METHOD AND SYSTEM FOR CELL EQUALIZATION WITH SWITCHED CHARGING SOURCES
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.
1 . A charging system for charging a rechargeable battery, comprising:
N battery cells coupled in series forming a series string, wherein said series string comprises:
a first battery cell coupled to a load end,
a N th battery cell coupled to a ground end, and
a cell junction between each pair of adjacent battery cells;
a first set of charging sources coupled to said series string, wherein each charging source in said first set comprises an input terminal and an output terminal, wherein an output terminal of a first charging source in said first set is coupled to said load end, and an output terminal of any other charging source is coupled to a respective cell junction occurring thereafter; and
a second set of charging sources coupled to said series string, wherein each charging source in said second set comprise an input terminal and an output terminal, wherein an input terminal of each charging source in said second set is coupled to a respective cell junction, and an output terminal of each charging source in said second set is coupled to said ground end.
2 . The charging system of claim 1 , wherein there are N charging sources in said first set of charging sources.
3 . The charging system of claim 2 , wherein there are (N-1) charging sources in said second set of charging sources.
4 . The charging system of claim 1 , further comprising:
a power source coupled to each input terminal of said charging sources in said first set, wherein each charging source in said first set is configured to provide charging current to at least one of said N battery cells.
5 . The charging system of claim 4 , wherein:
each charging source in said first set of charging sources is configured to operate in a first state and a second state; and when each charging source in said first set is operating in said first state, each charging source provides charging current to a respective cell junction and battery cell where each respective charging source is coupled, and when each charging source in said first set is operating in said second state, each charging source does not provide charging current to said respective cell junction and battery cell where each respective charging source is coupled.
6 . The charging system of claim 1 , wherein:
said ground end is a charging current common return path to each charging source in said second set.
7 . The charging system of claim 1 , wherein:
each charging source in said second set is configured to operate in a first state and a second state; and
when each charging source in said second set is operating in said first state, each charging source in said second set forms a low impedance electrical path between a cell junction where each particular charging source is coupled and said return path, and when each charging source in said second set is operating in said second state, each charging source forms a high impedance electrical path between said cell junction where each particular charging source is coupled and said return path.
8 . The charging system of claim 7 , wherein the charging system is configured such that when each charging source in said second set is operating in said first state, charging current bypasses a battery cell located:
(a) adjacent a cell junction where a particular charging source operating in said first state is coupled, and
(b) between said cell junction where a particular charging source operating in said first state is coupled and said ground end.
9 . The charging system of claim 1 , further comprising:
a plurality of cell monitors, wherein each cell monitor is coupled to a respective battery cell, and configured to measure a terminal voltage in an associated battery cell; and
a controller, wherein said controller is coupled to each of said cell monitors, each charging source in said first set, and each charging source in said second set, and wherein each charging source in said first set is operated by said controller to provide charging current to one or more battery cells containing a terminal voltage below a threshold amount, and each charging source in said second set is operated by said controller to divert charging current from one or more battery cells containing a terminal voltage above said threshold amount.
10 . A method for equalizing voltage of a secondary battery being charged, the method comprising the steps of:
coupling N battery cells in series to form a series string, wherein said coupling N battery cells step comprises the steps of:
coupling a first battery cell to a load end,
coupling a N th battery cell to a ground end, and
forming a respective cell junction between each adjacent pair of battery cells in said series string;
coupling a first set of charging sources to said series string, wherein said coupling a first set of charging sources step comprises the steps of:
coupling a first charging source of said first set to said first cell at said load end, and
coupling each remaining charging source in said first set to each cell junction formed on said series string; and
coupling a second set of charging sources to said series string, wherein said coupling a second set of charging sources step comprises the step of:
coupling each charging source of said second set to each cell junction formed on said series string.
11 . The method of claim 10 , wherein said coupling a first set of charging sources step comprises the step of:
coupling N charging sources to said series string.
12 . The method of claim 11 , wherein said coupling a second set of charging sources step comprises the step of:
coupling (N-1) charging sources to said series string.
13 . The method of claim 10 , further comprising the steps of:
configuring each charging source in said first set to operate in one of a first state and a second state, wherein said configuring step comprises configuring each charging source in said first set to provide charging current to a respective cell junction and battery cell where said first particular charging source is coupled when operating in said first state, and configuring each charging source in said first step to not provide charging current to said respective cell junction and battery cell where said first particular charging source is coupled when operating in said second state; and
configuring each charging source in said second set to operate in one of a first state and a second state, wherein said configuring step comprises configuring each charging source in said second set to form a low impedance electrical path between a return path and a respective cell junction where each charging source in said second set is coupled when operating in said first state, to form a high impedance electrical path between said return path and said respective cell junction where each charging source in said second set is coupled when operating in said second state.
14 . The method of claim 10 , further comprising the step of:
configuring each charging source in said first set to provide charging current to each battery cell containing a terminal voltage below a threshold amount; and
configuring each charging source in said second set to prevent each battery cell containing a terminal voltage above said threshold amount from receiving charging current.
15 . The method of claim 10 , further comprising the steps of:
coupling a plurality of cell monitors to each battery cell included in said series string, wherein said cell monitors are configured to:
monitor a respective voltage level of each of said N battery cells;
determine which of said N battery cells contains a terminal voltage above said threshold amount; and
determine which of said N battery cells contains a terminal voltage below said threshold amount.
16 . The method of claim 10 , further comprising the steps of:
configuring each charging source in said first set to selectively provide charging current to one or more of said N battery cells; and
configuring each charging source in said second set to selectively divert charging current from one or more of said N battery cells.
17 . A method for equalizing voltage of a secondary battery being charged, the method comprising the steps of:
providing charging current to at least one battery cell in a series string containing a terminal voltage below a pre-determined threshold amount, wherein said charging current is provided to charge said at least one battery cell; and
preventing charging current from being provided to any battery cell in said series string containing a terminal voltage above said pre-determined threshold amount utilizing a charging source, wherein said preventing step comprises the step of:
causing a first electrical path to possess an impedance lower than a second electrical path, wherein said second electrical path includes at least one battery cell containing a terminal voltage above said pre-determined threshold amount.
18 . The method of claim 17 , further comprising the step of:
switching ON said charging source to cause said charging current to flow through said first electrical path, and to not flow through said second electrical path.