I bought three super capacitors from Adafruit, model DDLE2R5LGN701KAA5S. They are rated at 700 Farad at 2.5V. (They have very little stock left, at time of writing, so if you want them too, act quickly!)
If I place them in series, I will end up with 233 Farad at the very maximum of 7.5V. However, charging to the max 7.5V is dangerous: one of the capacitors can easily overcharge and explode, that way. Unless you balance the capacitors.
Balancing a capacitor bank is not trivial. But there seems to be an alternative, as offered by Patrick of EngineeringShock.
Patrick's advice is to charge the capacitors to 80% of their nominal Voltage, so that even if there is a mismatch in internal resistance between the capacitors in series, none of them will overcharge. I like this approach, as it is simpler, does not require extra circuitry, at the cost of lost capacitance.
So if we charge a 7.5V bank to 80%, we get a charging voltage of 6 Volts. Let's make that our target. When we have have our bank charged to 6V, we can feed that into a voltage regulator, and as the bank drains to 3.3V, we should be able to ride from 6V to 3.3V and run a μcontroller off that, or some other low-power electronics.
And this leads me to my next objective: how to generate a steady 6V charging voltage? I would like to use solar panels for this. So let's dive into photovoltaics.
Reading the specification of a PV panel, there is the nominal voltage, which we can't really rely on. Then there is the Voc (Open Circuit Voltage) which we will measure if there is no current flowing. And also the Max Power Voltage, which is lower.
I need to read up on how to make sure that a photovoltaic panel output a steady 6V, and never any higher, at a tight tolerance. PV panels typically use charge controllers for that, but I want something simple. Maybe an opamp?
I am taking a different approach. I've decided to charge the capacitors in parallel, not in series, at a steady 2.5V.
To achieve this, I take input from a solar panel (with wildly varying voltage, and current) and convert that with a buck-converter to 2.5V that can be used to charge the capacitors.
I then use the capacitors as input for a boost-converter that brings a varying low voltage (It is only 2.5V when fully charged, and they discharge linearly) to a steady 3.3V. This I can then use in a client circuit, typically a μcontroller with some peripherals attached.