Patent ID: 7825629

Claim:
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 N th 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 N th battery cell and in parallel across the N th 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.