kdb341: RTC Battery System Design App Note (CTIN-00009)

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Introduction

In many embedded systems, there is a small battery called the RTC and/or CMOS battery. This battery provides power to the real time clock (RTC) while the embedded system is powered off. The real time clock maintains the system data and time. This battery also potentially preserves BIOS settings while the system is powered off; depending on the CPU/BIOS architecture.

These circuits typically have very small current draw, in the range of 1-10 micro amperes (μA). In a properly designed embedded system, the RTC battery is diode-ORed with a voltage regulator to ensure that the RTC battery is only used when the system is powered off, otherwise during operation the system would source from the on-board regulator.

Connect Tech’s COM Express and Qseven carriers provide either a soldered in battery, external battery connector, or the capability to select between the two options. The product’s User Guide will specify the implementation.

Since these batteries are not rechargeable, the system integrator / end-user must estimate the operating life of the RTC battery and be aware of the implications of a fully drained battery. When fully drained, behaviors could include loss of BIOS settings (system dependent) and/or an incorrect system clock (where the system clock is not synchronized to a network source).

This application note provides information on the characteristics of batteries and how to estimate the life time of RTC batteries when used in a COM Express or Qseven system.

Battery Discharge Characteristics

In general batteries begin discharging (draining) as soon as they are connected to any electronic circuit (load). The load can vary depending on usage.

In addition to discharge due to circuit loading, batteries also drain due to self-discharge. Self-discharge is a chemical reaction phenomenon that occurs in all batteries – regardless of type or chemistry. Self-discharge is constant, it occurs while the battery is in its package or in circuit.

The rate of self-discharge is highly dependent on the battery quality, chemistry, and most importantly, ambient temperature.

Connect Tech only uses high quality, industrial temperature rate lithium coin cell batteries with very low self-discharge characteristics.

In most cases, Connect Tech products will be populated with a Panasonic BR1225A. For a specific details on the battery used, refer to the product user guide.

Specification of the BR1225A battery

Nominal Voltage: 3V

Nominal Capacity: 48mAh

Operating Temperature: -40°C to +125°C

Self-Discharge:

At low temperatures, circuit discharge will be the dominant factor in determining battery life.

Estimating RTC Battery Life

Parameters

When calculating the RTC Battery life of a COM Express or Qseven based system, there are several parameters the system designer will need to know

(*) Connect Tech Inc. carrier products do not have any circuits that use V_RTC

Estimation Process

Estimating the battery life can be a complicated process when factoring in temperature, system on/off cycles, and battery discharge characteristics. In some cases, the battery manufacturer may not be release all the necessary information.

First, the battery life is a function of the capacity and the system load.

T_LIFE (hours) = CAP / I_RTCMAX

Example 1 with BR1225A where I_RTCMAX = 0.9 μA

T_LIFE (hours) = 48mAh / 0.9 μA = 53333 hours = 6.1 years

Then, the battery discharge curve needs to be considered to determine how long the voltage will remain above V_RTCMIN. If the battery discharge curve is unavailable and a cut-off voltage is not provided, then a safe assumption is that the battery will remain above V_RTCMIN for 90% of its life. This factor will be referred to as K_OP.

T_LIFE (hours) = ( CAP x K_OP)/ I_RTCMAX

Example 2 with BR1225A where I_RTCMAX = 0.9 μA

T_LIFE (hours) = (48mAh x 0.9) / 0.9 μA = 48000 hours = 5.5 years

If the carrier system is designed only to use the battery when it is off , as is the case with Connect Tech’s carriers, then off time percentage should be factored in. This effectively reduces I_RTCMAX

T_LIFE (hours) = ( CAP x K_OP) / [ I_RTCMAX x TOFF / (TON + TOFF) ]

Example 3 with BR1225A where I_RTCMAX = 0.9uA & TOFF / (TON + TOFF) = 20%

T_LIFE (hours) = ( 48mAh x 0.9) / [ 0.9uA x 20% ] = 240000 hours = 27.4 years

Panasonic actually publishes a battery discharge curve, available in the BR1225A datasheet – see link below:

https://industrial.panasonic.com/cdbs/www-data/pdf2/AAA4000/AAA4000C210.pdf

With a 90uA load @ 125°C, the battery will work for 490 hours above 2V. A rough extrapolation for 0.9 μA is 49000 hours at 125°C. This number is very similar to that calculated in Example 2.

As with any engineering estimation, it is a good practice to add some margin to the end result.

Specifying a battery

If a board mounted BR1225A battery is not sufficient; then the system designer will need to select and design an external battery cable.

Example:

A system designer has a Connect Tech’s CCG011 carrier and a COM Express module where I_{RTCMAX =} 5 μA and requires a ten year life, T_LIFE (hours) = 87600. The system is expected to be off 50% of the time.

CAP = T_LIFE (hours) x [ I_RTCMAX x TOFF / (TON + TOFF) ] / K_OP

CAP = 87600 hrs x [0.005mA x 0.3 ] / 0.9

CAP = 243 mAh

A Panasonic BR2330A batter with 255 mAh would be able to provide the exact life time needed

T_LIFE (hours) = ( CAP x K_OP) / [ I_RTCMAX x TOFF / (TON + TOFF) ]

T_LIFE (hours) = 91800

Applicable Products

The following is a list of Connect Tech’s carrier products and their battery capabilities. Refer to the product users guide for the most up to date information.

For assistance with integrating your embedded system, please contact [email protected]

References and Helpful Links

https://www.maximintegrated.com/en/design/tools/calculators/product-design/battery.cfm
http://www.congatec.com/fileadmin/user_upload/Documents/Application_Notes/AN9_RTC_Battery_Lifetime.pdf