Battery Life Calculator for RTCs

Battery Operating Time
Battery Capacity
mAh	°C Operating Temp
Hrs 1% Time to Fail at +110°C
IBAT (max)	IBAT (typ)
nA	nA
Days	Days
Years	Years
Years with Self-Discharge	Years with Self-Discharge

This calculator determines timekeeping operation time based upon battery capacity and load.

Where:

• I_BAT (max) is the maximum battery current, as noted in the RTC's data sheet.
• I_BAT (typ) is the typical battery current defined at both +25°C and the nominal supply voltage, as noted in the RTC's data sheet.

Estimated backup times in days and years are calculated using the formula Ah/A, where Ah is the battery capacity in ampere-hours and A is the load in amperes.

The backup time in years with self-discharge calculations is done using an Arrhenius model:



The time to 1% fail (TTF) hours and activation energy, E_A, have been estimated from accelerated life testing. Battery self-discharge becomes a significant contributor to overall operating life as the operating temperature increases.

The activation energy is assumed to be 1eV for all cases and the TTF for some common batteries is shown in the table below. The calculator assumes that the stress temperature, T2, is +110°C (+383°K). Note that the 1% TTF will be different across manufacturer and battery model numbers, and will vary with manufacturing tolerances. Contact the battery manufacturer for reliability data regarding a particular model.

Common Lithium Coin Cell Part Numbers and Capacities

Model	Capacity	1% TTF at +110°C
BR1632	120mAh	1900 hrs
CR1632	140mAh	1350 hrs
BR1225	48mAh	1900 hrs

For more information, please refer to application note 505, "Lithium Coin-Cell Batteries: Predicting an Application Lifetime."