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
|Ambient Temperature||Self-Discharge %.Year||Theoretical Shelf Life @ Temp in Years|
At low temperatures, circuit discharge will be the dominant factor in determining battery life.
Estimating RTC Battery Life
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
|System||IRTCMAX||The maximum current drawn by system while off. This is the sum of all devices using VRTC including the COM Express module and any ancillary circuits on the carrier (*)|
|System||VRTCMIN||The minimum voltage required for the RTC to function|
|System||TOFF / (TON + TOFF)||Percentage of time where the system is used|
|Battery||CAP||Capacity of the battery in mAH|
|Battery||Discharge Curve||This discharge curve shows Vbat vs. time. In some cases there will be a curve for each 20°C increment. The curve will show the batteries behavior at as it drains. For example, what is the voltage when the battery is at 5%|
|Battery||Self-discharge Curve||This will show how fast a battery will drain when not in use|
(*) Connect Tech Inc. carrier products do not have any circuits that use VRTC
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.
TLIFE (hours) = CAP / IRTCMAX
Example 1 with BR1225A where IRTCMAX = 0.9 μA
TLIFE (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 VRTCMIN. 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 VRTCMIN for 90% of its life. This factor will be referred to as KOP.
TLIFE (hours) = ( CAP x KOP)/ IRTCMAX
Example 2 with BR1225A where IRTCMAX = 0.9 μA
TLIFE (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 IRTCMAX
TLIFE (hours) = ( CAP x KOP) / [ IRTCMAX x TOFF / (TON + TOFF) ]
Example 3 with BR1225A where IRTCMAX = 0.9uA & TOFF / (TON + TOFF) = 20%
TLIFE (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:
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.
A system designer has a Connect Tech’s CCG011 carrier and a COM Express module where IRTCMAX = 5 μA and requires a ten year life, TLIFE (hours) = 87600. The system is expected to be off 50% of the time.
CAP = TLIFE (hours) x [ IRTCMAX x TOFF / (TON + TOFF) ] / KOP
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
TLIFE (hours) = ( CAP x KOP) / [ IRTCMAX x TOFF / (TON + TOFF) ]
TLIFE (hours) = 91800
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 firstname.lastname@example.org
|Part Number||On Board Battery||External Battery Header||User Guide|
|CCG030||No||On breakout board||CTIM-00457|
References and Helpful Links
Originally from kdb341: RTC Battery System Design App Note (CTIN-00009)