the second measurement circuit includes a successive approximation register (SAR) analog-to-digital converter (ADC). 
     Example 15. The circuit of one of examples 10 to 13, where the second measurement circuit includes a window comparator, and where the controller is configured to set an upper limit of the window comparator and a lower limit of the window comparator based on the output of the first measurement circuit. 
     Example 16. The circuit of one of examples 10 to 15, further including: a first sensing terminal coupled to the first measurement circuit and configured to be coupled to the first terminal of the first battery cell via the network; a second sensing terminal coupled to the first measurement circuit and to the second measurement circuit and configured to be coupled to the second terminal of the first battery cell; and a first power terminal coupled to the second measurement circuit and configured to be coupled to the first terminal of the first battery cell. 
     Example 17. The circuit of one of examples 10 to 15, further including: a first sensing terminal coupled to the first measurement circuit and configured to be coupled to the first terminal of the first battery cell via the network; a second sensing terminal coupled to the first measurement circuit and configured to be coupled to the second terminal of the first battery cell; a first power terminal coupled to the second measurement circuit and configured to be coupled to the first terminal of the first battery cell; and a second power terminal coupled to the second measurement circuit and configured to be coupled to the second terminal of the first battery cell. 
     Example 18. The circuit of one of examples 10 to 17, where a first total effective filter characteristic of the first path is as about equal to a second total effective filter characteristic of the second path. 
     Example 19. The circuit of one of examples 10 to 18, where the network includes a balancing network. 
     Example 20. The circuit of one of examples 10 to 19, where the first measurement circuit has a first step response, the second measurement circuit has a second step response, and the first response is substantially equal to the second step response. 
     Example 21. A battery management system including: a rechargeable battery including N battery cells coupled in series, where N is a positive integer greater than zero; a balancing network coupled to the rechargeable battery; and a battery monitoring circuit coupled to the balancing network, the battery monitoring circuit including: a sigma-delta analog-to-digital converter (ADC) having an input configured to be coupled to a first battery cell of the N battery cells via a first path of the balancing network, the sigma-delta ADC coupled to a first reference voltage generator; a measurement circuit having an input configured to be coupled to the first battery cell via a second path of the balancing network, the second path being different from the first path, the measurement circuit coupled to a second reference voltage generator different from the first reference voltage generator, the measurement circuit having a different architecture than the sigma-delta ADC; and a controller configured to: control the sigma-delta ADC to measure a first plurality of voltage samples across first and second terminals of the first battery cell during a first time period, control the measurement circuit to measure a second plurality of voltage samples across the first and second terminals of the first battery cell during the first time period, compare an output of the sigma-delta ADC with an output of the measurement circuit, and generate a diagnostic output signal based on the comparison. 
     Example 22. The battery management system of example 21, further including N sigma-delta ADCs and N measurement circuits, where the N sigma-delta ADCs include the sigma-delta ADC, and where each of the N sigma-delta ADCs is coupled to a respective battery cell of the N battery cells, and where the N measurement circuits include the measurement circuit, and where each of the N measurement circuits is coupled to a respective battery cell of the N battery cells. 
     Example 23. The battery management system of example 21, where the sigma-delta ADC has a first dominant pole of Nth order, where N is a positive integer greater or equal to 1, the measurement circuit has a second dominant pole of the Nth order, and the first dominant pole is substantially equal to the second dominant pole. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.