Abstract:
The novel cell balancing approach being disclosed minimizes the number of controlled active devices to enable Li-ion cell balancing in a battery arrangement. This is accomplished through a network of passive components associated with each cell. The network of passive components forms a bandpass filter. This allows selective charging of a cell purely based on the frequency components of a variable switching frequency controller.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and is a continuation-in-part of U.S. Provisional Patent Application Ser. No. 61/836,482, filed on Jun. 18, 2013 entitled MULTI-MODULE, SCALABLE, HIGH POWER DENSITY, RADIATION-HARDENED POWER CONVERTER INTERFACE SYSTEMS which is hereby incorporated by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under grant FA9453-12-M-0326 awarded by the United States Air Force. The United States government has certain rights in the invention. 
    
    
     REFERENCE TO A MICROFICHE APPENDIX 
     Not Applicable. 
     RESERVATION OF RIGHTS 
     A portion of the disclosure of this patent document contains material which is subject to intellectual property rights such as but not limited to copyright, trademark, and/or trade dress protection. The owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent files or records but otherwise reserves all rights whatsoever. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to improvements in power converters for battery charging systems. More particularly, the invention relates to improvements particularly suited for lightweight high-temperature high-stress environments for electronics with high reliability requirements. 
     2. Description of the Known Art 
     As will be appreciated by those skilled in the art, batteries and chargers for those batteries are known in various forms. Lithium-ion batteries require cell balancing to achieve proper charging and discharging over the life of the battery pack. In a typical application, lithium-ion battery cells are connected in series to achieve the desired nominal voltage from the resulting battery. One would think that a simple charger of the appropriate voltage could charge the series string of batteries. However, variations in individual cells&#39; maximum capacity prevent all the cells in a series string from being properly charged without damaging the weaker cells. In order to avoid damaging overcharging events, a cell balancing and bypass circuit is needed. Cell balancing circuits ensure that all Li-ion cells are optimally charged to their maximum level by monitoring each cell&#39;s state of charge and individually charging them to their optimal level while keeping already fully charged cells from overcharging. Currently, there are several hardware implementations to achieve cell balancing such as current bypass circuits and charge redistribution circuits. Current bypass or charge shunt circuits suffer from high inefficiency associated with the dissipative elements used to accomplish the current bypass. 
     Patents disclosing information relevant to batteries and chargers include: U.S. Pat. No. 8,421,400, issued to Khanna on Apr. 16, 2013, entitled Solar-powered battery charger and related system and method; U.S. Pat. No. 8,358,108, issued to Seman, Jr., et al. one Jan. 22, 2013 entitled System and method for re-initiating charge cycle for battery pack left in a charger; U.S. Pat. No. 8,098,048, issued to Hoff on Jan. 17, 2012 entitled Battery charger with integrated cell balancing; U.S. Pat. No. 8,044,640, issued to Cruise, et al. on Oct. 25, 2011 entitled Method for stepping current output by a battery charger; U.S. Pat. No. 7,880,435, issued to Ibrahim on Feb. 1, 2011 entitled Multi-battery charger with individual battery bypass control; U.S. Pat. No. 7,602,146, issued to Carrier, et al. on Oct. 13, 2009 entitled Protection methods, protection circuits and protection devices for secondary batteries, a power tool, charger and battery pack adapted to provide protection against fault conditions in the battery pack; U.S. Pat. No. 7,508,171, issued to Carrier, et al. on Mar. 24, 2009 entitled Protection methods, protection circuits and protective devices for secondary batteries, a power tool, charger and battery pack adapted to provide protection against fault conditions in the battery pack; U.S. Pat. No. 6,121,751, issued to Merritt on Sep. 19, 2000 entitled Battery charger for charging a stack of multiple lithium ion battery cells; and U.S. Pat. No. 5,969,502, issued to Beard on Oct. 19, 1999 entitled Battery charger capable of independently charging electromagnetic cells. Each of these patents is hereby expressly incorporated by reference in their entirety. 
     From these prior references it may be seen that these prior art patents are very limited in their teaching and utilization, and an improved power converter and charging system is needed to overcome these limitations. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved cell balancing methodology and device using a unique bandpass filter attached to each battery in the series stack. In this manner, each cell has a specific frequency for charging. A frequency shifting block attached to the entire series stack of all of the cells. The frequency shifting block generates one or more charging signals with each signal having a frequency such that that signal is specifically directed to an individual cell in the series stack. In this cell balancing methodology, each cell has two associated impedances. This block is able to produce specific charging energy signals to match individual cell&#39;s associated filtering impedances. This method assigns a particular frequency signature to each cell in order to bypass fully charged cells and charge the ones that are not fully charged. 
     Advantages of the present invention include minimizing the number of controlled active devices to enable cell balancing. This is accomplished through a network of passive components associated with each cell. The network of passive components forms a bandpass filter. This allows selective charging of a cell purely based on the frequency components of a variable switching frequency controller. 
     These and other objects and advantages of the present invention, along with features of novelty appurtenant thereto, will appear or become apparent by reviewing the following detailed description of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the following drawings, which form a part of the specification and which are to be construed in conjunction therewith, and in which like reference numerals have been employed throughout wherever possible to indicate like parts in the various views: 
         FIG. 1  is a functional block level representation of the proposed selective cell charging through charge energy frequency spreading. 
         FIG. 2  provides a selective cell charging schematic for three different battery cells. 
         FIG. 3  shows a waveform showing cells one and three in a series charging test at 42 kHz. 
         FIG. 4  shows a waveform showing cells one and three in a series charging test at 335 kHz. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIGS. 1 through 4  of the drawings, one exemplary embodiment of the present invention is generally shown as a selective cell charger  100  for charging individual cells V 1 , V 2 , V 3 , . . . VN. The selective cell charger  100  uses a frequency shifting block  102  to generate a frequency charge signal  104  for charge energy frequency spreading as a novel method seeking to minimize active device part count while yielding fast, dynamic response to individual cells&#39; state of charge. This approach enables the selective charge of individual cells such as V 1  based solely on the frequency components contained in the charge power signal  104 . In  FIG. 1 , a functional block level description of the selective cell charger  100  is shown. In this arrangement, each cell V 1 , V 2 , V 3 , . . . VN has an associated impedance network Z 1 , Z 2 , Z 3 , . . . ZN. Each associated impedance network Z 1 , Z 2 , Z 3 , . . . ZN is comprised of passive components including a take off component Z 1 A, Z 2 A, Z 3 A, . . . ZNA and a series stack component Z 1 B, Z 2 B, Z 3 B, . . . ZNB. The frequency shifting block  102  is connected across the battery terminals V+ and V−. This block  102  is able to produce specific charging energy signals  104  to match individual cell&#39;s V 1 , V 2 , V 3 , . . . VN associated filtering impedances set by the associated impedance network Z 1 , Z 2 , Z 3 , . . . ZN. This method assigns a particular frequency signature to each cell in order to bypass fully charged cells and charge the ones that are not fully charged. Unlike other cell level charge redistribution approaches, this approach can be implemented with minimal part count. Targeted cell charging can be achieved with software changes in the command signal to produce the desired frequency components found in the energy charging signal. 
     The circuit depicted in  FIG. 2  is one way to implement the in hardware for the selective cell charger  100 . In this representation only three cells V 5 , V 6 , V 7  are shown on the right side for simplicity although almost an infinite plurality of cells can be accommodated with this circuit. The design uses a forward converter transformer topology  200  to transfer power to the cell charging circuits Z 5 , Z 6 , Z 7  on the isolated side. These bandpass filters Z 5 , Z 6 , Z 7  selectively accept power transfer to the cell V 5 , V 6 , V 7  based on the frequency signature of the bandpass filters Z 5 , Z 6 , Z 7  given to the cell V 5 , V 6 , V 7 , while all other frequencies are rejected and power transfer is limited. 
     After filtering the input, a DC-restore diode (D 2 , D 4 , and D 6  in  FIG. 2 ) is necessary after each passband filter to transfer power through the rectifying diode (D 1 , D 3 , and D 5  in  FIG. 2 ). Finally,  FIG. 2  depicts a battery V 5 , V 6 , V 7  connected to each isolated filter output and connected in series to simulate a typical battery stack Vstack. 
     Proof of the operation was done with a demonstration and  FIGS. 3 and 4  provide the waveform evidence proving this demonstration. Cells one and three were placed in series to represent a basic battery stack. The bandpass center frequency for cell one is 42 kHz while the passband center frequency for cell three is 335 kHz. Next, both cells were connected directly to their respective filters. The system was powered by a 10 V power supply and the switching signal was controlled directly through a function generator. An isolated voltage probe was placed on cell one to measure its voltage, while a non-isolated voltage probe was used for cell three. Additionally, currents for each cell were monitored using Hall Effect current sensors. 
       FIG. 3  shows the results for the test for a 42 kHz signal with a 20% duty cycle. The current for cell one shows to be properly charging the battery at an average current of 55 mA. Furthermore, cell three shows the average current in the micro-amp range. Both results validate the proper operation of the filters to reject and accept charge depending on the frequency of the signal applied. Subsequently, a 335 kHz signal with 20% duty cycle was delivered to the control input. The results are shown in  FIG. 4  where the current through cell one is in the micro-amp range, which is indicative that it is not charging. The result for cell one validates the filter&#39;s ability to reject the 335 kHz frequency. As for cell three, an average current of 20 mA is being delivered to the cell. The filter associated with cell three is able to allow transfer of power to the specific cell. Thus, a novel cell balancing circuit has been shown and described and its operation proven. This novel balancing method enables the charging of battery cells with minimal part count of controllable active devices while enabling the ability to dynamically respond to cell-balancing needs. 
     Reference numerals used throughout the detailed description and the drawings correspond to the following elements:
         selective cell charger  100     frequency shifting block  102     frequency charge signal  104     First cell V 1     Second cell V 2     Third cell V 3     Nth cell VN   first impedance network Z 1     second impedance network Z 2     third impedance network Z 3     Nth impedance network ZN   First take off component Z 1 A   Second take off component Z 2 A   Third take off component Z 3 A   Nth take off component ZNA   First series stack component Z 1 B   Second series stack component Z 2 B   Third series stack component Z 3 B   Nth series stack component ZNB   Positive battery terminal V+   Negative Battery Terminal V−   First DC-restore diode D 2     Second DC-restore diode D 6     Third DC-restore diode D 4     First rectifying diode D 1     Second rectifying diode D 5     Third rectifying diode D 3     First demonstration battery cell V 5     Second demonstration battery cell V 6     Third demonstration battery cell V 7     Three cell battery stack Vstack       

     From the foregoing, it will be seen that this invention well adapted to obtain all the ends and objects herein set forth, together with other advantages which are inherent to the structure. It will also be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims. Many possible embodiments may be made of the invention without departing from the scope thereof. Therefore, it is to be understood that all matter herein set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense. 
     When interpreting the claims of this application, method claims may be recognized by the explicit use of the word ‘method’ in the preamble of the claims and the use of the ‘ing’ tense of the active word. Method claims should not be interpreted to have particular steps in a particular order unless the claim element specifically refers to a previous element, a previous action, or the result of a previous action. Apparatus claims may be recognized by the use of the word ‘apparatus’ in the preamble of the claim and should not be interpreted to have ‘means plus function language’ unless the word ‘means’ is specifically used in the claim element. The words ‘defining,’ ‘having,’ or ‘including’ should be interpreted as open ended claim language that allows additional elements or structures. Finally, where the claims recite “a” or “a first” element of the equivalent thereof, such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements.