Method and system for cell equalization with switched charging sources

• US 7,821,230 B2
• Filed: 07/06/2009
• Issued: 10/26/2010
• Est. Priority Date: 11/10/2004
• Status: Active Grant

• Current Assignee: Eaglepicher Technologies LLC (Eaglepicher Corporation)
• Original Assignee: Eaglepicher Technologies LLC (Eaglepicher Corporation)
• Inventors: Studyvin, William B., Miller, Greg, Shimp, Phillip, Pennock, Tim, Coatney, Eric
• Primary Examiner(s): Tso; Edward
• Assistant Examiner(s): RAMADAN, RAMY O
• Application Number:
  US12/498,210
• Publication Number:
  US 20090309544A1
• Time in Patent Office: 477 Days
• Field of Search: 320/118
• US Class Current: 320/119
• CPC Class Codes: H02J 7/0018 using separate charge circuits

Abstract

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.

Claims

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,

a N^th battery cell coupled to a ground, and

a cell junction formed 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 of charging sources comprises an input terminal and an output terminal, wherein an input terminal of each of the charging sources of the first set of charging sources is directly connected to a power source, an output terminal of a first charging source of said first set of charging sources is coupled to the load, and an output terminal of each of the other charging sources of the first set of charging sources is coupled to a respective cell junction, wherein each of the charging sources of the first set of charging sources provides a charging current in a first state of operation and does not provide a charging current in a second state of operation and wherein an N−

1 charging source of the first set of charging sources provides a charging current for an N−

1 battery cell and for the N^th battery cell to the cell junction formed between the N−

1 battery cell and an N−

2 battery cell when operating in the first state;

a second set of charging sources coupled to said series string comprising exactly N−

1 charging sources, wherein each charging source in said second set of charging sources comprises an input terminal and an output terminal, wherein an input terminal of each charging source in said second set of charging sources is coupled to a respective cell junction, and an output terminal of each charging source in said second set of charging sources is coupled to the ground, wherein each of the charging sources of the second set of charging sources diverts a charging current to the ground provided by a charging source of the first set of charging sources when operating in a first state and does not divert a charging current to ground when operating in a second state and wherein the N−

1 charging source of the second set of charging sources diverts to the ground the charging current for the N−

1 battery cell and the charging current for the N^th battery cell when operating in the first state and the N^th charging source of the second set of charging sources diverts to the ground the charging current for the N^th battery cell when operating in the first state of operation; and

a controller for selectively operating each charging source of the first set of charging sources and each charging source of the second set of charging sources in the first state or the second state to provide a charging current to each battery cell having a voltage less than a predetermined voltage.

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 1, further comprising:

the power source coupled to each input terminal of said charging sources in said first set of charging sources, wherein each charging source in said first set of charging sources is configured to provide charging current to at least one of said N battery cells.

4. The charging system of claim 3, 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 of charging sources 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 of charging sources 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.

5. The charging system of claim 1, wherein:

said ground is a charging current common return path to each charging source second set of charging sources to the power source and the first set of charging sources.

6. The charging system of claim 1, wherein when each charging source in said second set of charging sources is operating in said first state, each charging source in said second set of charging sources forms a low impedance electrical path between a cell junction where each particular charging source is coupled and the ground, and when each charging source in said second set of charging sources 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 the ground.

7. The charging system of claim 6, wherein the

charging system is configured such that when each charging source in said second set of charging sources 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 the ground.

8. 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

wherein the controller is coupled to each of said cell monitors, each charging source in said first set of charging sources, and each charging source in said second set of charging sources, and wherein each charging source in said first set of charging sources is operated by said controller to provide charging current to one or more battery cells having a terminal voltage below a threshold amount, and each charging source in said second set of charging sources is operated by said controller to divert charging current to the ground from one or more battery cells having a terminal voltage above said threshold amount.

9. 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,

coupling a N^th battery cell to a ground to form 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 of charging sources to said first cell at said load,

coupling each remaining charging source in said first set of charging sources to each cell junction formed on said series string, and

coupling an input terminal of each of the charging sources of the first set of charging sources directly to a power source; and

coupling a second set of charging sources to said series string, said second set of charging sources comprising exactly N−

1 charging sources, wherein said coupling a second set of charging sources step comprises the steps of;

coupling an input terminal of each charging source of said second set of charging sources to each cell junction formed on said series string and an output terminal of each charging source of the second set of charging sources to the ground; and

configuring each charging source in said second set of charging sources to operate in one of a first state and a second state, wherein in the first state, a charging source diverts a charging current from the input terminal of the charging source to the ground and in the second state, the charging source does not divert a charging current from the input terminal of the charging source to the ground.

10. The method of claim 9, wherein said coupling a first set of charging sources step comprises the step of:

coupling N charging sources to said series string.

11. The method of claim 9, further comprising the steps of:

configuring each charging source in said first set of charging sources 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 of charging sources 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

wherein said configuring each charging source in said second set of charging sources to operate in one of a first state and a second state step comprises configuring each charging source in said second set of charging sources to form a low impedance electrical path between a return path and a respective cell junction where each charging source in said second set of charging sources 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 of charging sources is coupled when operating in said second state.

12. The method of claim 9, further comprising the step of:

configuring each charging source in said first set of charging sources to provide charging current to each battery cell containing a terminal voltage less than a threshold amount; and

configuring each charging source in said second set of charging sources to prevent each battery cell containing a terminal voltage greater than said threshold amount from receiving charging current.

13. The method of claim 9, 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 has a terminal voltage above said threshold amount; and

determine which of said N battery cells has a terminal voltage below said threshold amount.

14. The method of claim 9, further comprising the steps of:

configuring each charging source in said first set of charging sources to selectively provide charging current to one or more of said N battery cells; and

configuring each charging source in said second set of charging sources to selectively divert charging current from one or more of said N battery cells.

15. A system for charging a rechargeable battery, said rechargeable battery comprising N battery cells configured in a series string, wherein N is at least three, 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;

a first set of charging sources coupled to said series string receiving and responsive to charging source control signals from the controller for selectively operating in a first or a second state, said first set of charging sources comprising;

a first charging source of the first set of charging sources comprising an input terminal directly 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;

a second charging source of the first set of charging sources comprising an input terminal directly 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 third charging source of the first set of charging sources comprising an input terminal directly connected to the power source and an output terminal connected to the cell junction between the second battery cell and the third battery cell, for providing to said cell junction 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 second set of charging sources comprising exactly N−

1 charging sources coupled to said series string receiving and responsive to charging source control signals from the controller for selectively operating in a first or a second state, wherein said second set of charging sources comprises;

a first charging source of the second set of charging sources 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 ground for diverting to the ground the charging current for the second battery cell and the charging current for the third battery cell provided by the first charging source of the first set of charging sources when operating in the first state and not diverting to the ground the charging current for the second battery cell and the charging current for the third battery cell when operating in the second state; and

a second charging source of the second set of charging sources 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 diverting to the ground the charging current for the third battery cell provided by either the first charging source of the first set of charging sources or the second charging source of the first set of charging sources when operating in the first state and not diverting to ground the charging current for the third battery cell when operating in the second state,

and wherein the controller transmits charging source control signals to operate the first charging source of the first set of charging sources in the first state until a voltage of one of the at least three battery cells exceeds a predetermined voltage, and then operates the first set of charging sources and the second set of charging sources to provide charging currents to the battery cells that have not exceeded the predetermined voltage and not the battery cells that have exceeded the predetermined voltage.

16. The system of claim 15 further comprising voltage monitors for measuring the voltage of each of the battery cells and providing the measured voltage to the controller, wherein the first charging source of the first set of charging sources further provides a current to the load when operating in the first state.