AN1012 datasheet(8/33 Pages) STMICROELECTRONICS

8 / 33 page

Battery backup current - predicting data retention time

AN1012

8/33

Doc ID 6395 Rev 4

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 k

Ω load

resistor.

The two lines, SL1% and SL50%, represent different failure rate distributions for the cell’s

storage life. At 60 °C, for example, the SL1% line indicates that the battery has a 1% chance

of failure 28 years into its life, and the SL50% line shows that the battery has a 50% chance

of failure at the 50 year mark. The SL1% line represents the practical onset of wear out, and

can be considered the worst case storage life for the cell. The SL50% 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, SL50% = 14270 x (0.91)

T, and SL

1% = 8107 x (0.91)

T, when

20 °C < T < 90 °C.

AN1012 Datasheet(PDF) 8 Page - STMicroelectronics