LTC3300-2
13
33002f
For more information www.linear.com/LTC3300-2
OPERATION
Battery Management System (BMS)
The LTC3300-2 multicell battery cell balancer is a key
component in a high performance battery management
system (BMS) for series-connected Li-Ion cells. It is de
-
signed to operate in conjunction with a monitor, a charger,
and a microprocessor or microcontroller (see Figure 1).
The
function of the balancer is to efficiently transfer charge
to/from a given out-of-balance cell in the stack from/to
a larger group of neighboring cells (which includes that
individual cell) in order to bring that cell into voltage or
capacity balance with its neighboring cells. Ideally, this
charge would always be transferred directly from/to the
entire stack, but this is impractical for voltage reasons
when the number of cells in the overall stack is large. The
LTC3300-2 is designed to interface to a group of up to 6
series cells, so the number of LTC3300-2 ICs required to
balance a series stack of N cells is N/6 rounded up to the
nearest integer. Since the LTC3300-2 address is 5 bits,
the maximum N can be is 192 cells. For connecting an
individual LTC3300-2 in the stack to fewer than 6 cells,
refer to the Applications Information section.
Because
the balancing
function entails switching large
(multiampere) currents between cells, precision voltage
monitoring in the BMS is better served by a dedicated
monitor component such as the LTC6803-2 or one of its
family of parts. The LTC6803-2 provides for high precision
A/D monitoring of up to 12 series cells. The only voltage
monitoring provided by the LTC3300-2 is a coarse “out-
of-range” overvoltage and undervoltage cell balancing
disqualification, which provides a safety shutoff in the
event Kelvin sensing to the monitor component is lost.
In the process of bringing the cells into balance, the over
-
all stack is slightly discharged. The charger component
provides
a means for net charging of the entire stack from
an alternate power source.
The last component in the BMS is a microprocessor/
microcontroller which communicates directly with the
balancer, monitor, and charger to receive voltage, cur
-
rent, and temperature information and to implement a
balancing algorithm.
There is no single balancing algorithm optimal for all
situations. For example, during net charging of the overall
stack, it may be desirable to discharge the highest voltage
cells first to avoid reaching terminal charge on any cell
before the entire stack is fully charged. Similarly,
during
net discharging of the overall stack, it may be desirable
to charge the lowest voltage cells first to keep them from
reaching a critically low level. Other algorithms may
prioritize fastest time to overall balance. The LTC3300-2
implements no algorithm for balancing the stack. Instead it
provides maximum flexibility by imposing no limitation on
the algorithm implemented as all individual cell balancers
can operate simultaneously and bidirectionally.
Unidirectional Versus Bidirectional Balancing
Most balancers in use today employ a unidirectional (dis
-
charge only
) approach. The simplest of these operate by
switching
in a resistor across the highest voltage cell(s)
in the stack (passive balancing). No charge is recovered
in this approach -instead it is dissipated as heat in the
resistive element. This can be improved by employing an
energy storage element (inductive or capacitive) to transfer
