Abstract:
A system includes a battery; an analog-to-digital converter coupled to the battery and capable of measuring an output voltage of the battery; a processor, receiving measured battery output voltages from the analog-to-digital converter; the processor using a first equivalent circuit model of the battery to estimate battery current when the battery operation is static; and the processor using a second equivalent circuit model of the battery to estimate battery current when the battery operation is dynamic.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/072,171, filed Oct. 29, 2014, for “METHOD AND APPARATUS OF BATTERY FUEL GAUGING USING A NOVEL HYBRID LITHIUM ION BATTERY MODEL”, for Githin Karippumannil Prasad, et al., which is incorporated herein in its entirety, 
     
    
     BACKGROUND 
       [0002]    In battery powered systems it is important to have an accurate estimate of the battery&#39;s usable capacity, called State of Charge (SOC). In an analogy to automotive fuel gauges, instrumentation to estimate SOC is called a battery fuel gauge. If a battery fuel gauge overestimates SOC, then the battery might unexpectedly stop functioning, forcing a system being powered by the battery to uncontrollably shut down, which in some cases might result in catastrophic data loss. If a battery fuel gauge underestimates SOC, then a system being powered by the battery might be warned that a battery is discharged when it is actually still has charge available, resulting in an inconvenient and unnecessary shut down or recharging operation. Accordingly, fuel gauge accuracy is important. 
         [0003]    Battery fuel gauges range from very simple to very complex. The simplest of gauges involves the method of voltage correlation, in which the SOC is determined using the strong correlation of the battery&#39;s open circuit voltage with its state of charge. However, accurate open circuit voltage values can be obtained only when the battery reaches equilibrium upon relaxation after a load, which can be very time consuming. Moreover, the battery will not reach equilibrium if it is always under load, so SOC may not be updated accurately. 
         [0004]    Another simple gauging technique, known as Coulomb Counting, uses a current-sense resistor connected in series with the output of the battery. The voltage across the resistor is used to measure current, and the current is integrated during charging and discharging to estimate SOC. However, an external resistor wastes energy and reduces the useable supply voltage. 
         [0005]    Another gauging approach is to model the battery as an equivalent circuit. The output voltage of the battery is monitored, current is estimated using the output voltage and the model, and the estimated current is integrated to determine an estimated change of charge. One simple equivalent circuit model (called an R model) assumes the battery is an ideal voltage source with an estimated internal resistance. The current can be estimated based on the open circuit voltage and the voltage drop resulting from current flowing through the estimated internal resistance. However, the R model does not accurately estimate the current because it does not capture the transient voltage behavior that occurs at the onset of a load change. 
         [0006]    An improved equivalent circuit model (called an RC model) has at least one parallel resistance/capacitance circuit, with the parallel resistance/capacitance circuit in series with an additional resistance.  FIG. 1  illustrates an example of an RC model  100  of a battery. In the example model, there is a voltage source  102  having a voltage V OC , which is the steady state open source voltage of the battery. There is a series resistor R SER . There is also a parallel circuit with a resistor R PAR  in parallel with a capacitor C PAR . The model circuit has an output voltage V OUT . A processor (not illustrated) monitors V OUT  and estimates the load current i LOAD  using the equivalent circuit model  100 . The processor integrates the estimated load current i LOAD  to obtain an estimated amount of change in charge. An example of how R SER , R PAR , and C PAR  are determined may be found in U.S. Patent Application Publication US 2012/0143585, published Jun. 7, 2012, by Barsukov et al., which is incorporated herein for all that it teaches and discloses. 
         [0007]    The most sophisticated and most accurate models are physics based, with complex differential equations modeling a large number of electrochemical parameters. Many of the electrochemical parameters are difficult to measure, and the computational complexity may be impractical for portable real-time electronic devices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a block diagram schematic of an example embodiment of a prior art equivalent circuit model of a battery. 
           [0009]      FIG. 2  is a block diagram of an example embodiment of a system that includes a battery and a battery fuel gauge. 
           [0010]      FIG. 3A  is a block diagram schematic of an example embodiment of a battery equivalent circuit model used by the battery fuel gauge of  FIG. 2 . 
           [0011]      FIG. 3B  is a block diagram schematic of an example embodiment of a battery equivalent circuit model used by the battery fuel gauge of  FIG. 2 . 
           [0012]      FIG. 4  is a flow chart of an example method estimating charge from a battery. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    The RC model improves the estimates of current during dynamic load changes, but still results in a degree of inaccuracy that is important in some systems, because the R and C parameters are a strong nonlinear function of SOC. The RC model can correctly represent the transient behavior of the battery at the onset of a load for a slight or no change in SOC. However, it cannot accurately predict the complete charge/discharge characteristics because an RC model, even with two or three parallel RC networks, does not correspond to the complex internal structure of the battery system when its parameters (resistances and capacitances) are changing with SOC, for example, during a long discharge. A more accurate load current estimation is obtained when a first model is used during static operation (that is, constant load current after a transient period or no load current) and a second model is used during dynamic operation (onset of changing load current). In addition, a more accurate result is obtained when each model has a separate transfer function for each temperature and each SOC. In one specific example embodiment, an R model is used during static operation (constant load current after an initial onset transient has subsided) and an RC model is used during dynamic operation (onset of changing load current). Accordingly, in the following example embodiment, a single resistor is used for an equivalent circuit model during static operation, and a series resistor with at least one parallel resistor and capacitor circuit is used for an equivalent circuit model during dynamic operation. This hybrid model improves precision in determining the load current by capturing the transient dynamics and the post transient behavior to give an accurate description of the entire battery discharge characteristic. The R model and RC model are just examples of different models that may be used, and other models may be used when needed by a particular transfer function at a particular temperature and SOC. 
         [0014]    For highest accuracy, the model circuit parameters (resistor and capacitor values) vary with temperature and SOC. Accordingly, in the following example embodiment, battery temperature is optionally measured and resistance and capacitance values of the equivalent circuit models are optionally changed as a function of temperature and SOC. 
         [0015]      FIG. 2  illustrates a system  200  that includes a battery  202  (depicted in the example as a stack of multiple batteries in series) and a battery fuel gauge  204 . The battery fuel gauge  204  includes an Analog-to-Digital Converter (ADC)  206  that measures the output voltage of the battery  202  and converts the battery voltage to digitized voltage samples. A processor (or controller)  208  receives the digitized voltage samples, estimates current, and integrates estimated current to estimate the SOC of the battery. An optional temperature sensor  210  measures the temperature of the battery  202  and the processor  208  optionally receives digital temperature measurements. The ADC  206  and processor  208  may be part of a separate battery fuel gauge module or chip, or they may be integral parts of the system  200 . For example, the processor  208  may be a processor that also controls the overall system  200 . The temperature sensor  210  may be integrated into the battery  202 , or it may be a separate device, or it may be part of a separate fuel gauge module. The equivalent circuit model parameters for the battery  202  may be measured as a function of temperature and SOC as a one-time measurement before manufacturing of the system  200  and the resulting values stored for use by the processor  208 . The processor  208  may use a curve fit to calculate resistance and capacitance values as a function of temperature and SOC or resistance and capacitance values as functions of temperature and SOC may be stored in tables for table look-up. 
         [0016]      FIG. 3A  illustrates an example of an equivalent circuit model  300  used by the processor  208  in  FIG. 2  during static operation (constant load current after a transient period or no load current). There is a voltage source  304  having a voltage V OC , which is the steady state open source voltage of the battery, and a series resistor R SER1 . The circuit model  300  has an output load current i LOAD  and an output voltage V OUT . If battery temperature is measured then the circuit model  300  models R SER1  as a function of temperature. 
         [0017]      FIG. 3B  illustrates an example of an equivalent circuit model  302  used by the processor  208  in  FIG. 2  during the dynamic phase (onset of changing load current). There is a voltage source  304  having a voltage V OC , which is the steady state open source voltage of the battery, a series resistor R SER2 , a first parallel RC circuit  306  comprising a resistor R 1  in parallel with a capacitor C 1 , and a second parallel RC circuit  308  comprising a resistor R 2  in parallel with a capacitor C 2 . The circuit model  302  has an output load current i LOAD  and an output voltage V OUT . If battery temperature is measured then the circuit model  302  models R SER2 , R 1 , and R 2  as a function of temperature. The circuit of  FIG. 3B  provides a more accurate model during the transient period of the battery. A single RC circuit may be used, but two RC circuits ( 306 ,  308 ) provide better accuracy. An example of determination of the parameters for the equivalent circuit of  FIG. 3B  may be found in U.S. Pat. No. 8,242,738 B2, issued Aug. 14, 2012, which is hereby incorporated by reference. 
         [0018]    In  FIG. 3B , V 1  is the voltage across the parallel combination of R 1  and C 1 . During a transient condition voltage V 1  is varying and then at the end of the transient period voltage V 1  converges to a constant value. For one example of how the processor may determine which model to use, both models ( FIG. 3A  and  FIG. 3B ) may be computed for every measurement of V OUT , and when V 1  is substantially unchanging then the model of  FIG. 3A  is used to estimate i LOAD , and when V 1  is varying then the model of  FIG. 3B  is used to estimate i LOAD . The choice of whether to measure the voltage across R 1  and C 1  or to measure the voltage across R 2  and C 2  is arbitrary. 
         [0019]      FIG. 4  illustrates an example method  400  of estimating charge from a battery. At step  402 , a processor measures an output voltage of the battery. At step  404 , during static conditions, the processor estimates current from the battery using a first equivalent circuit model. At step  406 , during dynamic conditions, the processor estimates current from the battery using a second equivalent circuit model. At step  408 , the processor integrates estimated current to estimate charge. 
         [0020]    While illustrative and presently preferred embodiments of the invention have been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.