Abstract:
A battery energy balance circuit and a battery charging bypass circuit is disclosed for batteries being charged at the same time to be charged equally. When the terminal voltages of the batteries are different, a controllable power device switch in the circuit switches on and off at a high frequency in order to reduce the input current to the batteries with higher terminal voltages and to increase the input current to the batteries with lower terminal voltages, achieving the goal of equal charging. The disclosed energy balance circuit can avoid damages to the batteries as a result of overcharging. When the number of batteries increases, one can expand the system in a modularized way to prevent inconvenience of circuit designs.

Description:
This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 092121816 filed in TAIWAN on Aug. 8, 2003, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The invention relates to a energy balance circuit and a charging bypass circuit for batteries and, in particular, to a energy balance circuit and a charging bypass circuit that can equally charge a set of batteries connected in series. 
   2. Related Art 
   Many applications require the use of many batteries connected in series. However, due to different characters and residual energy in batteries, they have to be properly matched. If the residual energy in the batteries connected in series is different, those with more residual energy are likely to be overcharged and damaged while those with smaller residual energy are not charged to full when they are charged in series. When discharging, those batteries not charged in full are likely to overdischarge to damage the batteries. Therefore, how to avoid such problems is an important issue for serial battery sets. 
   To protect the battery set, the simplest method is to connect to each battery a resistor and a control switch in parallel.  FIG. 1  shows an example with a battery set of three batteries. It includes a charging circuit CHR. The battery set is comprised of a first battery B 1 , a second battery B 2 , and a third battery B 3  connected in series. The first battery B 1  is connected in parallel a first resistor R 1  and a first switch SW 1 . The second battery B 2  is connected in parallel a second resistor R 2  and a second switch SW 2 . The third battery B 3  is connected in parallel a third resistor R 3  and a third switch SW 3 . 
   When charging, if the terminal voltage of any battery reaches a predetermined value, the corresponding switch is turned on. Suppose the voltage of the second voltage B 2  reaches the predetermined value, the second switch SW 2  is turned on. The second resistor R 2  can be shown as connected to the second battery B 2  in parallel. In such a way, the current originally flowing through the second battery B 2  is reduced because part of it flows through the second resistor R 2 . This can avoid overcharging. 
   The circuit shown in  FIG. 1  produces heat as the current flows through the resistor. Therefore, it cannot process large branch currents. 
   To solve such problems, the U.S. Pat. No. 5,479,083 improved the above-mentioned dissipative method and provided with a non-dissipative energy balance circuit. The circuit structure is shown in  FIG. 2 , including a first battery B 1  and a second battery B 2  connected in series, and a first switch SW 1  and a second switch SW 2  connected in series. The battery set and the switch set are connected in parallel. One end of an inductor L is coupled between the first battery B 1  and the second battery B 2 , the other end is coupled between the first switch SW 1  and the second switch SW 2 . The basic principle is that the first switch SW 1  and the second switch SW 2  are turned on and off in an alternative way so that the terminal with a higher voltage discharges while that with a lower voltage is charged in this circuit. When the first switch SW 1  is on, the first battery B  1  and the inductor L form a loop. When the second switch SW 2  is on, the second battery B 2  and the inductor L form a loop. Therefore, the bypass charging/discharging current is of pulse nature. 
     FIG. 3  illustrates another energy balance circuit with the similar idea of  FIG. 2 , which is disclosed in the U.S. Pat. Nos. 6,150,795 and 6,222,344. The switches SW 1  and SW 2  are on and off simultaneously, or only the switch corresponding to the battery with a larger terminal voltage is activated. If the terminal voltage of the first battery B 1  is larger, the first switch is turned on and off at a high frequency. In this way, the energy in the first battery B  1  is transferred to the second battery B 2  through the circuit. The energy transfer is possible only when the first switch SW 1  and the second switch SW 2  are on. Its transfer method is different from that of the U.S. Pat. No. 5,479,083. However, it also has pulse currents. 
   The U.S. Pat. No. 5,659,237 also discloses an energy balance circuit that distributes a total energy in an even way. Its main technical feature is to redistribute the energy in a battery set through a circuit to each of the batteries. Batteries with smaller terminal voltages get more energy while those with larger terminal voltages get less energy. Therefore, this circuit can achieve the goal of making the terminal voltage of each battery in the battery set the same. 
   The transformer in the U.S. Pat. Nos. 6,008,623 and 5,659,237 is changed to several independent ones. The basic idea of the U.S. Pat. No. 5,666,041 is the same as the U.S. Pat. No. 5,659,237. It also redistributes the serial battery set energy. The only difference is in the structure of the transformer. In order for the batteries with smaller terminal voltages to be given more energy, the U.S. Pat. No. 5,982,143 further includes a switch connected in series in front of a diode. 
   The battery energy balance circuit disclosed in the prior art few has the modularized property. When the number of battery sets increases, the design of the whole balance circuit has to be modified and the number of windings has to change too. Therefore, it is not economical in practice. Moreover, the charging and discharging currents in the circuit of the prior art are all pulse currents. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing, an objective of the invention is to provide a energy balance circuit for battery sets to improve technical problems in the prior art and to protect batteries from being damaged because of overcharging or overdischarging. 
   To achieve above objective, the disclosed energy balance circuit and the battery set are connected in parallel. When the terminal voltages of the batteries are different, the switches are turned on and off in a rapid way so that energy from batteries with higher terminal voltages are transferred to those with lower terminal voltages. Therefore, each battery in the battery set eventually has the equal voltage. 
   The disclosed energy balance circuit can be applied to the battery energy equalization device connected in series to the battery set. When the battery set is charging, such a device can be viewed as a bypass circuit so that no battery will be damaged because of overcharging. It has the advantage of being modularized. In comparison with the prior art, the charging current produced by the disclosed energy balance circuit is not a pulse current. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein: 
       FIG. 1  is a battery energy balance circuit in the prior art; 
       FIG. 2  is another battery energy balance circuit in the prior art; 
       FIG. 3  is yet another battery energy balance circuit in the prior art; 
       FIG. 4  shows the first embodiment of the disclosed energy balance circuit; 
       FIG. 5  shows a driving waveform and a charging current used in the first embodiment; 
       FIG. 6  shows simulated battery voltage and current storage device current in the first embodiment; 
       FIG. 7  shows the second embodiment of the disclosed energy balance circuit; 
       FIG. 8  shows simulated battery voltage and current storage device current in the second embodiment; and 
       FIG. 9  shows the second embodiment of the disclosed energy balance circuit, using a battery set with more than two batteries. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Please refer to  FIG. 4  for the disclosed battery set energy balance device. As shown in the drawing, the energy circuit E has an upper node N 1 , a middle node N 2 , and a lower node N 3 . The battery set B is connected to the energy balance circuit E via the upper node N 1 , the middle node N 2 , and the lower node N 3  in parallel. In the current embodiment, the battery set B is comprised of a first battery B 1  and a second battery B 2  connected in parallel. Corresponding to the number of batteries in the battery set, the energy balance device E includes a first current storage device L 1  and a second current storage device L 2 , a first switch SW 1  and a second switch SW 2 , and a voltage storage device C. 
   The first current storage device L 1  and the second current storage device L 2  are energy storage devices that can hold energy in a magnetic field. When current flows through such a device, energy is stored in the magnetic field generated by the device. A preferred embodiment uses inductors as such devices. In the first embodiment, the inductors are non-coupled inductors. 
   The voltage storage device C is an energy storage device that can hold energy in an electric field. When current flows through the device, energy is stored in the electric field generated by the device. Such a device is preferably a capacitor. 
   The first current storage device L 1 , the voltage storage device C, and the second current storage device L 2  are connected in series. The voltage storage device C is coupled between the first current storage device L 1  and the second current storage device L 2 . The other end of the first current storage device L 1  is connected to the upper node N 1 . The other end of the second current storage device L 2  is connected to the lower node N 3 . The upper node N 1  is connected to the positive output terminal of the battery set B. The lower node N 3  is connected to the negative output terminal of the battery set B. 
   The first switch SW 1  and the second switch SW 2  are controllable power element switches. They can be MOSFET, IGBT or BJT switch devices that are built in with a bypass diode. In the current embodiment of the invention, we use the MOSFET as an example. 
   One end of the first switch SW 1  is coupled between the first current storage device L 1  and the voltage storage device C. The other end is coupled between the first battery B 1  and the second battery B 2  through the middle node N 2 . One end of the second switch SW 2  is coupled between the second current storage device L 2  and the voltage storage device C. The other end is coupled between the first battery B 1  and the second battery B 2 . Using the MOSFET as a switch, the drain of the first switch SW 1  is coupled to the first current storage device L 1  and the voltage storage device C. Its source is coupled between the first battery B 1  and the second battery B 2 . The source of the second switch SW 2  is coupled between the first current storage device L 1  and the voltage storage device C. Its drain is coupled between the first battery B 1  and the second battery B 2 . In other words, the drains and sources of the first switch SW 1  and the second switch SW 2  are coupled to one another. 
   The on and off of the first switch SW 1  and the second switch SW 2  can be controlled by an oscillator. Taking the MOSFET in the current embodiment as an example, its gate is connected to the oscillator. 
   In the following, we describe how the disclosed energy balance circuit functions. The driving signal of the first switch SW 1  and the second switch SW 2  is shown in  FIG. 6 . 
   Before the switches actuate, the voltage different between the two ends of the voltage storage device C is VC and VC=Vb 1 +Vb 2 . Now consider the situation where only the switch corresponding to the battery with a higher voltage is driven. Without loss of generality, suppose the battery B 2  has a higher terminal voltage. When the second switch SW 2  is on, part of the electrical energy inside the second battery B 2  is transferred to the second current storage device L 2 . At the same moment, the energy in the voltage storage device C discharges its energy to the first battery B 1  via the first current storage device L 1 . Therefore, the first battery B 1  is being charged by the energy in the voltage storage device C. 
   When the second switch SW 2  is on, the terminal voltage across the second current storage device L 2  is VB 2 . The voltage difference between the two ends of the voltage storage device C is VC. The voltage across the first current storage device L 1  is then VB 1 −VC=−VB 2 . 
   When the second switch SW 2  is off, the first current storage device L 1  and the second current storage device L 2  cannot be instantaneously off, forcing the built-in bypass diode of the first switch SW 1  to be conductive. The voltage across the second current storage device L 2  is VB 2 −VC=−VB 1 . The voltage difference between the two ends of the first current storage device L 1  is VB 1 . 
   According to the volt-sec balance principle, the volt-sec is not balanced within one period. Therefore, the current in the current storage device L 2  increases. On the other hand, the current in the first current storage device L 1  increases in the negative direction. In this method, the energy in the second battery B 2  is transferred to the first battery B 1  via the circuit E. 
   As the first switch SW 1  and the second switch SW 2  alternate, the built-in bypass diode in the first switch SW 1  is on when the second switch SW 2  is off. Thus, let the first switch SW 1  turn on, making the voltage drop of the drain and source VDS of the first switch SW 1  smaller than the bypass diode. The current waveform is shown in  FIG. 6 , too. From  FIG. 5 , we know that the currents in the first current storage device L 1  and the second current storage device L 2  are non-pulse currents. The first current storage device L 1  is connected to the positive terminal of B; and the second current storage device L 2  is connected to the negative terminal of B. This means that the absorbed or feedback current of battery B 1  or B 2  from or to the bypass circuit is a continuous current. 
     FIG. 6  shows a simulated voltage of the battery and currents of the current storage devices. One can see from the drawing that the simulated experimental result is similar to  FIG. 5 . 
   When the battery set is charging, if the terminal voltages of the batteries are different high-speed on and off of a controllable power device switch can reduce the charging current to the battery with a higher terminal voltage, thereby increasing the charging current of that with a lower terminal voltage. This is why the disclosed energy balance circuit can be considered as a current bypass circuit. 
   Moreover, the first current storage device L 1  and the second current storage device L 2  can share one iron core, coiling to form a transformer. That is, the embodiment adopts a coupled inductor. The circuit is shown in  FIG. 7 . The operation detail is the same as the first embodiment. For the simulated battery voltage and the current storage device current, please refer to  FIG. 8 . 
   From  FIG. 6  and  FIG. 8  one can see that when the terminal voltages of the first battery B 1  and the second battery B 2  are different, the first switch SW 1  and the second switch SW 2  alternately turn on and off to reach equal terminal voltages on the batteries. Taking  FIG. 6  as an example, when the terminal voltage of the second battery B 2  is larger than that of the first battery B 1 , the second switch SW 2  is on, making the second battery B 2 , the second switch SW 2 , and the second current storage device L 2  form a loop. The energy inside the battery B 2  releases to charge the second current storage device L 2  via the loop. The second current storage device L 2  is thus being charged. At the same time, the energy in the voltage storage device C discharges to the first battery B  1  via the first current storage device. Therefore, the first battery B 1  is being charged. When the second switch SW 2  is off, the second current device L 2  charges the bypass diode via the first switch and the first current storage device L 1  keeps discharging to the first battery B 1 . Through the continuous charging and discharging process, the terminal voltages of the first battery B 1  and the second battery B 2  become equal. 
   Furthermore, refer to  FIG. 9  for the circuit diagram of the invention applied to a battery set with more than two batteries. As shown in the drawing, the battery set contains a first battery B 1 , a second battery B 2 , and a third battery B 3 . When one more battery is included, the energy balance circuit E also increases by one correspondingly. 
   In the battery set B with only two batteries, the upper node N 1  on the energy balance circuit E is connected to the positive pole of the first battery and the lower node N 3  is connected to the negative pole of the second (also the last) battery. The middle node N 2  is connected between the first and second batteries. 
   When the battery set B has three batteries, there is one more energy balance circuit correspondingly. If there are N batteries in the battery set, there should be N-1 energy balance circuits E. The N batteries are connected in series. Each of the N-1 energy balance circuits is connected to the N serial batteries in parallel in an overlapped way. Explicitly, the upper node N 1  of the first energy balance circuit is connected to the positive pole of the first battery in the battery set. Its lower node N 3  is connected to the negative pole of the second battery. Its middle node N 2  is connected between the first and second batteries. The upper node of the next energy balance circuit is connected to the middle node of the previous energy balance circuit, the middle node of the next energy balance circuit to the lower node of the previous energy balance circuit. Between each two consecutive batteries is connected with the middle node of a energy balance circuit and the lower node of its previous energy balance circuit. 
   Certain variations would be apparent to those skilled in the art, which variations are considered within the spirit and scope of the claimed invention.