RTC Battery Lifetime (AN09)

Affected Products	All congatec products featuring RTC

Preface

This application note is a short example of how to calculate the maximum battery service time when a specific RTC circuit battery type must be used. Additionally, it also describes how to determine the required RTC circuit battery capacity when a specific battery service time is required.

Terminology

Term	Description
RTC	Real Time Clock – battery backed circuit in PC-AT systems that keeps system time and date as well as certain system setup parameters. when the computer is not powered. It is normally supplied by a battery.

Introduction

The following sections describe how to calculate:

• the lifetime of the installed RTC battery

• the required RTC battery capacity when a specific life expectancy of the battery must be obtained

For detailed information about the power consumption of the RTC circuit on the various congatec products, refer to respective User's Guide on the congatec website at congatec web page]

The RTC Battery Power Consumption value is a measured value and may differ from the value listed in the chipset datasheet. The measurement for computer-on-modules (COM) is carried out with a congatec evaluation carrier board and therefore only valid with this setup.

To get the most accurate value for the RTC Battery Power Consumption, it is recommended to perform the measurement with customer’s carrier board and in worst case situation. This means you should measure the value when the board is in G3 mode (mechanical off – no AC power connected), at highest specified ambient temperature and when a new battery is connected.

How to Calculate

The section Calculating the maximum battery lifetime shows how to calculate the maximum battery lifetime with a specific battery. The next section Calculating the nominal battery capacity shows how to determine the nominal capacity when a specific battery lifetime has to be reached.

Calculating the maximum battery lifetime

The maximum lifetime of a battery supplying the RTC circuitry when AC power is switched off can be calculated by using following formula:

t_max: Maximum battery lifetime [years].

Bat_NomCap: Nominal capacity of the installed battery [Ah].

I_BatMax: Maximum RTC battery current [A].

Note:

Factor 0.8 considers the external impacts that can affect the battery lifetime. This factor can vary from 0.7 to 0.9 depending on the battery and operating conditions.

Caution:

The lifetime of the battery begins at the date of manufacturing. The RTC starts as soon as the battery is installed to the system. Additionally the battery will discharge even when it is not installed to a system. If a battery is not installed to a system the nominal capacity will be decreased by approximately 1% per year.

Calculating the nominal battery capacity

The nominal capacity of the battery supplying the RTC circuitry when AC power is switched off can be calculated by using following formula:

Bat_NomCap: Nominal capacity of the installed battery [Ah].

t_min: Minimum battery lifetime [years].

I_BatMax: Maximum RTC battery current [A].

Note:

Factor 1.2 considers the external impacts that can affect the battery lifetime. This factor can vary from 1.1 to 1.3 depending on the battery and operating conditions.

Examples

The following examples are based upon typical values. The first example determines the maximum battery lifetime for a specified battery type. The second example shows how to calculate the nominal battery capacity for a defined battery lifetime.

Example for specified battery lifetime

Battery type: CR 2032

Nominal battery voltage: 3V

Nominal capacity Bat_NomCap: 230 mAh

Maximum RTC battery current: 2.5 µA

Example for specified service time

Required service time for battery: 15 years

Nominal battery voltage: 3V

Maximum RTC battery current: 3 µA

→ The battery should have a nominal capacity equal to or greater than 473.04 mAh (i.e.the Panasonic CR2477).

Note:

If you plan to use batteries for a longer service time, ensure that the battery is new and has its full nominal capacity. Batteries stored for a long time should not be used.