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RF5151 Battery Life Calculation

RF5151 Battery Life Calculation


 The battery life of RF5151 mainly is determined by two fact ors: Transmit time / period and sensor power consumption.


1. Transmit Time / Period

The transmit time is dependent on the RF data rate and the length of data package. In other words faster data rate and shorter data package mean shorter transmit time, thereby, lower average power consumption. The transmit time can be calculated by the function below:


Transmit time (ms) = (data length in bytes + 10) * 8 / RF data rate (Kbps)


For example, in analogue sensor mode, the data format of transmit is totally five bytes [Group ID (1 bytes) + Slave ID (1 byte) + Data (2 bytes) + Bat (1 byte)] . If the RF data rate is 50K bps, the transmit time can be calculated as:

Transmit time (ms) = (5+10)*8 /50 = 2.4ms

The start-up time of crystal on RF5151 module is about 1ms and the current in the start-up stage of crystal is about 7mA. The transmit current of RF5151 is 14mA and the current in sleep mode is 1.5uA. If the tx transmit period is 1s, the average current is:  

Average current (mA) = (2.4ms / 1000ms) * 14mA + 1ms/1000ms * 7mA + 0.0015mA =0.0421mA.


2. Sensor Power Consumption


 In analogue sensor mode RF5151 module supplies power to sensor for 1ms. The power consumption is mainly determined by the sensor and the resistor in that period. What’s more the RF5151 consumes about 0.5mA current in 1ms.

 In DS18B20 mode RF5151 will start the measurement on DS18B20 in advance for 1s and then enter sleep mode. The data will be read in next second and be transmitted. Because DS18B20 consumes about 1mA current and the measurement is done in the sleep m ode of RF5151, the power consumption introduced by this sensor can be omitted.

 In SHT1x and SHT2x mode RF5151 module starts the measure and waits until the measurement is done. The module consumes about 0.5mA current in that period. The actual power consumption of sensors will be decided by the actual sensor type.