Download cell balancing

the tech
A key feature in modern battery management system design
By Murat Ozkan - Senior Design Engineer for Nuvation, Charged technical contributor,
and professional Coulomb counter
B
attery technology is an everchanging, ever-improving field that
has gained a great deal of momentum in recent years. With improved
energy density, power density, and
cost comes a huge variety of new
and exciting applications in a wide
range of industries. The future looks
bright.
But the future won’t be enabled
solely by advances in chemistry and
manufacturing processes. To get the
full benefit out of modern battery
technology, individual cells must be
grouped together into packs, and the
packs must be monitored for safety,
maintained for performance, and
given intelligence to connect with the
outside world. All of these functions
fall under the responsibility of a Bat-
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tery Management System, or BMS.
Building a BMS that unlocks the
potential of modern battery technology is no small task, and there are
many challenges to overcome along
the way.
The challenge of implementing a
cell balancing solution, for example,
is multi-faceted and highly systemdependent.
Balancing act
The level of “balance” (or “imbalance”) of a pack refers to the difference in state of charge (SOC)
between cells in the pack. With any
battery pack made up of more than
one cell, the usable SOC of the whole
pack is only as high as the cell with
the least energy. A good BMS will
balance cells accurately and quickly,
improving the usable capacity of the
pack.
When designing a multi-cell battery pack, two questions quickly
arise when considering the BMS:
Does it need balancing? And if so,
what method should be employed to
ensure that the pack remains balanced? As with any good engineering
analysis, attempting to answer these
will raise complicated issues that
need to be carefully examined.
There are many factors that affect
the level of imbalance one can expect
in a given system. The causes of
imbalance can be divided into two
categories. For the purposes of this
discussion, we’ll call them “intrinsic”
and “dynamic” sources.
Photo by Andrew Hudgins, NREL 17070
Cell
Balancing
...individual cells must be grouped together into
packs, and the packs must be monitored for
safety, maintained for performance,
and given intelligence
to connect with
the outside
world.
Intrinsic imbalance
Intrinsic causes of imbalance are
inherent to a given battery pack and
will always behave the same way. For
example, if one cell within a pack of
100 cells has a 5 percent lower capacity than the next lowest capacity cell,
it will always discharge to empty first,
and always charge to full first. And
there is no BMS remedy. For these
kinds of battery packs, cell balancing is less likely to be a cost-effective
approach to overcoming intrinsic
causes of imbalance. Conceivably, a
Engineering Notes
Common sources of cell imbalance
Intrinsic sources
• Process variation affecting
internal impedance
• Process variation affecting
capacity
• Process variation affecting rate
of degradation
• Differing age of cells
Dynamic sources
• Differing rates of self-discharge
(uneven heating)
• Differing rates of energy efficiency (uneven heating)
• Differing leakage currents (BMS)
• Resistance mismatch during
charge/discharge (pack design)
balancing circuit could be designed
that matched the maximum charge
and discharge rates of a pack, nullifying the effects of intrinsic causes of
imbalance, but this would undoubtedly be too costly and too complex to
be practical. If these intrinsic characteristics are the dominant sources of
imbalance in a given pack, then there
will be little benefit to balancing. In
other words, the answer to the original question of whether balancing is
necessary is, probably not.
Intrinsic imbalance is best addressed through careful pack design
and manufacturing (i.e., selecting
well-matched cells, designing in
well-matched pack components such
as bus bars and cabling, and ensuring
even heating/cooling).
JAN/FEB 2013 25
the tech
Dynamic imbalance
Dynamic sources of imbalance create a random distribution of cells
that differ in SOC change between
charge/discharge cycles. This kind of
imbalance can be mitigated through
the use of a BMS, resulting in a pack
that reaches its maximum possible
capacity cycle after cycle, over its
usable life.
Topology
There are two main classes of balancing circuit design: passive and active.
Passive
Passive balancing is fairly straightforward: energy from cells with a high
SOC is dissipated as heat through a
resistive shunt or load.
Typically, this kind of balancing is
only done during charging since the
imbalance is removed by throwing
away the excess energy.
Active
Active balancing refers to the process
of moving energy from cells with
higher SOC to cells with lower SOC.
This is done either in bulk, by taking
energy from a group of cells and
transferring it to a single lower cell,
or by shuttling energy between adjacent cells until it gets to the desired
cell. Active balancing can be used
At first glance,
active balancing
seems more
attractive. Why
throw energy
away when it can
be redistributed
within the
battery?
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during discharge as well as charge
conditions.
At first glance, active balancing
seems more attractive. Why throw
energy away when it can be redistributed within the battery? Some
further analysis, however, shows that
this decision isn’t quite as straightforward. The key difference between
active and passive balancing topologies is complexity, and by extension,
cost, size, and robustness.
Complexity
Component count is one measure
of complexity. Passive balancing
requires one switch (typically a
MOSFET) and one power resistor
for each cell to balance. By comparison, active topologies require one to
two switches per cell to manage the
energy in and out, and at least one
major energy storage element (with
the exception of the single capacitor
topology). Some of the more complex topologies also require lowlatency feedback control to manage
the energy transfer in a controlled
way (e.g., power converter type
topologies), which often requires an
additional processor or control IC.
Performance
Another important metric for
evaluating balancing topologies is
performance. The most significant
performance indicator is the time it
takes to balance a pack. With passive
balancing, the balance rate is a function of the resistance of the shunt
and the duty cycle with which the
shunt is switched across a cell. There
is often a balance-rate-versus-powerdissipation trade-off with passive
balancing since the energy from
high-SOC cells is removed entirely
as heat.
Some active balancing topologies,
such as the switched capacitor and
switched inductor, have decaying
charge/discharge profiles, resulting in
a more limited balance time. This, of
course, depends on the size of the capacitors/inductors selected, and how
much of the energy storage capacity
is used during the balancing procedure. These are typically the slowest
topologies. The fastest of the active
Engineering Notes
Common cell balancing methods
Passive balancing
Fixed shunt resistor
Variable shunt resistor/PWM
Active balancing
Switched
Capacitor
Inductive
Power
Converter
Single capacitor
Capacitor per cell
Switched inductor
Single wound transformer/coupled inductor
Multiple tap transformer
Buck
Buck/Boost
Flyback
Cuk
the tech
balancing topologies tends to be the
power converter-based designs, however, they also tend to be the most
expensive solutions.
Cost
There are many ways to evaluate
cost, because different systems will
be sensitive to different cost drivers.
Some useful cost metrics for battery
systems are:
• BMS cost per cell - Compare the
cost of each cell to the cost per
monitoring channel.
• BMS cost versus system cost
- What does the BMS cost in
relation to all of the system
components combined (cells,
power switches, cabling, housing,
thermal management, etc.)?
• BMS cost over the lifespan of the
system - A well-designed BMS
will last the lifetime of a battery
system. Cells within a system
may need to be replaced individually or system-wide as they age.
Extend the cost-per-cell metric to
all cells the system is expected to
consume over its lifetime.
• BMS cost as a function of cell
cost offset - A high-performance
BMS may enable the use of
cheaper cells (e.g., high-power
active charge may allow for lower
tolerances for matching cells
across a pack). Consider the cost
savings in using cheaper cells
against the increased cost of a
better-performing BMS.
Which topology is best? That,
of course, depends on the application. There are cases in which the
high costs of the better-performing
active topologies are negated, as in
systems that employ specialized cells
with low cycle life. Fast-charge ap-
plications may also benefit from the
costlier active topologies, since cell
mismatches are exaggerated at high
rates of charge. In applications where
the pack design is thermally limited,
it may be impossible to use passive
balancing. Typically there is not a
single reason for choosing a balancing topology - rather, it is a combination of multiple design constraints
that dictates the approach taken.
The tools and techniques available
to BMS engineers are changing as
rapidly and dynamically as battery
technology itself. Understanding the
processes and trade-offs involved in
each of the BMS design features is
vital to selecting the right solution
for a given system.
Contributions by Eliot Barker
and Bernard Smit
For a more in-depth analysis of the
performance and trade-offs for each BMS
topology, consult our sources:
[1] P. Ramadass, B. Haran, R. White and
B. N. Popov, “Capacity fade of Sony
18650 cells cycled at elevated temperatures,” Journal of Power Sources,
no. 112, pp. 606-613, 2002.
[2] R. P. Ramasamy, R. E. White and B. N.
Popov, “Calendar life performance of
pouch lithium-ion cells,” Journal of
Power Sources, no. 141, pp. 298-306,
2004.
[3] M. Daowd, N. Omar, P. Van Den Bossche and J. Van Mierlo, “A Review of
Passive and Active Battery Balancing
based on MATLAB/Simulink,” International Review of Electrical Engineering
(I.R.E.E.), 2011.
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