I want to build an autonomous outdoor temperature sensor that samples the temperature once per hour and stores the results over WiFi. I would use an ESP32 that will consume about 12 μA in stand-by (most of the time) and 200 mA when awake (15 seconds per hour, with peaks up to 600 mA when using WiFi.)
Because it is outdoors, I thought of using solar panels as an energy source and supercaps to store the energy (as batteries can't stand the temperature fluctuations). Several power harvesting ICs/supercap charging circuits exist, for example this module on Tindie, but they seem like overkill.
I am thinking of directly connecting a 3 V solar panel to 2.5 V supercaps, with a Schottky diode to avoid back flow and a Zener diode to limit the voltage. Then I would need a low quiescent current (maximum 10 μA) but high output current boost converter to convert the 2.5 V from the supercaps to the 3.3 V needed for the ESP32. Ideally, the input voltage of the converter would be as low as possible.
I can't find a converter to convert 1.0 - 2.5 V to 3.3 V with a low (<10 μA) quiescent current and a 600 mA maximum output current. Do they exist?
Ideally, the converter should be through-hole as my soldering skills are limited.
Some calculations for reference. Please correct me if I am wrong.
- Energy need is (200 mA × 15 s + 12 μA × 3585 s) / hour = about 0.95 mA / hour. This is about 24 mA / day.
- The sun doesn't shine every day. I would like 4 days autonomy = 100 mA energy storage.
- Assuming a useful voltage drop from 2.7 V to 1.0 V, this implies a capacitance of about 400 F to store the equivalent of 100 mAh @ 3.3 V. (0.5 × (2.7V2 - 1V2) × 400 F = 1258 J, 1258 J / 3.3 V / 3600 s = 0.106 Ah = 106 mAh)
- In the calculations, I neglect leakage current and other leakage loss, so I would put two 400 F in parallel.
- Re-charging the supercapacitors on a sunny day of 8 hours (in winter) would require 12.5 mA / h @ 3.3 V or 16.5 mA / h @ 2.5 V. So I would need at least a 16.5 mA solar panel (but again include a big margin.)