Extending battery lifetime in wireless applications using supercapacitors
It is estimated that there are 9.9 billion active IoT connections in 2020, rising to 21.5 billion in 2025, each with functional electronics, perhaps a microcontroller, and a wireless transceiver of some type. Power for these sensors, meters, remote data concentrators, gateways and more, is often only practically available from an integral battery which may need to be physically small with consequent limited energy storage. It is therefore important that the electronics consumes as little power as possible to avoid the inconvenience and manpower cost of periodic battery replacement and downtime.
Different wireless technologies have varying power draw
A major contributor to average and peak power consumption is the wireless chipset, with different protocols trading coverage and data rate with current draw. Table 1 shows some common technologies and how they compare.
|Technology||Frequency||Data rate||Range||Power usage||Cost|
|2G/3G||Cellular bands||10Mbps||Several km||High||High|
|802.15.4||subGHz, 2.4GHz||40,250kbps||>200 square km||Low||Low|
|LoRa||subGHz||<50kbps||1.5km to 4.5km||Low||Medium|
|LTE Cat 0/1||Cellular bands||1Mbps to 10Mbps||Several km||Medium||High|
|NB-IoT||Cellular bands||0.1Mbps to 1Mbps||Several km||Medium||High|
|Weightless||subGHz||0.1Mbps to 24Mbps||Several km||Low||Low|
|Wi-Fi||subGHz, 2.4GHz, 5GHz||0.1Mbps to 54Mbps||<100m||Medium||Low|
Table 1: Wireless technologies – how they compare
Power consumed has an average value and a higher peak value, typically when the wireless transmitter is active. This can vary strongly between technologies with some typical values given in Table 2.
|Receive (Rx) power||90mW||84mW||28.5mW||55mW|
|Transmit (Tx) power||350mW||72mW||26.5mW||80mW|
|Average power 10 messages/day||500μW||414μW||50μW||60μW|
Table 2: Indicative power consumption of wireless technologies
Average power obviously depends on the communication activity but you may think that this will set the battery size for a target lifetime. For example, Sigfox, with an average power consumption of 60µW from a 3V rail, represents 20µA average draw. For, say, one year or 8760 hours life, would a battery of 20 x 10-6 x 8760 = 175mAh be sufficient? This falls easily within the nominal rating of a standard Murata CR2032R manganese dioxide lithium coin cell of 200mAh capacity with a rated voltage of 3V. The capacity specified is for 1mA drain and the cell is rated at 3mA continuous load current. However, on transmit, when 80mW of power is taken, the current demand is nearly 30mA. As the battery has an internal resistance of around 6 ohms at 23°C, the voltage drops by 0.18V, and if the battery is near the end of its capacity, defined as Functional End Point (FEP), a terminal voltage of 2V, the voltage to the load could drop out of specification. Worse still, even if the 180mV drop is acceptable, pulse loads on lithium cells reduce the total available mAh capacity so the lifetime may not be anywhere near the target. As an example, Figure 3 shows that if pulse loads of 30mA are taken from the CR2032 battery, with a duty cycle of 0.1, (1ms on and 10ms off), capacity can be nearly halved from the 1mA value.
These conditions are similar to the shortest connection intervals for BluetoothTM low energy and ANT+TM. The middle plot shows that even with a constant load of 3mA, which is the average of the 30mA pulses, capacity is reduced, and full capacity is only available if the rated 1mA is taken.
Figure 1: Full CR2032 mAh battery capacity is only available at rated 1mA current.
The capacities shown are for typical equipment temperatures, the battery itself is rated for operation from typically -30°C to +70°C and at the low temperature loses around 10% of capacity. At -30°C terminal voltage also drops by around an extra 200mV under load and there is an additional self-discharge of about 1% per year at 23°C.
It’s clear that practical lifetime of a lithium battery in a wireless application could be disappointing. In our example with a target of one-year battery life, a single CR2032 cell would be insufficient for the wireless function and that is not taking into account the current draw of other electronics. In addition, high current peaks through the ESR of the battery will cause voltage dips that could easily fall below the minimum operating voltage of the chipset, well before the battery capacity is expended anyway.
Supercapacitors prevent loss of battery capacity
A way to improve the situation is to eliminate the peak demands on the battery which result in voltage drops and loss of capacity by using a parallel supercapacitor. ‘Supercaps’ have a fraction of the energy storage capacity of a battery (energy density) but have much higher power density. This is manifested by the ability to supply high peak currents with little voltage drop through ESR values measured in just milliohms. With capacitance values available now in the thousands of farads at 2.7V rated voltage, supercaps are able to deliver significant energy pulses, leaving the battery to supply just the average current to the load, recharging the capacitor relatively slowly. An added benefit is that the coin cell can be replaced without a break in operation with the supercapacitor temporarily providing power.
Some supercapacitors are available with 3V rating, such as the AVX SCC series with capacitance up to 50 farads, and can be fitted directly across a CR2032 coin cell. The range operates from -40°C to +65°C and to +85C if the voltage is derated to 2.5V. In our example of 30mA current pulses, using a 1F capacitor from the SCC range with an ESR of 860 milliohms across a CR2032 coin cell, the current taken from the cell can be expected to reduce approximately in proportion to the cell and supercap ESR value ratio, around 10:1, leaving just 3mA battery current pulses, greatly extending its capacity. The voltage will drop by about 30mA x 0.86 = 26mV because of the capacitor and cell ESRs and the 1F capacitor will lose energy 3V x 30mA x 1 ms = 90µJ during the pulse. If the initial voltage is 3V, this energy loss in the capacitor will result in an additional voltage drop of less than a millivolt. This is calculated by equating 90µJ with the difference between energy stored at 3V, 0.5C(3)2 and the energy stored at the reduced voltage Vr, 0.5C(Vr)2 and solving for Vr.
In our example above for a Sigfox application where the peak is 30mA and average 20µA, extra resistance can be added in series with the coin cell to reduce its contribution to the peak demand and extend its capacity further. The supercapacitor is then sized with this resistor to ensure it is fully recharged between current peaks.
Supercapacitor leakage current is important
A practical consideration is the leakage current of the supercapacitor. This can be significant for high capacitance values but the AVX 1 farad SCC type mentioned has a maximum of 6µA. This is measured after 72 hours as it starts higher and reduces with time. The leakage directly consumes battery power and needs to be factored in when sizing the battery. Where this is an issue, other supercap ranges have much lower leakage values, down to around 1µA which would have negligible effect on overall capacity. For minimum leakage, temperature rise should be maintained as low as possible but up to around 45°C there is no significant increase.
In applications where the supercapacitor is rated at 2.7V or less at high temperature, a low drop-out (LDO) regulator is necessary between a 3V coin cell and supercapacitor to avoid applying over-voltage. Excess voltage has a direct effect on supercapacitor lifetime. Tests by AVX showed that derating to 70% voltage of a typical supercapacitor part in their SCM range enabled the part to pass 4,000 hour/85°C acceptance tests with no failure and ending with stable capacitance and ESR values. Any LDO used should have very low quiescent consumption to avoid loss of battery capacity and low voltage drop to utilise the battery voltage to the maximum as it nears FEP.
Humidity can also affect supercapacitor reliability and performance. For demanding applications, high-reliability parts are available in the AVX SCM range which have shown excellent performance in 4,000 hour tests at 40°C and 95% relative humidity.
Supercapacitors in series may need balancing
Supercapacitors can be placed in series to increase voltage and avoid the need for voltage limiting with an LDO. In this arrangement, it is important that the supercaps are matched for leakage current, otherwise one part could settle with a higher voltage, reducing its reliability. Balancing can be achieved simply with resistors in parallel with the supercapacitors but this wastes power and battery capacity. An active circuit achieves balancing with lower loss if the op-amp used is a ‘micro-power’ type. Figure 2 shows a typical circuit, where IC2 balances capacitors C1 and C2. In this case the source is an energy harvesting PV array. IC1 and Q1 clamp the PV cell voltage to be 2.754V maximum.
Figure 2: Supercapacitor C1, C2 voltage balancing by IC2 in a PV energy harvesting application
Supercapacitors are available packaged in series and parallel arrangements without the need for further balancing, with elements guaranteed to be a close match during manufacturing, such as in the SCM series from AVX (Figure 3).
Figure 3: Series connected supercapacitors up to 9V in the AVX SCM series
Supercapacitors can extend the operational lifetime of typical batteries used for low power wireless applications up to 10-20 years, effectively eliminating the need for replacement, allowing sealed products and reducing maintenance costs significantly. For optimum benefit and reliability, supercap sizing, leakage current and applied voltage should be carefully considered.
 High pulse drain impact on CR2032 coin cell battery capacity