Decide Battery for IoT Devices

One of the fundamental requirements for IoT devices is its battery life and decide the right battery needs some inputs.

We will consider our RF Module to calculate the battery life and to give you some insights on how to size the requirement.

Any device will have its average current, peak current and standby/ sleep current defined or provided by the vendor. Let us take the case for the MKW01Z128 17dBm module. The MKW01Z128 is a SoC with radio integrated with a Cortex-M0+.

Radio:
Peak Transmit Current: 95mA
RX Current: 17mA
Sleep Current: Depends upon sleep state. Minimum is 0.1uA
MCU:
Typical Run Mode: 6mA
Various sleep mode currents: 0.1uA to 10uA

Now we define the MCU configuration/ use case while active as well as while on standby/ sleep:
In active mode the radio and the MCU are ON. There is an occasional transmit which we define at 1 packet every 10 minutes. We ignore any peripherals which are connected to the board to simplify the computation. The transmission will be of 100bytes at 50kbps  800bits – about 20ms of transmit time. Assume radio turns ON for 100ms for this 20ms of transmit time.
During sleep/ standby the radio is powered down and the MCU is in low power mode. We assume the MCU enters into active mode based on some stimulus from a peripheral since the radio is powered down.

Hence, the MCU power operations can be tabulated as below:
20ms of transmission every 10 seconds where radio + MCU will transmit (100mA, peak)
80ms for reception/ computation: 17 + 6 = 23mA
Assuming 250uA of sleep current for a stop mode for the remaining of the time 9.9 seconds

Now, since batteries are all available with mAH (milli Ampere Hours) we will use this unit for all our computation

1 hour – 60 * 60 * 1000 seconds.
20ms = 20/(3600*1000) = 5.5 * 10^(-6) H
100mA for 5.5 * 10^(-6) hours will be 5.5 * 10^(-4) mAH
25mA for 22 * 10^(-6) hours will be 5.5 * 10^(-4) mAH
250uA forever is 0.25mAH requirement

Hence, as evident above, the most significant impact is for the standby mode current.
Now considering capacities of common batteries:

Alkaline AA: 2000mAH,
Alkaline AAA: 1000mAH
Alkaline D: 16000mAH

(Reference: Duracell/ Energizer battery datasheets)

Considering an Alkaline AAA battery powering 0.25mAH load will power it for 4000 hours or roughly about 166 days/ approximately 5.5 months. A D battery will power it for 2 years.

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