Design for Reliability/Design for Service 57 factors mentioned in an earlier section. Therefore when a battery module is replaced, it should be done with a module of the same capacity and energy. That means that you cannot replace an old module with a brand new one as the new module will have higher capacity and energy than the aged one. The problem with doing this is that the new module will always be limited in its performance by the weakest cells in the system, so the new module will never be able to operate at its highest performance capability.
Optimally, the module should be replaced with one of equal capacity which can be achieved either by sorting “returned” modules and remanufacturing them so that they can be used as replacements or by artificially aging new cells to the same level as the ones in the module being replaced. Other system components that must be designed for serviceability include things like fuses, controllers, electronics, and fans. Many of these components may be mounted in a serviceable compartment that allows them to be accessed without having to open up the complete battery pack and exposing the high voltage system.
Fuses may be integrated into a manual service disconnect for larger PHEV or BEV battery packs but may be mounted into a separate fuse holder for small packs. In both cases they should be able to be accessed from outside of the pack. Controllers are also important to locate in places that allow them to be accessed in the event that a controller fails. For air cooled systems fans also present certain problems as most fans have lives of about three to five years. With the longer battery pack design and warranty life, fan selection can drive costs significantly for smaller systems.
In order to achieve a 10-year design life in an automotive application it may be necessary to select more expensive fans or trade-off with a shorter fan design life. Chapter Summary Both DFR and DFS pose challenges to the lithium-ion battery design due to the fact that lithium-ion batteries for automotive applications have not been in mass product for very long. The first lithium-ion batteries were commercially produced in 1991 for the portable power market, but it was not until about 2009 that the first lithium-ion batteries began hitting the market for automotive applications.
There simply are not enough data on lithium-ion battery performance in the automotive, stationary, industrial, or commercial fields to fully understand all of the facets of battery life. In addition to the lack of time in the market, many of the large applications are using “large format” lithium-ion cells that range from 10 Ah in capacity up to several hundred Ah in capacity which have only been introduced to the market in the around 2009 or so. However, through advances in cell technology, engineering, and testing, a battery can be designed to last the life of the application. The design life for an automotive quality lithium- ion battery ranges from 10 to 15 years.
This differs from the warranty period which is gener- ally 6–8 years for most applications. Since lithium-ion batteries are being designed to last the life of the vehicle, currently about 11–12 years in the United States, the battery design must