Using a supercapacitor for power management and energy storage with a small solar cell, Part 2
In Part 1 of this series, we have reviewed solar cell performance, how to select and size the supercapacitor, requirements of supercapacitor charging circuits and charging IC characteristics. We will now use two case studies to illustrate these properties in detail.
Case Study 1: Using a small solar cell indoors at low light, 100 lux, to power a Bluetooth low energy sensor using CAP-XX GA109
In this case we used a low power BLE sensor operating in low indoor light down to 100 lux. The sensor only operates when there is light, so the supercapacitor only needs to support data acquisition and transmission.
We used a Sensor Puck BLE sensor reporting temperature, relative humidity and light level to a phone app every second. The sensor max-min supply voltage range is 3.0V to 2.0V, therefore we will use a single cell supercapacitor with a maximum cell voltage = 2.5V. Figure 9 shows the current and voltage waveform while the sensor is acquiring and transmitting data. The peak current is ~22mA for 1ms duration, while the sustained burst has an average current of 4.5mA over 12ms, with a small peak of 5mA at the end of the pulse. Figure 5 shows this current is >> solar cell current of 260µA at the peak power point. In this case, we have chosen a CAP-XX GA109 supercapacitor, 180mF, 40mΩ. This is a small prismatic supercapacitor that enables a slender attractive industrial design. The test setup is shown in Figure 8.
The voltage drop at the end of the 22mA peak = 22mA × 1ms/180mF + 22mA × 40mΩ = 1mV.
The voltage drop at the end of the 12ms, 4.5mA pulse with a 5mA peak at the end = 4.5mA × 120ms/180mF + 5mA × 20mΩ = 5mV. Figure 5 shows a voltage drop over the pulse of ~6mV which confirms the calculation. This is a negligible drop and enables the GA109 to support the Sensor Puck for multiple transmissions. Figure 9 also shows the Sensor Puck transmitting every second, and the supercapacitor voltage decaying slightly between transmissions. This is because the solar cell charge power < average power drawn by the sensor transmitting once per second.
Figure 8 Lab test setup with small solar cell
Figure 9 BLE sensor current during transmission
To make the system sustainable, the load power must be limited to the solar cell charging power. We have done this, referring to Figure 10, by including a comparator with hysteresis, U1, controlling a FET, M1, between the supercapacitor and sensor. When the supercapacitor reaches 2.4V M1 turns ON, enabling the sensor to run. When the supercapacitor discharges to 2.2V M1 turns OFF disconnecting the sensor and enabling the supercapacitor to be re-charged to 2.4V. Figure 10 shows our charging circuit, with five solar cell strings in parallel supplying the AEM10940 charging the supercapacitor.
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Now that since we are disconnecting the power supply to the BLE sensor to regulate the average load power, when the sensor is turned on it will re-initialize. This draws an average of 12mA for 2.1secs. The GA109 voltage drop during initialization = 2.1s × 10mA/180mF + 12mA x 40mΩ = 117mV. Our 200mV hysteresis allows the GA109 to support sensor initialization plus several transmit bursts.
Figure 11 shows the effectiveness of this circuit charging the supercapacitor at 100 lux and then maintaining transmission. 100 lux is very dim light, insufficient to read by, illustrating that a solar cell solution is feasible even in such poor light.
Figure 11 shows that in such low light it took 45hrs to charge the GA109 supercapacitor from 0V and then 2.6hrs to re-charge after it has supplied a transmission burst from the sensor. Designers can overcome the slow initial charge time by pre-charging the supercapacitor at installation. Figure 12 shows how effectively the system works in reasonable light. 650 lux is the light level in a well-lit office, supermarket or factory floor. Figure 3 shows that at 650 lux, solar cell power is just over 0.4mW. It only takes 31 mins to charge the GA109 from 0V and 2 mins 5s to re-charge the supercapacitor after supplying a transmission burst. The 2.9s duration for the sensor on time indicates the GA109 is supporting initialization followed by a transmit burst.
This case study has shown how a small solar cell and efficient charging IC can charge a supercapacitor to support low power sensor acquisition and transmission. The e-peas AM10940 is an efficient IC that can charge the supercapacitor even when the solar cell provides only 150µW, and does so much more effectively with an increase in solar cell power to a little over 400µW. The GA109 is a small, thin supercapacitor, with sufficient C and low ESR to enable construction of an unobtrusive sensor suitable for wearables or indoors where cosmetic appearance is important.