Different fuel cell stack designs call for different cell contacting solutions

June 15, 2020 //By Dr. Markus Schuster, Norbert Witteczek, Smart Testsolutions
Different fuel cell stack designs call for different cell contacting solutions
The individual cell voltages of a fuel cell stack provide a deep insight into the inner workings of the stack and make it possible to recognise critical operating statuses and to react accordingly. For this reason vehicles with fuel cell systems tend to be equipped with an appropriate monitoring system. In the light of a cell spacing of less than 1 millimetre in modern fuel cells, cell contacting represents a major challenge. Potential tapping at the bipolar plates is made more difficult by the fact that stack designs differ.

Not all fuel cells are the same. The stacks differ in terms of both the materials employed and their geometry. A crucial criterion as regards cell contacting is the so-called cell pitch. This describes the thickness of an individual cell, in other words the distance between two bipolar plates. Bipolar plates are the key components of a fuel cell. They separate the gas compartments of adjacent cells. Arranged in layers to create a stack, the plates form the heart of a fuel cell system.

Three fundamental challenges

Today's metallic bipolar plates have a cell pitch of around 1 millimetre. The problem: Within a fuel cell stack, the spacing varies on account of manufacturing tolerances and also during the course of operation. What's more, the stack expands and contracts during operation. It is also subject to vibration when fitted in the vehicle. With regard to the cell voltage pickup (CVP), this means that the potential taps have to be somewhat flexible, whereas the mounting unit accommodating the contacts is a rigid structure. It is therefore a question of combining a measured value pick-up system of fixed size with variable potential taps.

A further basic challenge when developing cell contacting elements for fuel cell monitoring systems – known as Cell Voltage Monitoring Systems (CVM) – is how to produce an electrical contact to satisfy the requirements for use in vehicles: The contacting element must be electrically functional, vibration-resistant, suitable for automotive applications, of compact design, thermally stable and inexpensive.

To date, CVM systems have been used primarily in the development and testing of fuel cell vehicles. Given the relatively small quantities involved, the installation time required for cell contacting was of no great importance. But series production is a different matter again. The mass reproducibility of the cell contacting elements and the costs will come under increasing pressure. Aspects such as installation capability and time, as well as the level of production automation, will gain in significance. Whereas the installation of a CVM and cell contacting used to take half a day, a few minutes will have to suffice in future.

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