Data Sheet

AN1012 Battery backup current - predicting data retention time
Doc ID 6395 Rev 4 9/33
Figure 4. Predicted battery storage life versus temperature
3.2 Calculating storage life
Only the user can estimate predicted storage life in a given design because the ambient
temperature profile is dependent upon application-controlled variables. As long as the
ambient temperature is held reasonably constant, the expected storage life can be read
directly from Figure 4 on page 9. If the battery spends an appreciable amount of time at a
variety of temperatures, the following formula can be used to estimate predicted storage life:
where,
● t
i
/T is the relative proportion (of the total time) during which the device is at ambient
temperature TA
i
;
● SL
i
is the storage life at ambient temperature TA
i
as illustrated in Figure 4; and
● T is the total time = t
1
+ t
2
+ ... + t
n
.
For example, consider a battery exposed to temperatures of up to 90 °C for 600 hrs/yr, and
temperatures of 60 °C or less for the remaining 8160 hrs/yr. Reading predicted t
1%
values
from Figure 4,
● SL
1
is about 1.8 yrs;
● SL
2
is about 28 yrs;
● T is 8760 hrs/yr;
● t
1
is 600 hrs/yr; and
● t
2
is 8160 hrs/yr.
The predicted storage life evaluates to:
This predicts that the storage life, in this particular case, is at least 14 years. This is,
therefore, better than the normally accepted life time of 10 years.
AI01024b
20 30 40 50 60 70 80 90
1
2
3
4
5
8
6
TEMPERATURE (Degrees Celsius)
10
20
30
40
50
STORAGE LIFE (Years)
SL
50%
(AVERAGE)
SL
1%
1–
t
1
T
-----
1
SL
1
----------
×
⎝⎠
⎜⎟
⎛⎞
t
2
T
-----
1
SL
2
----------
×
⎝⎠
⎜⎟
⎛⎞
…
t
n
T
-----
1
SL
n
----------
×
⎝⎠
⎜⎟
⎛⎞
+++
1–
600
8760
-------------- -
1
1.8
---------
×
⎝⎠
⎛⎞
8160
8760
-------------- -
1
28
------
×
⎝⎠
⎛⎞
+