56 Chapter 5 The FMEA is most effective when it is based on a product that is in the market already. In this manner, you will already have a good idea of the types of failures that can or have occurred. However, with lithium-ion energy storage systems, for large applications there is little history from which to base your evaluations and so must be based on “expert” judgment of the engineering teams involved. The final thing I will mention on FMEAs is that they are a process that takes some time, dedication and knowledge to undertake. A typical DMFEA for instance may take several weeks or more to complete and is likely to be done in parallel to the design and engineering processes.
Often they are also led by a quality engineer who is a member of the design team who is an expert in conducting FMEAs and may be certified by one of the organizations that I mentioned earlier. Design for Service DFS is a parallel activity to DFR that is intended to integrate service features and lifetime serviceability into the product. The DFS process is focused on integrating features into the product that can be used by service and support personnel to repair a product without having to replace it entirely. In the design process, DFS involves evaluating what components may fail, or are designed to fail first, and making them easily and safely accessible for service personnel.
Part of the DFS process may entail conducting mean time to failure analysis on the compo- nents and systems of the energy storage system in order to understand how long they may last before catastrophic failure occurs. This is already a common process in printed circuit board design as well as in much of the electronics industries. The question when it comes to battery service life is what components should be serviceable. Many lithium-ion cells are connected together using a welded bus bar-type design. This means that once these are welded together they become a single unit, the SRU mentioned earlier. So in most cases the battery cells are not serviceable.
There are some companies that have developed mechanical connections for the battery cells, however, in automotive applica- tions there has been concern about meeting the reliability life of the product with these designs. Yet for other markets that do not have such challenging shock and vibration require- ments, mechanical solutions may be perfectly acceptable solutions. Frequently in the design process the battery module, the assembly of lithium-ion cells connected mechanically and electrically, is much easier to replace than replacing single cells. However, the cost of replacing a module will always be higher than the cost of replacing a single cell.
So there is always a trade-off that must be evaluated during the design process. Another challenge that must be considered in servicing of battery modules is the aging of the lithium- ion cells. As a battery ages during normal use the capacity begins to shrink due to natural