memory products - Al Kossow's Bitsavers

MKI48Z02/12(B)-15/20/25DATA RETENTION TIMEAbout Figure 7Figure 7 illustrates how expected MKI48Z02/12 batterylife is influenced by temperature. The life of thebattery is controlled by temperature and is virtuallyindependent of the percentage of time theMKI48Z02/12 spends in battery back-up mode.Battery life predictions presented in Figure 7 areextrapolated from temperature accelerated life-testdata collected in over 100 million device hours ofcontinuing bare cell and encapsulated cell batterytesting by SGS.:rHOMSON. Obviously, temperatureaccelerated testing cannot identify non-temperaturedependent failure mechanisms. However, in viewof the fact that no random cell failures have beenrecorded in any of SGS.:rHOMSON's ongoing batterytesting since it began in 1982, we believe thelikelihood of such failure mechanisms surfacing isextremely poor. For the purpose of this testing, acell failure is defined as the inability of a cell stabilizedat 25"C to produce a 2.0 volt closed-circuit voltageacross a 250K ohm load resistance.A Special Note: The summary presented inFigure 7 represents a conservative analysis ofthe data presently available. While SGS­THOMSON is most likely in possession of thelargest collection of battery life data of this kindin the world, the results presented should notbe considered absolute or final; they can be expectedto change as yet more data becomesavailable. We believe that future read-points oflife tests presently under way and improvementsin the battery technology itself will result in a continuingimprovement of these figures.Two end of life curves are presented in Figure 7.They are labeled "Average (t50%)" and "(tl%)".These terms relate to the probability that a givennumber of failures will have accumulated by a particularpoint in time. If, for example, expected lifeat 80"C is at issue, Figure 7 indicates that a particularMK148Z02l12 has a 1% chance of having a batteryfailure 10 years into its life and a 50% chanceof failure at the 17 year mark. Conversely, given asample of devices, 1% of them can be expected toexperience battery failure within 10 years; 50% ofthem can be expected to fail within 17 years.The t 1% figure represents the practical onset ofwear-out, and is therefore suitable for use in whatwould normally be though of as a worst-case analysis.The t50% figure represents "normal" or "average"life. It is, therefore, accurate to say that theaverage device will last "t50%".Battery life is defined as beginning on the date ofmanufacture. Each MK148Z02/12 is marked with afour digit manufacturing date code in the formYYWW (Example: 8502 = 1985, week 2).Calculating Predicted Battery LifeAs Figure 7 indicates, the predicted life of the batteryin the MKI48Z02112 is a function of temperature.The back-up current required by the memorymatrix in the MKI48Z02/12 is so low that it hasnegligible influence on battery life.Because predicted battery life is dependent upon applicationcontrolled variables, only the user can estimatepredicted battery life in a given design. As longas ambient temperature is held reasonably constant,expected life can be read directly from Figure 7. Ifthe MK148Z02l12 spends an appreciable amount oftime at a variety of temperatures, the follOWing equationsshould be used to estimate battery life.Predicted Battery Life = _________ 1 _________[(TA 1/TT)/BL 1)]+[(TA2/TT)/BL2]+··· +[(TAn/TT)/BLn)]Where TAl' TA2' TAn = Time at Ambient Temperature 1, 2, etc.TT = Total Time = TAl + TA2 + ... + TAnBL 1, BL2, BLn = Predicted Battery Lifetime at Temp 1, Temp 2, etc. (see Figure 7).EXAMPLE PREDICTED BATTERY LIFE CALCULATIONA process control computer operates in an environmentwhere the MK148Z02/12 is exposed to temperaturesof 50°C or less for 3066 hrs/yr; temperaturesgreater than 25 "C, but less than 6O"C, for 5256hrs/yr; and temperatures greater than 4O"C, but lessthan 85°C, for the remaining 438 hrs/yr.Reading predicted typical life values from Figure 7; BLl = 275 yrs., BL2 = 95 yrs., BL3 = 32 yrs.Total Time (TT) = 8760 hrs./yr. TAl = 3066 hrs./yr. TA2 =5256 hrs./yr. TA3 = 438 hrs./yr.Predicted Typical Battery Life ~ _________ 1 ________ _[(3066/8760)/275] +[(5256/8760)/95] +[(43818760)132]~ 109.2 yrs.7/11331