ST Life.augmented

Real-time clock battery life calculation

Battery Capacity Consumption

When Vcc is being held by the external power supply within its specified range, the current drawn from the battery is zero. When Vcc falls below the Battery Back-up Switchover Voltage (Vso), the device goes into battery back-up mode, and draws all its current from the battery.

The Vcc duty cycle represents the proportion of time, expressed as a percentage, that the device is supplied with power from the external supply, and hence not drawing current from the battery.

 
Calculating Battery Lifetime

Capacity consumption is simply calculated by:

  Battery Capacity
  8760 * (1) * Ibat

where:
– Battery Capacity is measured in ampere-hours;
– 8760 is the constant for the number of hours there are in a year;
– Vcc Duty Cycle is measured as a percentage (100% in this case); and
– Ibat is measured in amperes.
To calculate the typical and worse case data retention lifetimes, select the Serial RTC Device and then enter the battery capacity of your choice in the form below (in mAh), and click on Calculate. The calculated typical and worst case Battery Life (@25°C) in years will be then be displayed.

Device:
Enter Battery Capacity(1) (mAh):

Battery Lifetime(2) (in Years @ 25°C)

Calculated Typical:
Calculated Worst Case:
Note 1: M4T28-BR12SH = 48mAh;
M4T32-BR12SH = 120mAh
Note 2: Excluding Iout2 and/or Tamper Current for those devices with Vout or Tamper Inputs.
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