AN1011

Characterization and modeling

Doc ID 6254 Rev 4

3/11

Characterization and modeling

STMicroelectronics has conducted extensive tests on these cells, a great proportion being

destructive testing. Three main mechanisms of battery degradation were modeled to

establish the effects under various loads, temperatures (up to 85 °C) and other

environmental variables:

●

Active lifetime of the cell, as a result of the consumption of the lithium anode

●

Shelf life (storage lifetime) of the cell, as a result of the electrolyte drying up

(evaporation)

●

Self-discharge, as a result of small leakage currents inherent to the battery chemistry

The weight loss proved to be a valuable measure of electrolyte evaporation. Self-discharge

is specified by the battery manufacturer to be less than 0.3% per year.

Models were then developed to predict the rate of electrolyte loss and what effect

encapsulation of the cell would have. Tests on the cells, when encapsulated in the epoxy

used in the SNAPHAT process, have demonstrated a significant reduction in electrolyte

evaporation.

Figure 2.

(A) BR1225X discharge rate

(B) BR1632 discharge rate

Figure 3 shows the load discharge graphs taken at 20 °C. As can be seen, both cells

produce a nominal 3 V output with a flat discharge curve until the end of their effective lives,

and so confirms that both are suitable for providing battery backup to low leakage CMOS

SRAMs.

The battery lifetime of individual ZEROPOWER, TIMEKEEPER and RTC devices, as

described in AN1012, is a function of the current load, the battery selected and the system

duty cycle. System life, which even with 100% duty cycle can still be greater than ten years,

is specified in the datasheet for each device.

AI02519

0

1.0

1.5

2.0

2.5

3.0

3.5

1000

2000

3000

4000

5000

6000

Duration (Hrs.)

(B)

15k

30k

50k

100k

0

1.0

1.5

2.0

2.5

3.0

3.5

200 400 600

1200

1600 1800

Duration (Hrs.)

(A)

100k

800 1000

1400

2000

15k

30k