Cell Storage Life

Storage life is primarily a function of temperature.

Figure 9 illustrates the approximate storage life of

the M48Z08/18 battery over temperature. The re-

sults in Figure 9 are derived from temperature

accelerated life test studies performed at SGS-

THOMSON. For the purpose of the testing, a cell

failure is defined as the inability of a cell stabilized

at 25

°C to produce a 2.4V closed circuit voltage

across a 250 k

tions for the cell’s storage life. At 70

°C, for example,

chance of having a battery failure 28 years into its

sents the practical onset of wear out and can be

considered the worst case Storage Life for the cell.

life.

Calculating Storage Life

The following formula can be used to predict stor-

age life:

1

{[(TA1/TT)/SL1]+[(TA2/TT)/SL2]+...+[(TAN/TT)/SLN]}

where,

– TA1, TA2, TAN = time at ambient temperature

1, 2, etc.

– TT = total time = TA1+TA2+...+TAN

– SL1, SL2, SLN = storage life at temperature 1,

2, etc.

For example, an M48Z08/18 is exposed to tem-

peratures of 55

°C or less for 8322 hrs/yr, and

temperatures greater than 60

°C but less than 70°C

values from Figure 9,

– SL1

≅ 200 yrs, SL2 = 28 yrs

– TT = 8760 hrs/yr

– TA1 = 8322 hrs/yr, TA2 = 438 hrs/yr

Predicted storage life

≥

1

{[(8322/8760)/200]+[(431/8760)/28]}

or 154 years.

As can been seen from these calculations and the

results, the expected lifetime of the M48Z08/18

should exceed most system requirements.

Estimated System Life

Since either storage life or capacity consumption

can end the battery’s life, the system life is marked

by which ever occurs first.

Reference for System Life

Each M48Z08/18 is marked with a nine digit manu-

facturing date code in the form of H99XXYYZZ. For

example, H995B9431 is:

H = fabricated in Carrollton, TX

9 = assembled in Muar, Malaysia,

9 = tested in Muar, Malaysia,

5B = lot designator,

9431 = assembled in the year 1994, work week 31.

POWER SUPPLY DECOUPLING and UNDER-

SHOOT PROTECTION

switching, can produce voltage fluctuations, result-

be reduced if capacitors are used to store energy,

the bypass capacitors will be released as low going

spikes are generated or energy will be absorbed

when overshoots occur. A ceramic bypass capaci-

tor value of 0.1

µF (as shown in Figure 10) is

recommended in order to provide the needed filter-

ing.

In addition to transients that are caused by normal

SRAM operation, power cycling can generate

negative spikes can cause data corruption in the

SRAM while in battery backup mode. To protect

from these voltage spikes, it is recommeded to

1N5817 is recommended for through hole and

MBRS120T3 is recommended for surface mount.

AI02169

VCC

0.1

µF

DEVICE

VCC

VSS

Figure 10. Supply Voltage Protection

10/18

M48Z08, M48Z18