Cell contacting solutions are diverse

Not all fuel cell stacks are the same. This becomes apparent every time we set about developing potential tap solutions for the purpose of monitoring the single cell voltages of a stack. For a variety of reasons, the potential tap at the bipolar plates of a fuel cell stack presents a particular challenge.

To start with, only limited space is available in a vehicle engine compartment. A cell contacting element with an overall height of between 35 and 250 millimetres, as is the case with conventional solutions, takes up far too much valuable space. Secondly, it is important to minimise the work involved when installing the cell contacting element at the fuel cell stack – not least with a view to series production. At present, spring contacts are often employed for voltage tapping at fuel cells. These are not only very tall, they also have to be adjusted by hand. Such adjustment is a very awkward business given the cell spacing of less than a millimetre in modern fuel cells.

Spring contacts lend themselves to cell pockets

In the light of these challenges, we at SMART TESTSOLUTIONS have given a lot of thought to the question of how to improve cell contacting over the past few years. The first groundbreaking outcome of these considerations in 2016 was our cell contacting unit (cell voltage pickup, CVP) with spring contacts, which ensures reliable voltage tapping without the need for tedious re-adjustment by hand on installation. The entire contacting unit is attached to the end plates and – if applicable – to a central plate of the fuel cell stack. 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.

Bipolar plates with holes require pin contact solutions

But, as mentioned above, not all fuel cell stacks are the same. Whereas some types of stack are provided with cell pockets which make them suitable for the solution described above, other types feature holes or bores in the bipolar plate and so require the use of pin contact solutions.

We at SMART TESTSOLUTIONS support both customers who are looking for a better solution for an existing fuel cell stack with cell contacting unit and customers who require a CVP for a stack which is still at the development stage. In both instances the challenge we face is to find the appropriate CVP technology and to adapt our technical operating principle to the customer's stack design. In the latter case we also advise our customers on the design of the bipolar plates of their new stack with a view to obtaining a concept offering an ideal combination of bipolar plate and CVP.

Further challenges include operational reliability and cost-effectiveness

Alongside the limited space available in the engine compartment and a minimum amount of installation work, it is important not to lose sight of other challenging aspects. For example, the contacting units employed in vehicles have to be able to withstand the shocks and vibrations occurring during operation. To this end we have to technologically adapt applicable features, such as the means of attaching the CVP, to the fuel cell stack concerned. The choice of suitable materials also plays a part in this.

The mass production of fuel cell vehicles with integrated cell voltage monitoring will confront us with a whole range of further challenges to master in 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 were to prove successful, it would reduce the number of modules required for CVP solutions.
• 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.

The aim is to integrate CVM and CVP in a single component

On account of the greatly differing requirements associated with cell voltage monitoring (CVM) and cell contacting in fuel cell stacks, we are still working with separate systems at present. Our ultimate aim is however to successively merge the two elements into one single component. The crucial aspect of this is how to design the connection system to satisfy the requirements of IP safety class IP67. As a first step we are planning a wiring connection with single-side disconnection – on the CVM system end. The next stage would be to introduce rigid-flex pcb connections. We are expecting an even higher level of integration once ICs become state of the art for series CVM functionality - the CVM chips will then be located on the contacting unit as well.

Are you interested in our development work in the field of cell contacting? I would be pleased to provide you with information. Just send an e-mail to markus.schuster@smart-ts.de.