Cell Voltage Monitoring

Introduction

A fuel cell stack or battery string is a collection of individual cells electrically connected in series. Ideally, all the cells would be subjected to identical operating conditions and all would produce the same voltage. However, due to manufacturing variability of components, system architecture, and degradation with use, individual cells vary in performance depending on the specific operating conditions. Since fuel cells and batteries can be damaged by excessively high or low voltage, it is necessary to monitor individual cells so that corrective action can be taken before damage occurs. This is done with a device called a Cell Voltage Monitor, or CVM.

The CVM acquires voltages from each cell of the fuel cell stack or battery string. This information can be used simply as a protective measure, where current through the system is restricted to protect individual cells that are threatening to go out of the acceptable voltage range. Implementing this type of protection allows the system to handle more power than it otherwise could, since the alternative is to operate the system at low power levels in hopes that no cells will go out of range. It also has value as a diagnostic tool: for example, fuel cells show an abrupt drop in voltage with increasing current when their current approaches the maximum for the supplied air and hydrogen. Thus, if an individual cell in a fuel cell stack has a partially blocked air channel, it will show a distinctive behavior: its voltage will drop off sooner than the others in the stack, and the current at which this dropoff happens will increase when more air is supplied to the stack.

Cell voltage vs. current for partially obstructed and normal cells

Cell voltage monitoring is not trivial to implement, since it involves collecting voltage information from potentially hundreds of different cells, which may be at high voltages relative to the system ground. We have developed a unique solution to this problem that minimizes cost while providing accurate measurements and maintaining high voltage safety.

UD's CVM

Schematic for the latest revision of the UD CVM system
The most recent implementation of the patent-pending UD CVM system

Building a CVM system for a fuel cell stack containing many cells, such as the stacks used in UD's fuel cell buses with 110 cells each, requires some form of multiplexing to be cost effective. Unfortunately, semiconductor switches that can block the voltage of an entire stack all have significant leakage current that varies with temperature, over 1 microampere in some cases. This is a problem because in order to safely make connections from a high voltage fuel cell or battery system to a low voltage control system that receives data from the CVM, each connection must have a resistor to limit current in the event of an isolation failure (DC interrupting fuses are bulky and difficult to obtain for the current ranges that would be appropriate). The leakage current from inactive switches flows through the resistors attached to the active switches, causing errors that can be of the same order as the voltage being measured.

This problem can be mitigated somewhat by using multiple isolated multiplexers and analog-to-digital converters, each covering a small group of cells; however, this increases cost due to the multiple ADCs and isolated power supplies. We instead used a single ADC that is connected to the cells via reed relays, which have negligible leakage current, robust isolation between the signal and control terminals, and long service life (over 109 operations per relay are expected), and can be actuated directly by digital outputs from the on-board microcontroller.

To enhance the system's performance, the latest version of the CVM incorporates a holding capacitor for each cell it monitors. This means that rather than waiting for a single filter capacitor to settle at the cell voltage every time a new cell is selected, the isolated ADC and microcontroller can rapidly "climb" up and down the capacitor ladder and measure voltages that have stabilized during the time between scans. Thanks to this improvement, the entire stack can now be scanned in under 0.35 second, enabling studies of the effects of rapidly changing gas flow rates on the fuel cell's performance.

The UD CVM system is used to protect the stacks by signaling the voltage of the lowest cell in the stack to the fuel cell controller, which regulates stack current to maintain this voltage at an acceptable level. In addition, cell voltages are recorded by the data acquisition system, allowing voltage vs. current to be plotted for every cell. This allows easy diagnosis of problems with individual cells.

Brunner D.A., Peticolas B.W., "Cell voltage measuring systems and methods," US Patent application 20090181286, serial no. 12/329903 (filed December 8, 2008).