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Active Battery Cell Balancing
Kevin Scott and Sam Nork
Analog Devices, Inc.
With passive and active cell balancing, each cell in the battery stack
is monitored to maintain a healthy battery state of charge (SoC). This
extends battery cycle life and provides an added layer of protection by
preventing damage to a battery cell due to deep discharging because of
overcharging. Passive balancing results in all battery cells having a similar
SoC by simply dissipating excess charge in a bleed resistor; it does not,
however, extend system run time.1 Active cell balancing is a more complex balancing technique that redistributes charge between battery cells
during the charge and discharge cycles, thereby increasing system run
time by increasing the total useable charge in the battery stack, decreasing charge time compared with passive balancing, and decreasing heat
generated while balancing.
Active Cell Balancing During Discharge
The diagram in Figure 1 represents a typical battery stack with all cells
starting at full capacity. In this example, full capacity is shown as 90%
of charge because keeping a battery at or near its 100% capacity point
for long periods of time degrades its lifetime faster. The 30% discharge
represents being fully discharged to prevent deep discharge of the cells.
Charged
90%
Lower Capacity Cells
Discharge Faster
Charged
90%
Discharged
30%
Unused Capacity
Figure 2. Mismatched discharge.
It can be seen that even though there may be quite a bit of capacity left
in several batteries, the weak batteries limit the run time of the system. A
battery mismatch of 5% results in 5% of the capacity being unused. With
large batteries, this can be an excessive amount of energy left unused.
This becomes critical in remote systems and systems that are difficult to
access. As a result, there is a portion of energy that cannot be used, which
results in an increase in the number of battery charge and discharge cycles.
Furthermore, this unused energy reduces the lifetime of the battery and
leads to higher costs associated with more frequent battery replacement.
With active balancing, charge is redistributed from the stronger cells to
the weaker cells, resulting in a fully depleted battery stack profile.
Discharged
30%
Charged
90%
Figure 1. Full capacity.
Over time, some cells will become weaker than others, resulting in a
discharge profile, as represented by Figure 2.
Discharged
30%
Figure 3. Full depletion with active balancing.
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2
Active Battery Cell Balancing
Active Cell Balancing While Charging
When charging the battery stack without balancing, the weak cells reach
full capacity prior to the stronger batteries. Again, it is the weak cells that
are the limiting factor; in this case they limit how much total charge our system can hold. The diagram in Figure 4 illustrates charging with this limitation.
With active balancing charge redistribution during the charging cycle, the
stack can reach its full capacity. Note that factors such as the percentage of
time allotted for balancing and the effect of the selected balancing current on
the balancing time are not discussed here, but are important considerations.
Analog Devices Active Cell Balancers
Analog Devices has a family of active cell balancers, with each device targeting different system requirements. The LT8584 is a 2.5 A discharge current,
monolithic flyback converter used in conjunction with the LTC680x family of
multichemistry battery cell monitors; charge can be redistributed from one
cell to the top of the battery stack or to another battery cell or combination of
cells within the stack. One LT8584 is used per battery cell.
Lower Capacity Cells
Charge Faster
Charged
90%
The LTC3300 is a standalone bidirectional flyback controller for lithium
and LiFePO4 batteries that provides up to 10 A of balancing current. Since
it is bidirectional, charge from any selected cell can be transferred at high
efficiency to or from 12 or more adjacent cells. A single LTC3300 can balance
up to six cells.
Discharged
30%
Figure 4. Charging without balancing.
Module+
2.5 A Average
Discharge
+
Bat 12
•
•
Module+
Module–
V+
Read Cell Parameters
C12
S12
LT8584
Measurable Cell
Parameters
Enable Balancing
2.5 A Average
Discharge
+
Bat 2
•
•
Module+
Module–
LTC6804
Battery
Stack
Monitor
Read Cell Parameters
C2
S2
LT8584
2.5 A Average
Discharge
+
Bat 1
•
Module–
Read Cell Parameters
LT8584
Enable Balancing
Module–
Figure 5. 12 cell battery stack module with active balancing.
► Temperature
plus RCONNECTOR
Faults
► Switching
► Undervoltage
► Serial
Faults
Counting
► Coulomb
Module+
•
► VREF
► RCABLE
Extractable Cell
Parameters
Enable Balancing
► VCELL
► IDISCHARGE
C1
S1
V+/C0
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Next Cell Above
Charge Supply
(ICHARGE 1 to 6)
+
Charge Return
(IDISCHARGE 1 to 6)
+
•
IDISCHARGE +
Charge
Return
Cell 12
Serial
Data Out
3
to
LTC3300-1
3 Above
LTC3300-1
Cell 7
Cell 6
•
Charge
Return
3
LTC3300-1
•
+
ICHARGE
Conclusion
Both active and passive cell balancing are effective ways to improve system
health by monitoring and matching the SoC of each cell. Active cell balancing
redistributes charge during the charging and discharging cycle, unlike passive
cell balancing, which simply dissipates charge during the charge cycle. Thus,
active cell balancing increases system run time and can increase charging
efficiency. Active balancing requires a more complex, larger footprint solution;
passive balancing is more cost effective. Whichever method works best in
your application, Analog Devices offers solutions for both, which are integrated
into our battery management ICs (such as the LTC6803 and LTC6804) and
complementary devices that work in conjunction with these ICs to provide a
precise, robust battery management system.
References
Cell 1
•
3
Serial
Data In
from
LTC3300-1
Below
1
K evin Scott and Sam Nork. “Passive Battery Cell Balancing.”
Analog Devices, Inc.
Next Cell Below
Figure 6. High efficiency bidirectional balancing.
The LTC3305 is a standalone, lead acid battery balancer for up to four cells.
It uses a fifth reservoir battery cell (Aux) and continuously places it in parallel
with each of the other batteries (one at a time) to balance all battery cells
(lead acid batteries are rugged and can handle this).
10 µF
25 V
VREG CM
En1
En2
Mode
Term 1
Term 2
1 µF
6V
100 kΩ
Each
10 nF
100 nF
10 nF
CP
Boost
10 µF
25 V
Ngate 1 to 9
3.01 kΩ
10 µF
25 V
6.04 kΩ
10 µF
25 V
9
12.1 kΩ
Aux P
VH
Aux N
Ngate 1
Ngate 6
6.04 kΩ
Ngate 7
6.04 kΩ
PTC
27.4 kΩ
Ngate 4
Ngate 2
V1
Ct Bat
Ngate 5
Ngate 3
10 µF
25 V
V2
LTC3305
Ct Off
ISET
6.04 kΩ
V3
VL
42.2 kΩ
6.04 kΩ
10 µF
25 V
V4
UVFLT
OVFLT
DONE
BAL
PTCFLT
BATX
BATY
Ct on
Charger
Supply
1.33 kΩ
249 Ω
10 µF
25 V
+
Aux
6.04 kΩ
Ngate 8
Gnd
6.04 kΩ
Ngate 9
Figure 7. Four battery balancer with programmed high and low battery voltage fronts.
6.04 kΩ
ICHARGE
+
+
+
+
Bat 4
Bat 3
Bat 2
Bat 1
Battery
Stack
Charger
3
About the Authors
Kevin Scott works as a product marketing manager for the Power
Products Group
at Analog Devices, where he manages boost, buckAbout
the Author
boost and isolated converters, LED drivers, and linear regulators. He
Thomas
began
career
at Analog
Devices,
Inc., in creating
Munich
previouslyBrand
worked
as ahis
senior
strategic
marketing
engineer,
in
October
2015
as
part
of
his
master’s
thesis.
From
May
2016
to
technical training content, training sales engineers, and writing
January
2017,
he was
part of
a trainee
programadvantages
for field applicanumerous
website
articles
about
the technical
of the
tion
engineers
at Analog
in February
2017, he
company’s
broad
productDevices.
offering.Afterward,
He has been
in the semiconducmoved
into
the
role
as
field
applications
engineer.
Within
this role,
tor industry for 26 years in applications, business management,
and
he
is
mainly
responsible
for
large
industrial
customers.
Furthermore,
marketing roles.
he specializes in the subject area of industrial Ethernet and supportsKevin graduated
from
Stanford
University in 1987 with a B.S. in electrirelated
matters in
Central
Europe.
cal engineering and started his engineering career after a brief stint in
He
engineering
at the University of Cooperative
the studied
NFL. Heelectrical
can be reached
at kevin.scott@analog.com.
Education in Mosbach before completing his postgraduate studies
(now
Sam
Nork joinedSales
Linearwith
Technology
part of
Devices) as a
in
International
a master’s
degree
at Analog
the University
senior
product
engineer
the company’s
Milpitas,
CA headquarters
in
of
Applied
Sciences
in at
Constance.
He can
be reached
at
1988. In 1994, he relocated to the Boston area to start up and manage
thomas.brand@analog.com.
an analog IC design center where he continues to work today. Sam has
personally designed and released numerous integrated circuits in the
area of portable power management, and is inventor/co-inventor on
seven issued patents. As director of ADI’s Boston Design Center,
Sam leads a team of nearly 100 people and oversees day-to-day
development activity for a wide variety of analog integrated circuits in
areas including portable power management, high speed op amps,
industrial ADCs, system monitors, and energy harvesting. Previously,
Sam also worked for Analog Devices in Wilmington, MA as a product/
test development engineer. He received A.B. and B.E. degrees from
Dartmouth College. He can be reached at samuel.nork@analog.com.
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