Battery Life Calculator for RTCs

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, EA, 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.

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