Okay, so now that I understand how a capacitor works better, let’s try my hand at selecting a supercapacitor. How do you know what parameters are needed when selecting a supercapacitor? Remember, the capacitance is an indicator of how long your capacitor can supply a particular voltage before the voltage sag becomes too great for your computer to keep operating off of the remaining power. For simplicity, rather than using a boost switched-mode power supply and a smaller value supercapacitor, we can just use a larger value supercapacitor. Besides, it’s rechargeable and it’s short-term charge, so it’s not as essential to maximize the running time on a single battery or charge.
C = Q / V
V = Q / C
Q = C * V
C = 1 F
V = 5 V
Q = 5 C
C = 1 F
V = 3 V
Q = 3 C
Okay.. so what does that mean? It means I can drain 2 C of charge before the supercapacitor’s output voltage sags to 3 V. How much time is that in terms of Raspberry Pi Zero + Camera current draw, 600 mA?
I = 0.6 A
Q = I * t
t = Q / I = 2 / 0.6 = 3.333 sec.
And that is a maximum time… because of the buck switched-mode power supply, we will draw more current as the voltage sags.
Minimum time, compute effective current draw at 3 V.
P = V * I = 5 * 0.6 = 3 W
I = P / V = 3 / 3 = 1 A
t = Q / I = 2 / 1 = 2 sec.
So if we want to run 10 times as long, we want a 10 F supercapacitor. That allows us 10 times more time to be lazy in winding our manual winding charging coil. 50 times as long would be nicer. So now you’re talking a 50 F supercapacitor.
Alas, I look at the prices… and I say hey, a 40 F supercapacitor would be nice, but gosh that is relatively expensive. I can still live with a 25 F supercapacitor, 1 minute of Raspberry Pi runtime is plenty. Also, the fact is that getting supercapacitors capable of putting out 5 V is a lot harder than looking for the supercapacitors with a maximum voltage of 3.3 V, 2 V, or even lower.
Okay, another thing to remember… the larger the capacitance, the more you have to charge it in order to get up to the higher voltages.