Flex cards reduce monitoring costs

Fuel cells and electrolysers are core technologies for a hydrogen economy and for the energy transition. They consist of one hundred or more cells. Our cell voltage monitoring systems (CVM) monitor the individual cell voltages to ensure optimum operation. Each individual cell must be contacted to tap the voltage. For this purpose, we have developed various cell voltage pickup (CVP) solutions. In contrast to many other manufacturers of CVM systems, we are increasingly using flex cards for contacting instead of conventional individual cables with crimp contacts. This has many advantages.

If you change the ink cartridge in an inkjet printer or remove jammed paper, you have probably noticed the thin, long film with the fine conductive tracks that moves back and forth with the print head: That's a flex card. Flex cards have many advantageous properties such as low space requirements, simple connection, high robustness even when moving and, of course, the flexibility that gives them their name. This is why they are used in countless applications in electronics. They are usually used to bridge geometrically difficult gaps and to compensate for movements or vibrations.

Fuel cells and electrolysers consist of dozens, sometimes even hundreds of individual cells that are separated by bipolar plates. Until now, manufacturers thought that simply measuring the overall voltage of the stack was sufficient for monitoring purposes. However, it is increasingly being recognised that it is best to measure the voltage of each individual cell so that the operator can detect a drop in the performance of the stack at an early stage and react immediately - and not just in the laboratory on prototypes, as was previously the case, but on each individual cell in each stack, for example in production vehicles.

The cell voltage signals are picked up by CVPs (cell voltage pickups). What exactly these CVPs look like depends largely on the bipolar plates of the cell stack to be contacted: they may have V-shaped recesses or so-called flags for the contact, but often they are so-called pinholes for holding round needle contacts. Because each cell is only between one and two millimetres thin, these round needle contacts are just as close together. As a result, a good hundred tiny needle contacts are packed into the width of a hand - contacting them is a real challenge. The developments from SMART TESTSOLUTIONS are setting standards here; no competitor has developed such a wealth of CVP solutions.

Important detail of the cell contacts: The signals must be forwarded from each contact to the CVM, where the signals are read out and pre-processed. The catch: CVP and CVM are located on different circuit boards because there is usually no space for both directly on the cell stack; a kind of wiring harness is therefore required. Crimp contacts are very prone to faults and tend to break under vibration, for example in a fuel cell vehicle. With several hundred contacts, troubleshooting is very time-consuming.

This is where the flex cards come into play. Their conductor tracks lead from the CVP to the CVM with the measuring electronics. The number of connections per board or flex card can vary. The flex card allows a great deal of design freedom. The developer is largely free to determine where the contacts are located, how many tracks are combined and which CVM modules they lead to. If changes to the stack or the CVM system are required, the flex card can be adapted in no time at all. This makes it easy to design the flex cards for different cell pitches. Even with the same manufacturer, the thickness of the cells varies depending on the model.

In a specific solution for an automotive supplier's fuel cell, the flex card elegantly bridges a height difference of a few millimetres between the CVP and CVM with around 200 connections. Imagine if you had to do this with individual cables. Each cable would need a different length, or you would have excess cable at the short connections and thus a tangle of cables. In addition, crimping the tiny contacts would be extremely demanding and error-prone.

Here are the advantages of a flex card for connecting CVP and CVM at a glance:

• Long service life: The flexibility of flex cards means that the conductor tracks never break. The contacts can be permanently protected against corrosion with a lacquer during soldering.
• High dielectric strength.
• High availability: Flex cards, often referred to as flexible PCBs, are available on the market at favourable prices and every PCB manufacturer supplies them in large quantities in good quality.
• The foil of the flex card can be labelled directly by the supplier, with type number, properties, etc. This would be difficult with individual cables; small adhesive flags would be an enormous amount of work with a high risk of confusion.
• Low costs: An exact cost calculation is difficult as it depends heavily on the design. It can be estimated that the flex card solution saves around 50 percent of the costs per measuring channel compared to individual wiring of the contacts.
• High quality standards: Numerous norms and quality standards (IPC standards) exist for PCBs and polyamide foils, which create security for developers and customers.