Download This memo covers the design choices involved in choosing a cell

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.