For many small, portable Internet of Things (IoT) applications, the “Holy Grail” is wireless connectivity with a battery life of up to 10-years, using a coin cell. This is not an easy task, as most inexpensive coin cells offer a maximum capacity of only ~ 240 mAh. By selecting a radio SoC with low sleep current consumption, 10 years is very achievable for both short and long-distance wireless connections.
In order to obtain 10-year battery life for low coin cell capacities like 240 mAh, wireless devices typically spend the majority of their time asleep, only waking up occasionally to make a wireless transmission, as depicted in Figure 1 below. For example, a 7 msec wake time, relative to a 5 second transmit interval (120 wireless transmits per hour), yields a duty cycle of 0.14% wake time and 99.86% sleep time. This is why low deep sleep power consumption is imperative for achieving 10-year battery life from a coin cell.
Figure 1. Sleep and Wake Time Duty Cycles
To achieve a 10-year battery life, Bluetooth Low Energy is the preferred choice for short to medium distance wireless connectivity, typically 30 to 50 meters depending on Transmitting (Tx) and Receiving (Rx) power. For longer distances, anything over 1000 meters, Sub Gigahertz Software defined radios (SDR) are ideal.
Bluetooth® Low Energy
Bluetooth Low Energy operates in 2.4 GHz ISM band using a 40-channel partition, spaced 2 MHz apart. Three RF channels (37, 38 & 39) are dedicated for advertising functions that allow the discovery of devices available in the vicinity. Channels 0-36 are dedicated for data. The advertising channels (Figure 2) are allocated in different parts of the spectrum to provide immunity against interference from 802.11/Wi-Fi.
Figure 2. Bluetooth Low Energy Advertising Channels (Source Accton Marketing)
The data unit of an advertisement packet called the Protocol Data Unit (PDU), has a two-byte header that specifies the type and length of data payload up to 37 bytes (6 bytes for the advertisement address and up to 31 bytes for data).
Figure 3. Bluetooth Low Energy Advertising Packet (Source Accton Marketing)
Connectable vs. Non-connectable
Bluetooth Low Energy advertisement packets can be either Connectable or Non-connectable. Figure 4 depicts the RSL10 System in Package (RSL10 SIP) Bluetooth Low Energy module captured by a Power Analyzer for the Connectable (left) and Non-connectable (right) “3 Advertisement” events, both at 0 dbm Transmit Power. While both events use channels 37, 38 and 39, and last 7 msec, the Connectable event includes an RX pulse for each channel. This makes sense since the Connectable event also wants to receive. The resulting Power Analyzer measurement reveals the average current for each- 711.624uA for Connectable, and 504.307uA for Non-connectable. Meanwhile, the RSL10 SIP’s deep sleep current is 160 nA for a 16 kbB RAM retention for the Bluetooth Low Energy stack, and running an internal timer to wake itself up.
Figure 4. Connectable vs. Non-Connectable Advertising Packets
RSL10 SIP Battery Life (5 Bytes)
Given the conditions described above, Figure 5 demonstrates that the RSL10 SIP Actual Battery Life will range from 10.97 Years (2.5 second Advertisement Interval, Connectable) to 27.26 years (5 second Advertisement Interval, Non-Connectable). These calculations are based off the use of a 240 mA CR2032 coin cell and 5 byte data transfer (PDU).
Figure 5. RSL10 SIP Actual Battery Life
Ideal Battery Life vs. Actual Battery Life
Li-ion coin cells come with data sheets that plot the Continuous Discharge Characteristics for a given load. In the CR2032 battery example in Figure 6, the plot captures the discharge curve for a constant 19 0uA load. The RSL10 SIP average current captured in Figure 5 ranges from 865 nA to 1.57 uA, much lighter than the 190 uA curve.
When I calculate the “Ideal VBAT”, I coulomb count 240 mAh from 100% full to 0% empty, represented by the red dashed line labeled “Ideal VBAT”. A coin cell will never actually behave like the red dashed line. Knowing that the “Actual VBAT” discharge curve lies somewhere between the red dashed line and the blue CR2032 190uA discharge curve, I’ve de-rate the “Ideal VBAT” by 15% to arrive at the green “Actual VBAT” discharge curve.
Figure 6. Continuous Discharge Characteristics
Figure 7 below exposes the RSL10 SIP battery life if we increase the data size from 5 Bytes to 31 Bytes.
Figure 7. RSL10 SIP Battery Life (31 Bytes)
Proprietary RF Protocols
Proprietary Sub-GHz radios are designed for longer distance wireless transmissions. With a link budget of 153 db (16 dbm Tx power with a -135 Rx sensitivity), the AXM0F24 narrow-band SoC can transmit a distance of 37km, or 23 miles (915MHz, 30db fade margin). For a 1.1 km distance, the AXM0F243 exceeds the desired 10-year Actual Battery Life.
Figure 8. AXM0F243 Actual Battery Life
With the right radio SoC, achieving a 10-year battery life for short and long-ranges is entirely possible. Learn more about ON Semiconductor’s solutions for connectivity today.
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