April 2010 - Maximize Cycle Life of Rechargeable Battery Packs with Multicell Monitor IC

Maximize Cycle Life of Rechargeable Battery Packs
with Multicell Monitor IC
Jon Munson
Rechargeable battery packs prematurely deteriorate in performance if any cells are
allowed to overdischarge. As a pack becomes fully discharged, the ILOAD • RINTERNAL
voltage drop of the weakest cell(s) can overtake the internal VCELL chemical potential
and the cell terminal voltage becomes negative with respect to the normal voltage. In
such a condition, irreversible chemical processes begin altering the internal material
characteristics that originally provided the charge storage capability of the cell,
so subsequent charge cycles of the cell do not retain the original energy content.
Furthermore, once a cell is impaired, it is more likely to suffer reversals in subsequent
usage, exacerbating the problem and rapidly shortening the useful cycle life of the pack.
With nickel-based chemistries, an overdischarge of a set of series-connected
cells does not necessarily lead to a safety
hazard, but it is not uncommon for one or
more cells to suffer a reversal well before
the user is aware of any significant degradation in performance. By then, it is too
late to rehabilitate the pack. In the case
of the more energetic lithium-based cell
chemistries, reversals must be prevented
as a safety measure against overheating or
fire. Monitoring the individual cell voltages is therefore essential to ensure a long
pack life (and safety with lithium cells).
Figure 1. Simple load-disconnect circuit to prevent excessive discharge of a nickel-cell pack.
FDS6675
1k
1.25V
100nF
1k
1.25V
100nF
1k
1.25V
100nF
1k
1.25V
100nF
1k
1.25V
100nF
1k
1.25V
100nF
1k
1.25V
100nF
1k
1.25V
100nF
100nF
V+
C12
C11
C10
C9
C8
C7
C6
C5
C4
C3
C2
C1
V–
7.5k
OV1
OV0
UV1
UV0
HYST
CC1
CC0
SLTB
SLTOK
DC
EOUT
EOUT
SIN
LTST-C190KGKT
2N7002K
CMHD457
1M
SIN
VTEMP1
VTEMP2
100nF
10k
SOUT
SOUT
VREF
EIN
VREG
EIN
LTC6906
V+
OUT
GND GND
1µF
30 | April 2010 : LT Journal of Analog Innovation
1µF
SPST
ENABLE
DIV
100nF
FEATURES OF THE LTC6801
The operating modes and programmable threshold levels are set by pinstrap connections. Nine UV settings (from
0.77V to 2.88V) and nine OV settings
(from 3.7V to 4.5V) are available. The
number of monitored cells can be set
from 4 to 12 and the sampling rate can
be set to one of three different speeds
to optimize the power consumption
versus detection time. Three different
hysteresis settings are also available to
tailor behavior of the alarm recovery.
100k
LTC6801
9.6V
Enter the LTC6801, developed to provide integrated solutions for these
specific problems. The LTC6801 can
detect individual cell overvoltage (OV)
and undervoltage (UV) conditions of up
to twelve series connected cells, with
cascadable interconnections to handle
extended chains of devices, all independent of any microprocessor support.
SET
1M
To support extended configurations of
series-connected cells, fault signaling
is transmitted by passing galvanically
isolated differential clock signals in both
directions in a chain of “stacked” devices,
providing excellent immunity to load
design ideas
Cell reversal is a primary damage mechanism in traditional
nickel-based multicell packs and can occur well before
other noticeable charge-exhaustion symptoms appear.
noise impressed on the battery pack. Any
device in the chain detecting a fault stops
its output clock signal, thus any fault
indication in the entire chain propagates
to the “bottom” device in the stack. The
clock signal originates at the bottom of
the stack by a dedicated IC, such as the
LTC6906, or a host microprocessor if one
is involved, and loops completely through
the chain when conditions are normal.
In many applications, the LTC6801 is used
as a redundant monitor to a more sophisticated acquisition system such as the
LTC6802 (for example, in hybrid automobiles), but it is also ideal as a standalone
solution for lower-cost products like
portable tools and backup power sources.
Since the LTC6801 takes its operating
power directly from the batteries that it
monitors, the range of usable cells per
device varies by chemistry in order to provide the needed voltage to run the part—
from about 10V up to over 50V. This range
supports groupings of 4–12 Li-ion cells
or 8–12 nickel-based cells. Figure 1 shows
how simply an 8-cell nickel pack can be
monitored and protected from the abuse
of overdischarge. Note that only an undervoltage alarm is relevant with the nickel
chemistries, though a pack continuity fault
would still be detected during charging
by the presence of an OV condition.
AVOIDING CELL REVERSALS
Cell reversal is a primary damage
mechanism in traditional nickelbased multicell packs and can actually occur well before other noticeable
charge-exhaustion symptoms set in.
Figure 2. Pack discharge conditions
that promote cell reversals may not
be apparent from load potential.
IDEAL CELLS
5V AT LOAD
3 WEAK CELLS
5V AT LOAD
1 WEAK CELL
8V AT LOAD
0.63V
–0.1V
1.16V
0.62V
1.0V
1.15V
0.63V
1.1
1.16V
0.62V
–0.1V
LOAD
LOAD
1.15V
0.63V
1.1V
1.16V
0.62V
–0.1V
1.16V
0.63V
1.0V
–0.1V
0.62V
1.1V
1.16V
Consider the following scenario. An 8-cell
nickel-cadmium (NiCd) pack is powering
a hand tool such as a drill. The typical
user runs the drill until it slows to perhaps
50% of its original speed, which means
that the nominal 9.6V pack is loading
down to about 5V. Assuming the cells are
perfectly matched as in the left diagram
of Figure 2, this means that each cell has
run down to about 0.6V, which is acceptable for the cells. However, if there is a
mismatch in the cells such that perhaps
five of the cells are still above 1.0V, then
the other three would be below zero
volts and suffer a reverse stress as shown
in the middle diagram of Figure 2.
Even assuming that there is only one weak
cell in the pack (a realistic scenario) as in
the right diagram in Figure 2, the first cell
reversal might well occur while the stack
voltage is still 8V or more, with just a subtle reduction in perceived pack strength.
Because of the inevitable mismatching
that exists in practice, users unknowingly
reverse cells on a regular basis, reducing
LOAD
the capacity and longevity of their battery
packs, so a circuit that makes an early
detection of individual cell exhaustion
offers significant added value to the user.
USING THE LTC6801 SOLUTION
The lowest available UV setting of the
LTC6801 (0.77V) is ideal for detecting
depletion of a nickel-cell pack. Figure 1
shows a MOSFET switch used as a load
disconnect, controlled by the output state
of the LTC6801. Whenever a cell becomes
exhausted and its potential falls below
the threshold, the load is removed so that
cell reversal and its degradation effects
are avoided. It also allows the maximum
safe extraction of energy from the pack
since there are no assumptions made as to
the relative matching of the cells as might
be the case with an overly conservative
single pack-potential threshold function.
A 10kHz clock is generated by the LTC6906
silicon oscillator and the LTC6801 output status signal is detected and used to
control the load disconnect action. Since
April 2010 : LT Journal of Analog Innovation | 31
The LTC6801 simultaneously monitors up to 12 individual
cells in a multicell battery pack, making it possible to
maximize the pack’s capacity and longevity. It can
also be cascaded to support larger battery stacks.
9.6V
Figure 3. Alternative circuit provides an audible
warning of the need to recharge the pack without
interrupting service to the load.
100k
LTC6801
1k
1.25V
100nF
1k
1.25V
100nF
1k
1.25V
100nF
1k
1.25V
100nF
1k
1.25V
100nF
1k
1.25V
100nF
1k
1.25V
100nF
1k
1.25V
100nF
V+
C12
C11
C10
C9
C8
C7
C6
C5
C4
C3
C2
C1
V–
OV1
OV0
UV1
UV0
HYST
CC1
CC0
SLTB
SLTOK
DC
EOUT
EOUT
SIN
SPST
ENABLE
LED1
2N7002K
47k
2N7002K
CMHD457
1M
VREG
100nF
10k
SIN
VTEMP1
VTEMP2
VREF
7.5k
PKB24SPCH3601-B0
SOUT
SOUT
EIN
EIN
LTC6906
V+
100nF
OUT
GND GND
1µF
1µF
100nF
PDZ5.1B
DIV
SET
1M
LED1: LTST-C190KGKT
this example does not involve stacking of
devices, the cascadable clock signals are
simply looped-back rather than passed to
another LTC6801. An LED provides a visual
indication that power is available to the
load. Once the switch opens, the voltage
of the weak cell tends to recover somewhat and the LTC6801 reactivates the load
switch (no hysteresis with 0.77V undervoltage setting). The cycling rate of this
digital load-limiting action depends on
the configuration of the DC pin; in the
fastest response mode (DC = VREG), the
duty cycle of the delivered load power
drops and tapers off, with pulsing becoming noticeable and slower as the weakest
cell safely reaches a complete discharge.
32 | April 2010 : LT Journal of Analog Innovation
In some applications it is not acceptable
to spontaneously interrupt the load when
the weakest cell is nearing full discharge
as depicted in Figure 1. For those situations, the circuit of Figure 3 might be a
good alternative. This circuit does not
force a load intervention, but simply
provides an audible alarm indication that
the battery is near depletion. Here the
LED provides an indication that the alarm
is active and that no cells are exhausted.
An LTC6801 idle mode is invoked whenever the source clock is absent, and
power consumption then drops to a
miniscule 30µA, far less than the typical self-discharge of the pack. In both
figures, the circuits show a switch
that disables the oscillator (and other
peripheral circuitry) in order to place the
circuit into idle mode when not being
used so that battery drain is minimized.
CONCLUSION
The LTC6801 simultaneously monitors up
to 12 individual cells in a multicell battery
pack, making it possible to maximize
the pack’s capacity and longevity. It can
also be cascaded to support larger battery stacks. The device has a high level
of integration, configurability and well
thought out features, including an idle
mode to minimize drain on the pack
during periods of inactivity. This makes
the LTC6801 a compact solution for
improving the performance and reliability of battery powered products. n
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