Lithium Coin-Cell Batteries: Predicting an Application Lifetime
Abstract: This application note discusses various contributors to battery capacity consumption in a battery-backed application, and how to predict battery lifetime in a system. A designer should be able to use this application note to determine the battery requirements for his system.
Maxim builds a large number of products
that incorporate lithium coin-cell batteries to provide
nonvolatile (NV) memory or real-time clock (RTC) functionality
in the absence of system power. The typical
specification for these products has been to provide a
10-year battery lifetime in the absence of system power.
Because of end-application uncertainties, the lifetime
prediction is conservative.
End users should evaluate the anticipated lifetime in
their specific application, especially for applications
that exceed typical commercial environments or that
need to reach lifetimes beyond 10 years. An understanding
of the reliability model is also beneficial for
users opting to purchase discrete battery controllers and
combine them with a battery, rather than purchasing the
module product containing the controller and battery.
This article gives the reader an overview of the major
factors affecting the lifetime of an integrated circuit (IC)
that can be powered by either the system power or a
lithium battery for a backup supply.
Why Battery Backup?
There are several alternatives for data retention while the
system is powered-off. Battery-backed SRAMs are a
reliable alternative when the read-write speed or number
of cycles is important. Flash or EEPROM also provide
NV data storage, but at the cost of simplicity or speed.
The major disadvantage of battery-backed SRAM is that
the battery is a consumable. Therefore, the product
selection must consider the available charge within the
battery to determine the end product's lifetime. For
devices that need to maintain time in the absence of
system power, some form of electrical energy needs to
be available to maintain a crystal oscillator. This current
demand is well suited to being maintained by a battery.
IC Current Demands
If an IC (SRAM or RTC) is going to be battery powered,
there needs to be a match between the current demands
of the IC, the expected lifetime, and the energy available
in the battery. If the IC and battery are being purchased,
the data sheet specifications will provide the information
required to predict the lifetime of the battery as a
function of the IC load. If the IC and battery are being
purchased as a module, end users can rely on the module
manufacturer to have the appropriate screens in place to
ensure that the system lifetime meets the specification.
Figure 1. Lifetime based on amount of current being pulled
from the battery.
Maxim has established screening limits on
all of its battery-backed products that allow the available
battery capacity to power the end part for specified
lifetimes up to 10 years. In the case of Maxim
ICs, the design and fab processes have been
optimized to produce low-current demands. In the case of
higher density SRAMs purchased from outside vendors,
special screening is sometimes required to ensure that the
module lifetime specifications are met. Figure 1 is
produced from battery capacities reported by Panasonic.
The four lines shown in Figure 1 represent four of the most
common battery sizes (BR1225, BR1632, BR2330, and
BR3032). The battery manufacturers' rated electrical
capacity (in mAh) is shown with each battery size.
Battery Construction/Attributes
Maxim has chosen to use primary lithium
coin-cell batteries in modules that require battery
backup. These cells, which have a rated voltage of 3V
and a typical in-system voltage around 2.7V, make them
well suited as a backup supply. The voltage also remains
stable during the battery discharge (Figure 2), so the
voltage at the end of life is nearly the same as with a
fresh battery. While a flat discharge curve is desirable
for backup supply voltage, it does make predicting the
remaining electrical capacity difficult.
Figure 2. The output voltage remains constant during discharge.
Primary lithium coin-cell batteries have a very predictable
behavior. Distributions of such key parameters as
open-circuit voltage or internal impedance are very
tightly grouped. These tight distributions allow the
battery manufacturers to set aggressive test limits in
their process to ensure that abnormal cells are excluded
from the population. These tight distributions also allow
the user of the batteries to identify IC/battery systems
that contain a defect. For example, since the voltage
distribution and the voltage vs. battery load is very
predictable, the battery voltage after a load has been
attached can be an indicator of the load placed on the
battery. If the battery load is the current demand of a
well-behaved distribution of ICs, the resulting loaded
voltages will also be tightly distributed. Any loaded
voltage that is seen outside of the normal distribution is
then an indication of an abnormal IC or battery. This
result can be used to reject the resulting module as a
potential reliability risk.
Battery Testing/Screening
The battery manufacturer's 100% testing creates an
extremely consistent product. However, anyone using
batteries as an integral part of his system should employ
testing methods to ensure that only properly functioning
cells are included in the end product. There are three
types of defects that can be detected by a properly
defined screen. First are the test escapes from the battery
manufacturers' system. These are the easiest to detect.
The second form of defect is low-level internal leakage.
It is possible for a battery to have an internal defect that
would manifest itself only after some period of time.
Detecting these cells requires a thorough understanding
not only of the proper testing levels but also the anticipated
distribution of results. The third type of defect is a
handling or manufacturing defect by the battery user.
Because of the limited amount of electrical capacity
available, inadvertent loads placed on the cell for even
short periods of time can result in reduced electrical
lifetimes.
A thorough screening program will involve 100% tests
for electrical characteristics at key steps in the manufacturing
process. Because of the predictable nature of the
electrical performance, measuring the battery voltage
before and after load-attach will identify cells that are
abnormal. Such screening also will identify loads that
are not typical. In addition to the electrical screening, a
visual sampling of the batteries will help identify manufacturing
variations that could result in degraded leak
resistance.
Battery Reliability Model
The battery is a "balanced construction" with the
reactive components included in quantities that should
result in full reaction. The key components to the electrical
reaction are the metallic lithium, cathode, and electrolyte.
The battery manufacturers' goal is to maximize
the available energy placed in the cell. Since the internal
volume of the battery is limited, the maximum energy
density is achieved when the components are in exactly
the correct ratios. Therefore, any component loss limits
the available reaction of the other components. The reliability
model for batteries takes the balanced construction
into account and seeks to determine what will cause
any key component depletion.
Because the battery is a consumable in the system, the
most obvious limitation of the lifetime will be an electrical
load placed on the battery. The lifetime based on
an electrical load is easy to calculate. Simply divide the
available battery capacity in milliamp hours by the
current demand in milliamps to get the lifetime in hours.
Determining the battery's lifetime as a function of electrical
load also requires consideration of the power-on
duty cycle. In a properly designed system, the battery is
electrically isolated while system power is applied. This
eliminates any battery current draining or charging. The
reduced duty cycle will effectively extend the lifetime of
the battery in systems that are powered up a high
percentage of the time and are relying on battery backup
for only a short time.
Because these batteries are being used in very low or
zero-current applications, users also need to look for
other possible mechanisms that will deplete any of the
reactive components. One such mechanism is electrolyte
loss through the crimp seal. This mechanism has been
shown to be temperature-accelerated with an activation
energy of approximately 1.0eV. At room temperature
the batteries will exhibit an electrical loss rate of less
than 0.5% per year, and this mechanism can safely be
ignored. However, at elevated temperatures the loss rate
of the electrolyte can become significant and must be
considered.
Because of the reactive components' balanced nature, it
does not matter whether the electrical reaction consumes
the electrolyte or it is expelled through the seal because
of elevated temperatures. When the battery does not
have enough electrolyte to continue the reaction, the
battery will no longer provide current. Therefore, we
recommend using a parallel model for lifetime predictions
that considers the electrical demand and temperature
when predicting the system lifetime (Figure 3).
There are models that treat the electrical and temperature
depletion legs as independent and predict a lifetime as if
there were no interaction between the two components
of electrolyte loss. Using such a model will overstate the
true lifetime if the system is exposed to temperatures
much higher than room temperature.
Calculating the lifetime of the battery is similar in
concept to calculating the effective resistance of two
parallel resistors. The user has control over whether the
IC is consuming power from the battery or the system's
power supply, so the current consumption leg is shown
to include a switch. While the IC is being powered from
the system power supply, the lifetime due to current
consumption can be approximated as infinite.
The manufacturer of the IC/battery system has control
over the selected components and manufacturing
process. Properly selected components and manufacturing
screens should result in adequate system-level
lifetime. However, end users have control over the
ultimate lifetime performance based on the actual use of
the system. End users can control both legs of the model.
The electrical load leg is controlled through the power-on
duty cycle of the equipment. While system power is
applied, the Maxim components include
battery isolation circuitry that electrically isolates the
battery and eliminates all current drain from the battery.
Therefore, the electrical load leg of the reliability model
is only active while the system is in battery backup. The
system ambient temperature controls the temperatureaccelerated
leg. Providing adequate cooling and proper
component placement can help reduce the temperature
exposure of the battery and, thereby, extend the system's
lifetime.
Figure 3. Battery lifetime based on electrolyte evaporation and
electrical consumption.
Figure 4. The self-discharge rate increases as temperature increases.
Sample Lifetime Calculations
Case I—The system is designed to be in battery backup
100% of the time at room temperature. The electrolyte
evaporation at room temperature is so low that it can
virtually be ignored. The lifetime is limited by the IC's
current drain.
Electrical Consumption Leg
Battery capacity (BR1632) = 120mAh
IC current drain = 1.2µA
Duty cycle = 100%
Battery lifetime = (0.12Ah) / (1.2 × 10-6A) =
100,000 hours = 11.4 years
Electrolyte Evaporation Leg
Battery lifetime at +25°C = 230 years
Calculation : (230 × 11.4) / (230 + 11.4) = 10.9 years
Case II—The system is designed to be in battery backup
50% of the time at +60°C. The lifetime due to either the
electrical consumption or electrolyte evaporation would
appear to be approximately 20 years. The combination
of the two mechanisms will cause the electrolyte to be
consumed in 10 years.
Electrical Consumption Leg
Battery capacity (BR1632) = 120mAh
IC current drain = 1.2µA
Battery lifetime = (0.12Ah) / (1.2 x 10-6A x 50%) =
200,000 hours = 22.8 years
Electrolyte Evaporation Leg
Battery lifetime at +60°C = 19.1 years
Calculation: (19.1 x 22.8) / (19.1 + 22.8) = 10.4 years
Integrated Battery Controllers
If a system is to contain battery-backed SRAM or RTC,
it is important to use an appropriate battery controller.
These controllers handle the switching from system
power supply to the battery in the event of power failure.
They also provide the on-chip reverse-charging protection
required by Underwriters Laboratory or other
testing agencies. Maxim sells standalone
battery controllers that allow the system designer to
customize a system based on battery-capacity demands
or layout constraints.
While the standalone battery controllers are well suited
for certain applications, their use comes with some additional
costs. Not only must end users select and acquire
an appropriate battery, but the manufacturing process
must also accommodate the particular battery requirements.
Because of the limited available capacity in a
battery, the manufacturing process must ensure that no
inadvertent loads are placed on the battery. This requires
that the batteries be handled with insulated or nonconductive
tools while many other components in the design
are ESD sensitive and should be handled with conductive
tools.
The materials used in lithium battery construction limit
their temperature exposure capabilities. A single pass
through a reflow solder operation destroys the battery,
which raises the question of whether the battery should
be attached to the PCB with a mechanical holder or
soldered to the PCB. A mechanical holder can be
attached to the PCB using automated equipment and
reflow solder. The battery is then inserted after the hightemperature
processing is complete. The mechanical
holder eliminates any temperature exposure to the
battery, but the resulting system depends on the mechanical
contacts holding the battery in place. Attaching the
battery to the PCB with solder requires purchasing a
tabbed battery and hand soldering that component after
all reflow solder operations have been completed.
A final concern with using a battery controller and
separate battery is the cleanliness of the manufacturing
process. Even trace amounts of ionic contaminants can
result in electrical leakage paths that can place loads on
the batteries equal to the designed IC load. This will
greatly shorten the system's effective life.
Battery Module Products
Using a module product that contains the battery
controller and battery will avoid some of the problems
discussed above. The module manufacturer will have the
required processes to handle the batteries without degradation,
and the module construction will also help
isolate the battery from the end user's environment,
thereby avoiding some of the ionic contamination issues.
The end result can maximize battery lifetime.
In addition, many of the Maxim modules
incorporate a "Sleep Mode" function that isolates the
battery until system power is first applied. This feature
allows the module products to be assembled and fully
tested; the electrical load is then removed from the
battery. Thus, the parts can be left in inventory for an
extended period without removing any charge from the
battery.
Conclusion
Maxim's battery-backup products are
designed and manufactured to provide end users with a
specified lifetime. This lifetime has been calculated
under "worst case" conditions and assumes that the part
will be in battery-backup 100% of the time. By understanding
the mechanisms involved in the depletion of the
battery, end users can reasonably and accurately predict
system lifetimes based on the power-on duty cycle and
the battery temperature exposure.
If users decide to select one of the battery controllers
sold by Maxim and provide their own
battery in the system, they should consider the battery's
nature in the selection process. Proper IC screening and
battery testing are required to ensure the available
capacity is adequate to provide the desired lifetime.
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