Abstract:
The invention provides a method for balancing the electrical voltages of at least two electrical accumulator units that are connected in series. According to the invention, one accumulator unit is connected to the winding of a coil in order to excite the coil, and the other accumulator unit is charged by the excited coil by the subsequent connection of the winding to the other accumulator unit. In addition, the invention provides a corresponding electrical accumulator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP 2009/065469 filed on Nov. 19, 2009. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a method for balancing the electrical voltages of at least two serially connected electrical accumulator units. The invention also relates to a corresponding electrical accumulator. 
     Description of the Prior Art 
     It is clear that in future, in both stationary applications, such as wind farms and non-stationary applications, such as in vehicles, for example hybrid and electric vehicles, new battery systems of which very stringent demands for reliability will be made will increasingly come into use. The background of these demands is that a failure of the battery systems can lead to either a failure of an entire system pertaining to the application, or to a safety-relevant problem. One conceivable example of such a failure is an electric vehicle that if its traction battery fails is “dead in the water”, since it is no longer capable of propelling itself. As an example of a safety-relevant problem, a wind farm is conceivable, in which electrical accumulators are used for protecting the farm against impermissible modes of operation by adjusting the rotor blades under strong wind conditions. Failure of these electrical accumulators can then lead to safety-relevant problems. 
     When many individual accumulator units, such as battery cells, connected in series are used, the individual accumulator units are not automatically equal. As a result, particularly over the service life of the accumulator units, this leads to unequal electrical voltages among the individual accumulator units, unless appropriate countermeasures are taken. Especially with lithium-ion batteries, excessive charging or deep discharging of individual accumulator units leads to irreversible damage. Such excessive charging or deep discharging can result when a battery management system regulates a charging or discharging operation based on one of the accumulator units, which is not representative all of the accumulator units. For that reason, balancing of the electrical voltages of the electrical accumulator units among one another must be done at regular intervals. This balancing is known as “cell balancing”. To that end, the individual accumulator units are discharged, by external wiring provisions, in such a way that after the balancing, they all have the same electrical voltage. 
     It is known for that purpose to perform so-called resistance balancing. To that end, an ohmic resistor or a resistor combination is assigned to each accumulator unit via switches. By means of the resistors, the accumulator units are discharged until such time as the accumulator units have the electrical voltage. It is disadvantageous here that energy stored in the electrical accumulator units is converted into heat by the resistors and is carried away unused, for the sake of achieving the desired charge balance. Hence there is a need for a way in which balancing the electrical voltages of a plurality of accumulator units among one another is attained with little energy loss and in which a substantial improvement in the efficiency of a complete electrical accumulator system is brought about. 
     SUMMARY OF THE INVENTION 
     According to the invention, it is provided that the one accumulator unit is connected to the winding of a coil for the excitation of that coil, and that next, by means of the excited coil, the other accumulator unit is charged by the connection of the winding to that other accumulator unit. It is provided that the same winding that is used for exciting the coil is also used for charging the other accumulator unit. In this way, it becomes possible for the energy stored in the accumulator units not to be merely converted into heat, but to be transferred from the one accumulator unit to the other, so that the electrical voltages of the various accumulator units are balanced with each other. Charging the other accumulator unit should be understood to mean that the coil is excited, and by means of the electrical energy that is thus available, the other accumulator unit is further charged. Charging should accordingly be understood to mean not full charging of the entire electrical accumulator, but rather transporting an electrical charge between the accumulator units and the coil for the sake of balancing the electrical voltages. 
     In a further feature of the invention, it is provided that the one accumulator unit, which is connected to the winding of the coil for its excitation, has a higher electrical voltage than the accumulator unit that is charged next, by the connection of the winding. 
     In a further feature of the invention, it is provided that as the accumulator units, one accumulator cell each, in particular a battery cell, is used. 
     In a further feature of the invention, it is provided that the coil is charged by closure of a switch. Using the switch makes it possible to charge at least one coil in a targeted manner. In this way, the method can be employed in targeted manner to individual accumulator units, without always having to include all the accumulator units in the method. 
     In a further feature of the invention, it is provided that the two accumulator units are located adjacent one another. Being located adjacent one another should be understood to mean that the accumulator units are connected directly in series with one another, and a positive pole of one of the accumulator units is connected directly to a negative pole of the other accumulator unit via a line. 
     In a further feature of the invention, it is provided that each adjacent accumulator unit is assigned a coil. When there is a plurality of adjacent accumulator units, it is provided in particular that each adjacent accumulator unit is assigned its own coil, with a winding. 
     In a further feature of the invention, it is provided that the coil charges the other accumulator unit by opening the switch. By appropriate interconnection, it becomes possible to end the charging of the coil by opening the switch, and by reinduction, or in other words de-excitation, the coil makes the energy stored in it available. In that case, the coil outputs the stored electrical energy, and that energy is taken up by the other accumulator unit, which is being charged. The combination here of closing the switch to charge the coil and opening the switch to charge the accumulator unit is advantageous, since by means of only two positions of the switch, both the charging of the coil and the charging of the accumulator unit can be brought about in succession in a simple way. 
     In a further feature of the invention, it is provided that the other accumulator unit is charged by the coil via at least one diode. This is especially advantageous whenever a flow of current, which flows into the coil upon charging, is reversed and flows out of the coil again, for charging the accumulator unit in the reverse manner. Thus the coil can be connected to two accumulator units simultaneously, and the charging of the other accumulator unit depends on whether the coil is being charged or discharged. 
     The invention relates further to an electrical accumulator having at least two serially connected electrical accumulator units and one electrical balancing circuit, in particular for performing the method described above, in which the balancing circuit has at least one coil having a winding and its winding is connectable to one of the accumulator units for exciting the coil, and in which, for charging the other accumulator unit, the winding can be connected to that accumulator unit. 
     In a further feature of the invention, it is provided that the balancing circuit has at least one diode and/or at least one switch. 
     In a further feature of the invention, it is provided that the switch is embodied as a semiconductor switch, in particular a transistor, thyristor, or the like. By the use of semiconductor elements, very easy automation is made possible, by means of electrical components, such as circuits. Moreover, in this way the device of the invention can be embodied in a space-saving way and can be produced economically. 
     In a further feature of the invention, it is provided that each of the accumulator units has an accumulator cell, in particular a battery cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings illustrate the invention in terms of an exemplary embodiment; in the drawings: 
         FIG. 1  shows an electric switch with a balancing circuit; 
         FIG. 2  shows the accumulator with the balancing circuit of  FIG. 1  in a first method step; and 
         FIG. 3  shows the accumulator with the balancing circuit of  FIG. 1  in a second method step. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  shows a detail of an electrical accumulator  101 , comprising three accumulator units  102  in the form of accumulator cells  103  that are connected in series adjacent to one another. The electrical accumulator  101  is embodied as a battery  104 , and the accumulator cells  103  are embodied as battery cells  105 . The accumulator units  102  form the electrical accumulator  101  by means of the fact that a first accumulator unit  106  is connected via its negative pole  106 ″ and a line  107  to a node point  108 , which leads via a line  109  to a positive pole  110 ′ of a second accumulator unit  110 . The second accumulator unit  110  is connected in turn via its negative pole  110 ″ and a line  111  to a node point  112 , which leads via a line  113  to a positive pole  114 ′ of a third accumulator unit  114 . A balancing circuit  115 , a detail of which is shown in  FIG. 1 , is assigned to the accumulator  101 . The balancing circuit  115  is connected via a line  116  to a positive pole  106 ′ of the first accumulator unit  105 . It is also connected to the node point  108  via a line  117 , to the node point  112  via a line  118 , and to a negative pole  114 ″ of the third accumulator unit  114  via a line  119 . The balancing circuit  115  has a plurality of electric coils  121 , each with a winding  120 . The balancing circuit  115  furthermore has diodes  122  and switches  123 . The line  116  is connected via a node point  124  to a further line  125 , which leads to a first winding  126 . The first winding  126  is connected via a line  127  to a node point  128 , which has a further line  129  which leads to a first switch  130 . Beginning at the first switch  130 , a line  131  leads to a node point  132 . From the node point  132 , a line  133  leads to a further node point  134 , which in turn is connected via a line  135  to a second winding  136 . The second winding  136  is connected via a line  137  to a node point  138 , which is connected via a line  139  to a further node point  140 . The node point  140  is also connected to the line  117 . Via a line  141 , a first diode  142  is connected to the node point  140 . The diode  142  is also connected to the node point  128  via a line  143 . The diode  142  is disposed between the lines  141  and  143  in such a way that it has a conducting direction from the line  141  to the line  143 . The node point  134  is connected via a line  144  to a second diode  145 , which in turn is connected via a line  146  to the node point  124 . The second diode  145  has a conducting direction from the line  144  to the line  146 . Beginning at the node point  132 , a further line  149  leads to a second switch  150 . From the switch  150 , a line  151  leads to a node point  152 . From the node point  152 , a line  153  leads to a further node point  154 , which in turn is connected via a line  155  to a third winding  156 . The winding  156  is connected via a line  157  to a node point  158 , which is connected via a line  159  to a further node point  160 . The node point  160  is also connected to the line  118 . Via a line  161 , a third diode  162  is connected to the node point  160 . The diode  162  is also connected to the node point  134  via a line  163 . The diode  162  is disposed between the lines  161  and  163  in such a way that it has a conducting direction from the line  161  to the line  163 . The node point  154  is connected via a line  164  to a fourth diode  165 , which in turn is connected via a line  166  to the node point  138 . The fourth diode  165  has a conducting direction from the line  164  to the line  166 . Beginning at the node point  152 , a further line  169  leads to a third switch  170 . Beginning at the switch  170 , a line  171  leads to a node point  174 . The node point  174  is in turn connected via a line  175  to a fourth winding  176 . The winding  176  is connected via a line  177  to a node point  180 . The node point  180  is also connected to the line  119 . Via a line  181 , a fifth diode  182  is connected to the node point  180 . The diode  182  is also connected to the node point  154  via a line  183 . The diode  182  is disposed between the lines  181  and  183  in such a way that it has a conducting direction from the line  181  to the line  183 . The node point  174  is connected via a line  184  to a sixth diode  185 , which in turn is connected to the node point  158  via a line  186 . The sixth diode  185  has a conducting direction from the line  184  to the line  186 . The switches  123  are assigned to an electronic control unit  190 . For that purpose, they are embodied as semiconductor switches  191  in the form of transistors  192 , so that the control unit  190  forms an integrated circuit  193 . Dashed lines  179  indicate that both the electrical accumulator  101  and the balancing circuit  115  are continued logically onward in the direction of the lines  179 . 
       FIG. 2  shows the electrical accumulator  101  and the balancing circuit  115  of  FIG. 1  with all their features. Unlike in  FIG. 1 , in  FIG. 2  the second switch  150  is closed, for performing a first method step. The accumulator unit  110  furthermore has a higher voltage than the other accumulator units  106  and  114 . As a result, there is a closed electric circuit  195  for the second accumulator unit  110 , the second winding  136  and the third winding  156 . The electric circuit  195  in  FIG. 2  is shown in heavy lines and is provided with current direction arrows  196 . The electric circuit  195  extends from the positive pole  110 ′ of the second accumulator unit  110  via the line  109  and onward via the line  117  to the node point  140 , so that by means of the lines  137  and  139 , the coil  121  of the second winding  136  is charged. The electric circuit  195  also extends via the lines  135 ,  133  and  149  to the closed second switch  150 . It continues via the lines  151 ,  153  and  154  to the third winding  156 . Beginning at the third winding  156 , the electric circuit  195  is closed via the lines  157 ,  159 ,  118  and  111  to the negative pole  110 ″ of the second accumulator unit  110 . By means of this closed electric circuit  195 , a charge is transferred into the second and third windings  136  and  156  and stored there. The closure of the second switch  150  is effected by the control unit  190 . It is provided that each accumulator unit  102  charges two windings  120 . By closure of the switch  150 , the electric circuit  195  is closed, and it is opened again either after a certain period of time or after a certain level of the current that flows through the switch is reached. 
       FIG. 3  shows the accumulator  101  and the balancing circuit  115  of  FIG. 1  with all their features. The switches  123  are all opened for a second method step, and the coils assigned to the second winding  136  and the third winding  156  are excited. The result is two electric circuits  197  and  198 ; the electric circuit  197  is assigned to the first accumulator unit  106 , and the second electric circuit  198  is assigned to the third accumulator unit  114 . The electric circuit  197  extends from the second winding  136  to the second diode  145 , via the lines  135  and  144 . Beginning at the second diode  145 , the electric circuit  197  extends onward via the lines  146  and  116  into the positive pole  106 ′ of the first accumulator unit  106 . From the negative pole  106 ″ of the accumulator unit  106 , the electric circuit  197  extends via the lines  107 ,  117 ,  139  and  138  back to the second winding  136 . The electric circuit  198  begins at the third winding  156 , which is connected via the lines  157 ,  159 ,  118  and  113  to the positive pole  114 ′ of the third accumulator unit  114 . From the negative pole  114 ″ of the third accumulator unit  114 , the fifth diode  182  is connected via the lines  119  and  181 , so that beginning at the diode  182 , the electric circuit  198  is closed via the lines  183  and  155 . Within the electric circuits  197  and  198 , the various current directions are represented by current direction arrows  196 . The current directions run in the directions that correspond to the conducting directions of the second diode  145  and the fifth diode  182 . Via the electric circuits  197  and  198 , the coils of the windings  136  and  156  can be de-excited; that is, they give up their charges, which flow into the corresponding accumulator units  106  and  114  and as a result further charge those accumulator units. For that purpose it is unnecessary to employ further control means for the second method step, since this process ensues automatically because of the balancing circuit shown. 
     The method steps shown in  FIGS. 2 and 3  describe the possibility of charging the first accumulator unit  106  and the second accumulator unit  114  with an electrical charge from the second accumulator unit  110 . This process is highly energy-efficient, since electrical charges are transferred among the accumulator units. 
     The foregoing relates to the preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.