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.
In the light of these challenges the Smart Testsolutions team, known in the electromobility sector above all for cell voltage monitoring systems on fuel cells, has put a lot of thought into ways of improving cell contacting in recent years. The first outcome of these considerations was a contacting unit with spring contacts mounted on one side in 1997.
Graphite bipolar plates - durable but expensive
In automotive engineering, a distinction can basically be made between three different types of removable contact: Pin contacts, spring contacts and tab contacts (refer to Fig. 1). The type of contact used in each case is governed not least by the nature of the bipolar plates in the stack. Graphite plates possess a high level of electrical conductivity and exhibit great resistance to highly corrosive conditions. They are however mechanically vulnerable, so far difficult to mass-produce, and heavy. With a thickness of 4 to 4.5 millimetres, experts also consider graphite bipolar plates to be responsible for roughly 80 per cent of the weight and up to 45 per cent of the cost of a stack. Graphite plates have started to become more popular again in recent times. This is because progress in production techniques means they can now be made far thinner, thus enabling the advantage they offer in terms of durability to be exploited to the full.
Metallic plates are much lighter, far thinner and offer better cold starting properties. As the high-volume production of metallic bipolar plates is also associated with considerable cost savings, these have since become established on the fuel cell market. Metallic plates do however also have their drawbacks, as they tend to consist of several parts which are welded together. Producing a tight joint is a very difficult matter and the plates are usually less durable than graphite bipolar plates.
Pin contacts are primarily used with metallic bipolar plates
As graphite is a brittle material, it is difficult to drill holes in thin plates in particular. Pin contacts are therefore not an option for graphite bipolar plates. It is however possible to drill holes in metallic plates. But bearing in mind the general challenges outlined above, it soon becomes apparent why Smart Testsolutions did not pursue this approach any further. To achieve contacting, the pin contacts have to be located in a specific fixed position.
This means having to already decide on a certain cell pitch when designing the contacting unit. Furthermore, manufacturing tolerances in the bipolar plates and thus slight differences in spacing can soon cause the pins to exhibit lateral offset and not fit ideally in the holes provided. This leads to undesirable shear forces which impair the functionality and reliability of the monitoring system.
For this reason, many manufacturers of fuel cells with metallic bipolar plates prefer to use tab contacts rather than pin contacts. This involves forming the plate with an additional tab. Potential tapping then takes place by way of a suitable clip-type connecting element which has to be fitted over the 0.1 to 0.3 millimetre thin tabs. The advantage: Clip components are available on the market in the form of so-called on-board clips, which can be directly assembled on PCBs. Suitable contacting units are thus relatively simple and inexpensive to make.
Here again, installation does however present a major challenge. Firstly on account of the often varying cell spacing and secondly due to the small component dimensions. Such on-board clips are extremely small and the tabs very thin. The contact can no longer be used if the tab becomes bent.
In the opinion of Smart Testsolutions, spring contacts are the best option from the point of view of installation and reliability for contacting solutions to be employed in mobile applications. Pin contacts without a spring should only be used for stationary applications in research and development work. Such pin contacts are commercially available components to which the measurement leads are secured using shrink-down tubing. The leads provide the necessary flexibility.
Spring contacts offer many advantages
The previously mentioned cell contacting element from 1997 (Fig. 2) works with spring contacts mounted on one side. Ten contacts in each case are attached to a small PCB, which can then be combined to form a complete system depending on the number of cells to be contacted. These contact modules are flexibly mounted in guides. They can thus be adjusted after unfastening the screw connections of the clamping arms. The individual contacts are also free to move both sideways and – thanks to the spring force – upwards and downwards. This permits adaptation of the contacting unit to different stack types, compensation for manufacturing tolerances and accommodation of stack movement in operation.
The design of the assembly illustrates how to a certain extent variable CVP modules can be linked to a rigid CVM system with specified connectors. From the contacting modules, the measurement signals are relayed via wire-to-board connections to a rigid PCB with the connectors appropriate to the CVM system. Special design feature: The wire-to-board connections are permanently soldered on one side but provided with connectors on the other. As a result, the module PCBs with the contacts can simply be replaced depending on the cell pitch.
The advantage of the S-shaped contacts is that the bipolar plates are protected against abrasion. A larger contact surface also ensures more reliable connection. By contrast, measurement of the individual cell voltages is more likely to fail if there is only a single point of contact with the bipolar plate. The signal could be interrupted for instance if the spring contact lifts off slightly on account of the vehicle running over a pothole.
Spring contacts mounted on one side are employed with bipolar plates which are provided with a recess to accommodate the contacts. The drawback to this type of contacting is that the corresponding CVP systems take up a lot of space. The overall height is between 35 and 250 millimetres. That is a lot given the confined space available in the engine compartment of a vehicle. Another factor is the work involved when performing installation at the fuel cell stack. Depending on the stack, a considerable amount of manual readjustment may be required to ensure that every single contact rests in a cell groove – a tedious business given a cell spacing of less than 1 millimetre.
Spring contact mounted on two sides minimises overall height
In the light of these challenges, Smart Testsolutions have developed an improved cell contacting unit with spring contacts over the last three years. This CVP guarantees reliable voltage tapping without the need for any awkward manual readjustment on installation. The contacts are for the most part automatically centred in the cell pockets, thus ensuring short installation times. And a height of just five millimetres is an added advantage of the contacting unit.
This minimal height is achieved by employing two-sided mounting of the spring contact and a special contact design. The contacting unit consists of gold-plated copper-beryllium wires which are spring-mounted at both ends in modular holders. This provides compensation for the manufacturing tolerances of the stack, as well as for impact and vibration acting in vertical direction. The holders are threaded onto sections and are flexibly mounted. This also makes it possible to accommodate horizontal changes in the length of the stack.
The entire contacting unit is attached to the end plates and – if applicable – to a central plate of the fuel cell stack. Once the contacts are aligned in the cell pockets, the system is secured in its final position by a hood which presses the copper-beryllium wires into the cell pockets and ensures constant contact pressure as well as reliable signal tapping. The system is not affected by either vibration or temperature and can be integrated into a wide range of existing fuel cell packages on account of its minimal overall height.
Providing spring contacts with a gold coating is a good way of avoiding increased contact resistance due to corrosion in operation. A cleaning effect is also obtained from the shock and vibration occurring in mobile operation and the associated slight movement of the spring-loaded contacts on the bipolar plate. This keeps the contact resistance between the contact and the plate to a minimum.
A drawback to spring contacts mounted on one side or on two sides is that assembly of these on PCBs cannot be automated. Solutions need to be found here – particularly with a view to the mass production of fuel cell vehicles with integrated cell voltage monitoring. Certain other challenges will present themselves in the near future:
- Reduction of costs: One-off CVP units are currently still extremely expensive to make. The unit price will have to come down for use in mass-produced vehicles – even if the numbers are fairly small.
- Fewer different modules: In view of the varying number of cells in the fuel cell stacks, use still has to be made of many different modules at present. The aim is to standardise the number of cells in a fuel cell stack in future. If this proves successful, it will be possible to reduce the number of different CVP modules, which should have a positive effect on costs.
- Automated installation: The greatest challenge is to create CVP concepts which involve a minimum amount of installation work at the fuel cell stack in combination with the highest possible degree of automation of the installation process. This means having to make allowance for the applicable production and automation requirements when developing the CVP.
On account of the greatly differing requirements associated with cell voltage monitoring and cell contacting in fuel cell stacks, the corresponding systems still tend to be developed separately at present. More than twenty years ago, Smart Testsolutions already presented a combined CVM and CVP product solution in the second generation of fuel cell monitoring systems. We expect combined solutions to assert themselves in the future. One crucial aspect, for example, is how to design the connection system to satisfy the requirements of IP safety class IP67. We are expecting an even higher level of integration once ICs become state of the art for series CVM functionality.
About the authors:
Dr. Markus Schuster is Business Line Manager e_Cell Electronics at Smart Testsolutions GmbH.
Norbert Witteczek is Business Line Manager Test Systems & Applications at Smart Testsolutions GmbH.
Smart Testsolutions is a supplier of solutions for performing tests on automotive control units, electronic systems and renewable energy systems such as batteries and fuel cells. The company is based in Stuttgart, Germany.
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