Impact of dc-dc converter distribution and redundancy on reliability of battery-integrated-converter systems

Battery-integrated-converter systems were first proposed as a solution for charge balancing in series connected battery systems. Modularizing the battery cells/blocks with individual converters also provides the opportunity for redundancy and thus increased reliability. By adding additional series modules, modules with failed battery cells/blocks can be bypassed by their associated converter while maintaining system operation. In this paper, we assess the effect of converter distribution on system reliability. The results show that for low battery cell and converter failure rates, by proper redundancy choice, it is possible to design a highly reliable system. However, there is a compromise between the module failure rate (based on the cell and the converter failure rates); and redundancy level. For the modules with higher failure rates, the system reliability can be improved by applying annual scheduled replacement of failed modules. Using this approach, the six-sigma criterion can be achieved for a system of single/two strings of 30 V rated modules and the system of two parallel strings of 40 V rated modules. Further reliability-cost assessment is suggested to choose the optimum design for a specific application based on the influential factors such as: module voltage rating, module reliability, redundancy level, and scheduled-maintenance intervals.