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A system and method for charging a rechargeable, or secondary, battery including a series string of cells, includes a topology of charging sources that selectively provides charging current to cells that need to be charged, but avoids overcharging cells that are already charged above a predetermined

A system and method for charging a rechargeable, or secondary, battery including a series string of cells, includes a topology of charging sources that selectively provides charging current to cells that need to be charged, but avoids overcharging cells that are already charged above a predetermined voltage threshold. Based on individual cell voltage measurements, the charging current is controlled in a manner to direct charging current to the battery cell(s) needing charge until these cells are fully charged, and by-passes battery cells that are fully charged or become fully charged.

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What is claimed is: 1. A system for charging a secondary battery comprising a plurality of battery cells connected in a series string, said series string comprising a first battery cell connected to a load and an Nth battery cell connected to a ground, and a cell junction formed at a connection bet

What is claimed is: 1. A system for charging a secondary battery comprising a plurality of battery cells connected in a series string, said series string comprising a first battery cell connected to a load and an Nth battery cell connected to a ground, and a cell junction formed at a connection between each respective pair of adjacent battery cells, said system comprising: a plurality of charging sources comprising exactly (N+2)/2 charging sources when N is an even number or (N+1)/2 charging sources when N is an odd number, each of the plurality of charging sources having a first state and a second state, wherein in the first state, the charging source provides a charging current and in the second state, the charging source does not provide a charging current, said plurality of charging sources further comprising: a first charging source electrically coupled to said load end; a second charging source electrically coupled to a first cell junction between said first battery cell and a second battery cell located adjacent to said first battery cell; and an additional charging source electrically coupled to each cell junction occurring every two cells thereafter; and a plurality of shunt regulators, wherein a respective shunt regulator is connected in parallel across each battery cell connected between the first battery cell and the Nth battery cell and in parallel across the Nth battery cell, wherein each of said plurality of shunt regulators is configured to operate in a first state and a second state, wherein in the first state, a shunt regulator clamps a voltage of a battery cell with which said shunt regulator is connected in parallel across to a predetermined voltage and bypassing current around the respective cell with which the shunt regulator is connected in parallel, and in the second state, the shunt regulator does not clamp the voltage of the battery cell, and wherein said plurality of shunt regulators comprises exactly N−1 shunt regulators; and a controller connected to each of the plurality of charging sources and each of the plurality of shunt regulators, said controller comprising circuitry to selectively operate each of the plurality of charging sources in the first state or the second state, and each of the plurality of shunt regulators in the first state or the second state to provide charging current to each battery cell having a voltage less than the predetermined voltage and to avoid providing charging current to each battery cell having a voltage equal to or greater than the predetermined voltage, wherein the controller is configured to allow only one charging source to be switched on at a time. 2. The system of claim 1, wherein the controller is connected to each of the plurality of shunt regulators, wherein the controller is configured to cycle each of said plurality of shunt regulators between said first state and said second state. 3. The system of claim 2, wherein said controller is configured to cycle each of said plurality of shunt regulators to said first state if a battery cell with which said shunt regulator is coupled across in parallel has a voltage equal to a predetermined voltage, and cycle each of said plurality of shunt regulators to said second state if said battery cell has a voltage less than the predetermined voltage. 4. A method for equalizing voltage of a secondary battery being charged, the battery comprised of N cells connected in a serial string from a first cell at a load end to an Nth cell at a ground end with a respective cell junction formed at the connection between each pair of adjacent cells, the method comprising: coupling a plurality of switched charging sources to the serial string, wherein said plurality of switched charging sources coupled to the serial string comprises (N+2)/2 switched charging sources coupled to the serial string when N is an even number, wherein a first switched charging source is electrically coupled to the load end of the serial string, and second switched charging source is electrically coupled at the cell junction between the first cell and a second cell adjacent said first cell; and a respective switched charging source is electrically coupled at the cell junctions occurring every two cells thereafter, wherein each of the switched charging sources selectively operates in one of a first state and a second state, wherein when a switched charging source is in the first state, the switched charging source provides a charging current to the respective cell junction or load end to which that switched charging source is electrically coupled; and when the switched charging source is in the second state, the source does not provide a charging current to the respective cell junction or load end to which that switched charging source is electrically coupled; coupling a plurality of shunt regulators to the serial string, wherein said plurality of shunt regulators coupled to the serial string comprises exactly N−1 shunt regulators, wherein a respective shunt regulator is connected in parallel across each of the second cell through the Nth cell, wherein each of said plurality of shunt regulators is configured to operate in a first state and a second state, wherein each of the shunt regulators selectively operates in a first state and a second state and when in a first state a shunt regulator clamps a voltage of the respective cell with which it is connected in parallel to a predetermined voltage by diverting a charging current received at the respective cell junction to a cell junction at another terminal of said respective cell, and when in a second state, the shunt regulator operates as a high-impedance electrical path; and selectively operating the switched charging sources and the shunt regulators in the respective first state or in the respective second state to provide charging current to each of the cells having a voltage less than the predetermined voltage and to avoid providing charging current to each cell having a voltage equal to or greater than the predetermined voltage, wherein only one charging source operates in the first state at any given time. 5. The method of claim 4, wherein said plurality of switched charging sources coupled to the serial string comprises (N+1)/2 switched charging sources coupled to the serial string when N is an odd number. 6. The method of claim 4, further comprising: monitoring a respective voltage level of each of the N cells; and determining which of the N cells is at or above a predetermined voltage threshold. 7. A method for equalizing voltage of a secondary battery comprising a plurality of series connected battery cells, said method comprising: a) monitoring an amount of charge contained within each of the plurality of battery cells utilizing at least one cell monitor to determine if at least one battery cell of the plurality of battery cells needs charging, said monitoring comprising monitoring a voltage of each of the plurality of cells and determining that at least one battery cell of the plurality of battery cells needs charging if the monitored voltage is less than a predetermined threshold voltage, wherein said plurality of battery cells comprises N series connected battery cells; b) transmitting a signal to begin charging operations from said at least one cell monitor, to a controller, when at least one of said plurality of battery cells needs charging; c) determining, by said controller, which charging source, of a plurality of charging sources, to utilize to charge said at least one of said plurality of battery cells needing charging, wherein said plurality of charging sources comprises switched charging sources coupled to the plurality of series connected battery cells comprises (N+1)/2 switched charging sources coupled to the serial string when N is an odd number; d) switching on, by said controller, an appropriate charging source of said plurality of charging sources to charge said at least one of said plurality of battery cells needing charging, wherein said appropriate charging source is determined by its connection within the plurality of series connected battery cells with respect to the at least one battery cell needing charging, wherein only one charging source is switched on at any given time; e) switching on at least one shunt regulator of a plurality of shunt regulators, each shunt regulator of the plurality of shunt regulators coupled in parallel with at least one of said plurality of battery cells, wherein said plurality of shunt regulators comprises exactly N−1 shunt regulators, and wherein said at least one shunt regulator is a shunt regulator coupled in parallel across a battery cell having a voltage greater than or equal to the predetermined threshold voltage, wherein said switching on comprises operating the at least one shunt regulator in a first state, wherein the shunt regulator, when operating in the first state, clamps the voltage of the respective battery cell with which it is connected in parallel to the predetermined voltage by diverting a charging current received at the battery cell around said respective battery cell, and when in a second state, the shunt regulator operates as a high-impedance electrical path. 8. The method of claim 7, wherein said step of switching on at least one shunt regulator occurs prior to said step of switching on an appropriate charging source. 9. The method of claim 7, further comprising the steps of: g) continuing to monitor, by said cell monitor, said plurality of battery cells until at least one battery cell receiving charging current is charged to a threshold amount of charge, wherein said threshold amount of charge is determined by monitoring the voltage of the battery cell and determining that the battery cell is charged to the threshold amount of charge when the monitored voltage of said battery cell is equal to or greater than the predetermined voltage; and h) cycling to said first state, by said controller, a shunt regulator coupled in parallel to said battery cell receiving charging current when said battery cell receiving charging current contains said threshold amount of charge. 10. The method of claim 9, further comprising: i) repeating steps g) and h) until each of said plurality of battery cells contains said threshold amount of charge. 11. The method of claim 10, further comprising the steps of: i) switching off, by said controller, said charging source when each of said plurality of battery cells contains said threshold amount of charge; and j) cycling any shunt regulator to said second state, by said controller, any shunt regulator cycled to said first state by said controller. 12. A system for charging a rechargeable battery, said rechargeable battery comprising at least three battery cells connected in a series string, wherein a first battery cell of the at least three battery cells is connected to a load, a third battery cell of the at least three battery cells is connected to a ground, and a second battery cell of the at least three battery cells is connected between the first and third battery cells, and wherein a cell junction is formed between each pair of battery cells, said charging system comprising: a controller transmitting charging source control signals and shunt regulator control signals; a plurality of charging sources coupled to said series string receiving charging source control signals from the controller, each charging source of said plurality of charging sources responsive to charging source control signals from the controller for selectively operating in a first or a second state, wherein in the first state, the charging source provides a charging current and in the second state, the charging source does not provide a charging current, said plurality of charging sources comprising: a first charging source of the plurality of charging sources comprising an input terminal connected to a power source and an output terminal coupled to the load for providing, via the output terminal, a charging current for the first battery cell, a charging current for the second battery cell, and a charging current for the third battery cell when operating in the first state and no charging current when operating in the second state; and a second charging source of the plurality of charging sources comprising an input connected to the power source and an output terminal connected to the cell junction between the first battery cell and the second battery cell, for providing, to said cell junction, the charging current for the second battery cell and the charging current for the third battery cell when operating in the first state and no charging current when operating in the second state; and a plurality of shunt regulators coupled to said series string receiving and responsive to shunt regulator control signals from the controller for selectively operating in a first or a second state, said plurality of shunt regulators comprising: a first shunt regulator of the plurality of shunt regulators comprising an input terminal connected to the cell junction between the first battery cell and the second battery cell and an output terminal connected to the cell junction between the second battery cell and the third battery cell for clamping a voltage of the second battery cell to a predetermined voltage and diverting current around the respective cell with which the shunt regulator is connected in parallel when operating in the first state and not clamping the voltage of the second battery cell when operating in the second state; and a second shunt regulator of the plurality of shunt regulators comprising an input terminal connected to the cell junction between the second battery cell and the third battery cell and an output terminal connected to the ground for clamping a voltage of the third battery cell to the predetermined voltage and diverting current around the respective cell with which the shunt regulator is connected in parallel when operating in the first state and not clamping the voltage of the third battery cell when operating in the second state, wherein the controller transmits charging source control signals to operate the first charging source of the plurality of charging sources in the first state until a voltage of one of the at least three battery cells exceeds the predetermined voltage, and then operates the plurality of charging sources and the plurality of shunt regulators to provide charging currents to the battery cells that have not exceeded the predetermined voltage and not to the battery cells that have exceeded the predetermined voltage, and wherein the controller transmits charging source control signals such that only one of the plurality of charging source operates in the first state at any given time. 13. The system of claim 12 further comprising voltage monitors for monitoring the voltage of each of the battery cells and providing the monitored voltage to the controller, wherein the controller operates only one charging source of the plurality of charging sources in the first state at any time and wherein the rechargeable battery comprises exactly N battery cells, the plurality of charging sources comprises exactly (N+2)/2 charging sources when N is an even number or (N+1)/2 charging sources when N is an odd number, and the plurality of shunt regulators comprises exactly N−1 shunt regulators.