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OBPP BYPASS DESIGN SOLUTIONS LPRDS-CMS-2011 TECHNICAL MEMO SUBJECT: OBPP BYPASS DESIGN SOLUTIONS AUTHOR: WILL SCHLANSKER DATE: 2/17/2011 ABSTRACT This memo covers the design choices involved in choosing a cell bypass method and the reasoning behind choosing a passive balancing solution. TECHNICAL FINDINGS There are two major categories of cell balancing: active and passive. Active balancing utilizes some method of shuttling charge from a higher charge cell to lower charge cells. The most common method uses inductive shuttling, using an inductor as the intermediate stage between cells, but there are also approaches using capacitors or other batteries. This method has the advantage of retaining much more power within the system, as opposed to converting it to heat in a dissipation path. Conversely, it has scalability issues, as it necessitates connections to and from each cell to each other cell through the charge shuttle. Once the number of cells grows beyond a certain point, the sheer number of interconnects are a significant design obstacle. Also, in a high power scenario, adding an inductive load to the system brings up several safety issues. The other cell balancing method is passive balancing. This technique utilizes resistive bypass paths to divert current away from cells that are fully charged. Any excess power is burned off through a power resistor, allowing the other cells in a series stack to continue charging while preventing overcharging in those that are at capacity. Obviously, the first issue with passive balancing is the largely wasteful bypass path, where power is not only wasted, but can easily generate a large amount of heat. That said, it is much easier to design and create a balancing algorithm for this method. It can also scale much more easily as more cells are added to the system. RECOMMENDATIONS AND DECISIONS Currently, passive cell balancing is the most reasonable design approach. Based on the limited timeframe for design and prototyping, active balancing has too many unknowns with concern to actual operation and even theoretical simulations. Passive balancing, while less efficient, is much easier and safer to implement without exhaustive simulation and testing.