Abstract:
This invention includes a method for sensing the parasitic impedance in a battery charging system and compensating for these parasitic impedances. In one embodiment, the voltage of the system is measured with no charging current applied. Next, a charging current is applied and a second voltage is measured. The parasitic impedance is then extrapolated and multiplied by a predetermined rapid charging current. The product is added to a predetermined cell termination voltage. The charging means remains in a rapid charge current mode until a voltage equal to the sum of the impedance-current product and the predetermined voltage is reached, wherein the current is reduced to a maintenance charging level.

Description:
TECHNICAL FIELD 
     This invention relates generally to battery charging methods, and more particularly to charging battery cells while dynamically compensating for parasitic cable impedance. 
     BACKGROUND 
     Rechargeable battery cells are typically charged with chargers and power supplies that connect to a host device through a copper cable. For example, in a cellular telephone application, a power supply might plug into a wall receptacle, with a three to six foot cable running from the supply to the phone. These cables, like any metal conductor, have a parasitic impedance. They are not perfect conductors in that when current passes through the cable, power is dissipated due to the characteristic resistivity (ρ) of the conductor. This dissipated power is lost as heat, which reduces the efficiency of the charging system. 
     The parasitic power loss is problematic to charger designers. Lithium batteries have very sensitive voltage thresholds that cannot be exceeded. For example, the integrity of a lithium cell rated at 4.1 V can be compromised if charged to a voltage above this limit. At the same time, the voltage of a lithium battery is a direct indicator of battery capacity. Thus, the goal is to charge the battery as quickly as possible to 4.1 V, without exceeding this limit. 
     Chargers that employ a cable to connect to the battery must measure the termination voltage of the battery at the charger side of the cable. As a result, the actual voltage measured by the charger is the battery cell voltage plus the voltage drop across the cable. As the cable impedance can be as high as 600 mΩ, and as the charging current can be as high as 1 amp, the actual measured voltage at the charger can be in error by as much as 600 mV! Additionally, the impedance of the cable can vary with the quality of the connection and temperature of the conductor. 
     One possible solution to this erroneous measurement is to take the voltage measurement on the battery side of the cable. The problem with this method is that an extra conductor in the cable is required to transmit the voltage information back to the charger. This adds cost and weight to the charging system. 
     Chargers generally charge initially at a very high current (rapid charge) until the cell reaches its termination voltage, and then at a very low rate (trickle charge) until the battery is fully charged. The longer that a charger remains in rapid charging mode, the quicker the battery will charge. Erroneous voltage measurements mean that the charger will begin to taper charging current before charging is complete. In other words, the charger will sense 4.1 V (and therefore terminate rapid charging) before the cell actually hits 4.1V because there will be a 600 mV parasitic voltage drop across the cable. Since the charger does not know what the impedance of the cable is, it must assume a zero impedance and thus terminate rapid charging when the charger side of the cable reaches 4.1 V. The net effect is that a battery takes longer to charge than it should. 
     The charger&#39;s goal is to accurately and quickly charge the cell. Parasitic cable impedance interferes with this goal. There is thus a need for a faster charging algorithm which compensates for cable impedance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an exemplary charging circuit applicable to the present invention. 
     FIG. 2 illustrates graphical data of battery charging without compensation for parasitic impedance. 
     FIG. 3 illustrates graphical data of battery charging in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” 
     This invention includes a battery charging algorithm and method for compensating for the parasitic impedance of a cable. This invention can be employed on conventional charging hardware by integrating the steps of the algorithm into the charger&#39;s operating firmware. 
     Referring now to FIG. 1, a typical charging circuit  100 , with parasitic impedances  108 - 110 ,  113 - 115 , is illustrated therein. The charging circuit includes a control circuit  101 , which may be a microprocessor, programmable logic, state machine, pulse width modulator controller, or the like. For exemplary purposes, we will describe the control circuit  101  as a microprocessor, as microprocessors are very flexible with respect to configuration. Any number of microprocessors could be used in this application, including the 78 KOS family manufactured by NEC. 
     The control circuit  101  includes a charge control output  102 . This charge control output  102  can be a binary hi-lo output for controlling a pass transistor  106  in an on-off state, a pulse width modulated switching output, or a linear output to control a linear regulator. A pass transistor  106  serves as the voltage converter and a blocking diode  107  ensures that the charging circuit  100  does not discharge the battery cell  112 . Also included is a current sense resistor  116  so that the control circuit  101  may monitor the charging current through the second analog to digital (A/D) converter  104 . A first A/D converter  103  senses the voltage at the charger by comparing the first A/D converter  103  value and the second A/D converter  104  value. 
     Various parasitic impedances are also shown. These impedances include the first contact impedances  108 , 115  where the cable connects to the charger, the cable impedances  109 , 114  and the second contact impedances  110 , 113  where the cable connects to the battery. By way of Kirchoff&#39;s Voltage Law, as current flows from the blocking diode  107  to the current sense resistor  116 , there are six parasitic voltage drops across the parasitic impedances and the voltage drop across the cell. The voltage measured by the control circuit  101  includes these six parasitic drops and is thus not-equal to the cell voltage while current is flowing through the cable. 
     One embodiment of this invention determines the equivalent circuit impedance by applying predetermined charging currents, measuring the voltage with the control circuit  101  and then mathematically extrapolating the equivalent resistance. Once this resistance is known, the charger may apply a rapid charging current until the voltage sensed by the control circuit  101  is equal to the termination voltage of the cell  112  plus the charging current times the equivalent resistance, thereby reducing the charging time of the cell  112 . 
     By way of example, a first charging current of 100 mA may be applied to the cell  112 , and a corresponding voltage V 1  would be determined by subtracting the voltage at the second A/D  104  from the voltage at the first A/D. Next, a 500 mA current may be applied and a second voltage, V 2 , could likewise be measured. From Ohm&#39;s Law, it is known that: 
     
       
         
           V 
           chgr 
           =V 
           cell 
           +I 
           chg 
           *R 
           equiv 
         
       
     
     where V chgr  is either V 1  or V 2 , V cell  is the voltage of the cell  112 , I chg  is the applied charging current and R equiv  is the equivalent resistance. 
     When the first charging current and second charging current are applied across a short period of time, the battery cell voltage remains effectively constant. Thus, the equivalent impedance, Requiv, can be determined by having the control circuit  101  simultaneously solve two equations by substituting V 1  and V 2  for Vchgr and the corresponding currents for Ichg. The corresponding firmware algorithm for the control circuit  101  might look as follows: 
     
       
         Set  V   1 = Vcell +100 mA* Requiv   
       
     
     
       
         Set  V   2 = Vcell +500 mA* Requiv   
       
     
     
       
         Set  Requiv =( V   2 − V   1 )/(500−100) 
       
     
     If the cell voltage can be reliably measured with the current off, the calculation of Requiv can be further simplified. There would only need to be one charging current. The equation would simply be: 
     
       
           Requiv= ( V   2 − V   1 ) Ichg   
       
     
     Once the equivalent resistance is known, the control circuit  101  can be programmed to charge in rapid mode until the charger voltage reaches the cell termination voltage plus the rapid charge current times the equivalent resistance. This method of charging greatly reduces the charging time of a cell. 
     As previously noted, when the equivalent resistance is not compensated for, the charger prematurely reduces the charging current from a rapid charge to a trickle charge. Referring now to FIG. 2, illustrated therein is a charging curve for a 650 mA*h, 4.1 V lithium-ion cell where no cable compensation is taken into account. As can be seen at point  200 , a rapid charge current is applied until the cell voltage reaches 90% of its termination value (4.1V) at point  201 . This voltage includes not only the cell voltage, but also the equivalent resistance (represented by curve  202 ) times the charging current. 
     Battery capacity was measured by an external device and is plotted on the same graph as curve  203 . Once 90% of the termination voltage is reached at point  201 , the charging current is reduced to a trickle current at point  204 . By charging in this mode, after 250 minutes, the cell has reached 460 mA*h, or 70% of its rated capacity. 
     Referring now to FIG. 3, illustrated therein is a graph showing the same parameters as in FIG. 2, but with cable compensation. While the battery was charging, a 500 mA test current was applied and a voltage was measured and stored by the control circuit as V 1 . The current was then turned off and a second voltage was stored by the control circuit as V 2 . Using the method above, the equivalent impedance was calculated as 1.2 Ohms. Thus, the termination voltage (the voltage at which rapid charge is changed to trickle charge) was increased from 4.1V (the termination voltage of the cell) to 7.1V (4.1V+1.2 Ohms*2.5 A). 
     As can be seen from the graph, the rapid charging current is applied for nearly 50 minutes, as opposed to the less than 5 minutes of FIG.  2 . Consequently, the battery reaches 90% of its capacity in only 60 minutes, as opposed to more than 250 minutes in FIG.  2 . Thus, the method yields more than a 400% improvement in charging time. 
     While the preferred embodiments of the invention have been illustrated and described, it is clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions, and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the following claims. For example, while the invention has been described as using a microprocessor capable of mathematical computation, the circuit could easily be designed with analog parts using comparators to reach thresholds in the detection of the equivalent impedance.