AN1012

Battery technology

Doc ID 6395 Rev 4

7/33

2

Battery technology

STMicroelectronics uses both the BR1225 and the BR1632 lithium button cell batteries.

These have charge capacities of 48 mAh and 120 mAh, respectively. Their constituents

have non-toxic and non-corrosive characteristics, and are chemically and thermally stable

before, during, and after discharge. This makes these cells particularly attractive for use in

electrical components.

They contain a solid carbon cathode that is pressed into a tablet of predetermined weight

and height. The anode consists of high-purity lithium metal. The electrolyte is based on an

organic solvent instead of the corrosive alkaline or acidic solution found in most

conventional batteries. This greatly reduces the likelihood of internally-induced cell leakage,

and reduces the ill effects in cases of externally-induced cell leakage. The cell is then crimp-

sealed with a polypropylene grommet.

ST has conducted extensive tests on these cells, at temperatures up to 85 °C. Destructive

analysis was conducted (post-stress), in order to measure such factors as weight loss and

remaining charge capacity. The analysis determined that the cells were drying out, and that

the weight loss was due to electrolyte evaporation. Models were developed to predict the

nominal rate of electrolyte loss, and how this would be reduced by adding a second level of

encapsulation. This proprietary secondary seal encapsulation, adopted by ST, has been

found to provide up to a two-fold reduction of the electrolyte loss rate.

Both cells produce a nominal 2.9 V output with a flat discharge curve until the end of their

effective lives, and thus confirms that both are suitable for providing battery backup to low

leakage CMOS SRAMs (see Figure 3).

Figure 3.

(A) BR1225 discharge rate and (B) BR1632 discharge rate

AI02519

0

1.0

1.5

2.0

2.5

3.0

3.5

LOAD CHARACTERISTICS

Temp: 20°C

1000

2000

3000

4000

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6000

Duration (Hrs.)

(B)

15k

Ω

30k

Ω

50k

Ω

100k

Ω

0

1.0

1.5

2.0

2.5

3.0

3.5

LOAD CHARACTERISTICS

Temp: 20°C

200 400 600

1200

1600 1800

Duration (Hrs.)

(A)

100k

Ω

800 1000

1400

2000

15k

Ω 30kΩ