# US 6,724,194 B1 · Cell voltage monitor for a fuel cell stack > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 6,724,194 B1 - Document ID: US6724194B1 - Status: Granted - Kind: B1 - Application number: 09/607,796 - Filed: 2000-06-30 - Granted: 2004-04-20 - Earliest claimed: 2000-06-30 - Assignees: Ballard Power Systems Inc. - Inventors: Russell Howard Barton - Web page: https://patentgrep.com/patent/US6724194B1 ## CPC - H01M 8/04552: {of the individual fuel cell} - G01R 19/16538: {in AC or DC supplies (G01R19/16519 and G01R19/16528 take precedence)} - G01R 31/396: Acquisition or processing of data for testing or for monitoring individual cells or groups of cells within a battery - H01M 2008/1095: {Fuel cells with polymeric electrolytes} - Y02E 60/50: Fuel cells - H01M: PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY - G01R: MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES (indicating correct tuning of resonant circuits H03J3/12) - Y02E: REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION ## Abstract Voltage reversal in fuel cell stacks may be detected and/or prevented by monitoring each cell or small groups of cells in the stack for low voltage conditions. An improved cell voltage monitor is constructed using a plurality of voltage monitoring units in which each unit comprises a heating resistor, a rectifier in series therewith, and a sensing thermistor in thermal communication with the heating resistor. The heating resistor is electrically-connected across a cell or cells and heats the associated sensing thermistor in accordance with the voltage of the cell(s). Since the resistance of the sensing thermistor is indicative of its temperature and hence also of voltage of the cell(s), the sensing thermistor resistance can be used to signal a low voltage condition. ## Claims ### Claim 1 A voltage monitoring unit comprising: at least one cell in a fuel cell stack; a heating resistor; a rectifier in series with said heating resistor; said heating resistor and rectifier in series being connectable across said fuel cell, wherein the polarity of said rectifier is such that current is prevented from flowing through said rectifier when said fuel cell is in voltage reversal; and a sensing thermistor, said sensing thermistor being electrically isolated from and in thermal communication with said heating resistor, wherein the resistance of the sensing thermistor is indicative of the voltage of at least one fuel cell. ### Claim 2 (depends on 1) The voltage monitoring unit of claim 1 additionally comprising a reference thermistor in series with said sensing thermistor and thermally isolated from said heating resistor. ### Claim 3 (depends on 2) The voltage monitoring unit of claim 2 wherein said reference thermistor and said sensing thermistor have the same resistance as a function of temperature characteristics. ### Claim 4 (depends on 2) The voltage monitoring unit of claim 2 additionally comprising a diode electrically-connected at one end between said series-connected sensing and reference thermistors. ### Claim 5 (depends on 1) The voltage monitoring unit of claim 1 wherein the resistance of said sensing thermistor changes substantially with an increase in temperature above ambient. ### Claim 6 (depends on 5) The voltage monitoring unit of claim 5 wherein said sensing thermistor is a resettable thermal switch. ### Claim 7 (depends on 6) The voltage monitoring unit of claim 6 wherein said sensing thermistor is a polymeric positive temperature coefficient fuse. ### Claim 8 (depends on 1) The voltage monitoring unit of claim 1 wherein said heating resistor is a positive temperature coefficient thermistor. ### Claim 9 (depends on 1) The voltage monitoring unit of claim 1 wherein said heating resistor, said rectifier and said sensing thermistor are each coated with a thermosetting polymer. ### Claim 10 (depends on 1) A cell voltage monitor for a fuel cell stack comprising a plurality of the voltage monitoring units of claim 1. ### Claim 11 (depends on 10) The cell voltage monitor of claim 10 wherein the series-connected heating resistors and rectifiers in each of said plurality of voltage monitoring units are connected in series. ### Claim 12 (depends on 2) A cell voltage monitor for a fuel cell stack comprising a plurality of the voltage monitoring units of claim 2. ### Claim 13 (depends on 12) The cell voltage monitor of claim 12 wherein the series-connected reference thermistors and sensing thermistors in each of said plurality of voltage monitoring units are connected in parallel across a reference voltage supply. ### Claim 14 (depends on 4) A cell voltage monitor for a fuel cell stack comprising a plurality of the voltage monitoring units of claim 4. ### Claim 15 (depends on 14) The cell voltage monitor of claim 14 wherein the series-connected reference thermistors and sensing thermistors in each of said plurality of voltage monitoring units are connected in parallel across a reference voltage supply, and the other ends of the diodes in each of said plurality of voltage monitoring units are connected to a common signal line. ### Claim 16 (depends on 15) The cell voltage monitor of claim 15 wherein the polarity of said diodes is such that current is prevented from flowing from an unheated one of said sensing thermistors to a heated one of said sensing thermistors via said common signal line. ### Claim 17 (depends on 6) A cell voltage monitor for a fuel cell stack comprising a plurality of the voltage monitoring units of claim 6. ### Claim 18 (depends on 17) The cell voltage monitor of claim 17 wherein the resettable thermal switches in each of said plurality of voltage monitoring units are connected in parallel. ### Claim 19 (depends on 10) The cell voltage monitor of claim 10 wherein said plurality of voltage monitoring units are mounted on a flexible printed circuit board. ### Claim 20 (depends on 1) A fuel cell stack comprising the voltage monitoring unit of claim 1 wherein the series-connected heating resistor and rectifier in said voltage monitoring unit are electrically-connected across at least one fuel cell in said fuel cell stack. ### Claim 21 (depends on 20) The fuel cell stack of claim 20 wherein the polarity of said rectifier is such that current is prevented from flowing through said series-connected resistor and rectifier when said fuel cell is in voltage reversal. ### Claim 22 (depends on 20) The fuel cell stack of claim 20 wherein said stack comprises a plurality of series-connected fuel cells and a cell voltage monitor comprising a plurality of voltage monitoring units. ### Claim 23 (depends on 22) The fuel cell stack of claim 22 wherein each of said voltage monitoring units is connected across a pair of said fuel cells in said stack. ### Claim 24 (depends on 20) The fuel cell stack of claim 20 wherein the fuel cells in said stack are solid polymer electrolyte fuel cells. ### Claim 25 A method for detecting a low voltage in a fuel cell in a fuel cell stack comprising: connecting a heating resistor and a rectifier in series across said fuel cell wherein the polarity of said rectifier is such that current is prevented from flowing through said rectifier when said fuel cell is in voltage reversal; positioning a sensing thermistor in thermal communication with and electrically isolated from said heating resistor such that said sensing thermistor is heated when current flows through said heating resistor; and obtaining a signal indicative of the resistance of said sensing thermistor, said signal detecting a low voltage in at least one fuel cell. ### Claim 26 (depends on 25) The method of claim 25 wherein said signal is obtained by: connecting a reference thermistor in series with and thermally isolated from said sensing thermistor; applying a reference voltage across the series-connected sensing and reference thermistors; and measuring the voltage across said sensing thermistor. ### Claim 27 (depends on 25) The method of claim 25 wherein said sensing thermistor is a resettable thermal switch and said signal indicates whether said resettable thermal switch is open or closed. ### Claim 28 (depends on 25) The method of claim 25 comprising thermally isolating said sensing thermistor from said fuel cell stack. ### Claim 29 (depends on 25) A method of monitoring for voltage reversal in a fuel cell stack comprising using the method of claim 25 to detect for any low voltage fuel cell in said fuel cell stack.