Data Sheet

Battery backup current - predicting data retention time AN1012
8/33 Doc ID 6395 Rev 4
3 Battery backup current - predicting data retention
time
A ZEROPOWER
®
, TIMEKEEPER
®
, supervisor, or serial RTC device will reach the end of
its useful life for one of two reasons:
● Capacity consumption
It becomes discharged, having provided current to the SRAM (and to the oscillator in
the case of the TIMEKEEPER) in the battery backup mode.
● Storage life
The effects of aging will have rendered the cell inoperative before the stored charge
has been fully consumed by the application.
The two effects have very little influence on each other, allowing them to be treated as two
independent but simultaneous mechanisms. The data retention lifetime of the device is
determined by which ever failure mechanism occurs first.
3.1 Storage life
Storage life, resulting from electrolyte evaporation, is primarily a function of temperature.
Figure 4 illustrates the predicted storage life of the BR1225 battery versus temperature. The
results are derived from temperature-accelerated life test studies performed at
STMicroelectronics. For the purpose of testing, a cell failure is defined as the inability of a
cell, stabilized at 25 °C, to produce a 2.4 V closed-circuit voltage across a 250 kΩ load
resistor.
The two lines, SL
1%
and SL
50%
, represent different failure rate distributions for the cell’s
storage life. At 60 °C, for example, the SL
1%
line indicates that the battery has a 1% chance
of failure 28 years into its life, and the SL
50%
line shows that the battery has a 50% chance
of failure at the 50 year mark. The SL
1%
line represents the practical onset of wear out, and
can be considered the worst case storage life for the cell. The SL
50%
line can be considered
to be the normal, or average, life. As indicated by the curves in Figure 4 on page 9, storage
life does not become a limiting factor to overall battery life until temperatures in excess of
60 °C to 70 °C are involved.
As an approximation, SL
50%
= 14270 x (0.91)
T
, and SL
1%
= 8107 x (0.91)
T
, when
20 °C < T < 90 °C.