This recommended practice is intended to serve as a procedure to verify the functional performance, design specifications or vendor claims of any PEM (Proton Exchange Membrane) type fuel cell stack sub-system for automotive applications. In this document, definitions, specifications, and methods for the functional performance characterization of the fuel cell stack sub-system are provided. The functional performance characterization includes evaluating electrical outputs and controlling fluid inputs and outputs based on the test boundary defined in this document.

In this document, a fuel cell stack sub-system is defined to include the following:

• Fuel cell stack(s) – An assembly of membrane electrode assemblies (MEA), current collectors, separator plates, cooling plates, manifolds, and a supporting structure.

• Connections for conducting fuels, oxidants, cooling media, inert gases and exhausts.

• Electrical connections for the power delivered by the stack sub-system.

• Devices for monitoring electrical loads.

• Devices for monitoring cell voltage.

• Humidification devices.

• Instrumentation for detecting normal and/or abnormal operating conditions.

• Enclosures (that may qualify as pressure vessels), and ventilation systems for the enclosure.

Not included in the sub-system are the following:

• Fuel and air processors

• Thermal management system

• Power conditioner and distributor

• Controllers

Limitations of Recommended Practice

The test article of this document is the fuel cell stack sub-system that is the primary component within the fuel cell power system. Therefore, it may not represent the performance characterization of either the fuel cell system or the stack sub-system components. Among different types of fuel cell stack sub-systems, only the PEM type is covered in this document.

A generic test boundary that specifies the power inputs and outputs is defined in this document (Section 5.1.1). If other configurations are used, modifications to the test boundary should be made based on the agreement of the testing parties.

This document is designed to determine the functional performance characterization of a PEM fuel cell stack sub-system at the beginning of life. This document does not cover the following:

• Start up characteristics of a fuel cell as this property is highly dependant on the system to which it is connected. All start ups should be carried out according to a protocol provided by the stack manufacturer/supplier, unless a different protocol is agreed to by the testing parties.

• Robustness performance characteristics such as the effects on the fuel cell stack sub-system by tilt, vibration, and harshness are not included in this document. These should be considered in conjunction with fuel cell vehicle requirements

• Cumulative effects due to fluid contamination. The cleanliness of fluid inputs such as maximum parts per million of sulfur and chlorine should be determined by the testing parties prior to stack sub-system testing. For minimizing the effect of carbon monoxide residuals within the fuel cell stack, the fuel input line may be equipped with an air bleed.

• Performance related to governmental regulations or certification.

• Shelf life, aging, and piece-to-piece variance of the fuel cell stack sub-system.

• Environmental effects such as temperature, humidity, and altitude of the area around the stack.

The discussion of general safety is not included. However, general fuel cell system safety is covered in SAE J2578. Safety concerns and precautions specific to the fuel cell stack sub-system and its testing methods are addressed in this document.

In this document, the formula used to calculate stack sub-system efficiency is based on stack power produced relative to the lower heating value of the hydrogen gas consumed (see Equation 4 in Section 7.2.3). This approach was chosen instead of one based on calculating Gibbs' free energy for the following reasons:

a. The Gibbs' free energy must assume a single temperature throughout the stack, and this is not always the case.

b. The Gibbs' free energy makes use of an ‘ideal voltage' that is not strictly valid in a system where temperatures are not fixed.

c. The approach taken can be verified empirically, without making any assumptions.

d. The approach taken can be easily integrated into a fuel cell system calculation and thus can be related mathematically to the work in SAE J2615 and SAE J2616.

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