SuperCap Charging and Delivery System using Solar Panels

A super cap is not a battery.
A super cap discharging voltage drops steeply. A rechargeable battery keeps its voltage up until it is nearly dead.

 

It might help if we knew more about your project, for example why the need for supercapacitors and why you settled on using 3 of them and 3x 5V, 250mA solar panels, and what you seek to do with the microcontroller and voltage monitor.

Regardless, the solar panels are not a constant 5V, depending on the accuracy (and how) they're rated it may vary from that. Generally you want to keep voltage as high as possible to reduce vDrop losses when there's a charging regulation and delivery regulation involved.

With what you have, this limits you to 3 x 2.7V caps in series for (max) 8.1V and 100F capacitor bank. 2x series of 5V solar panels might achieve this (then you have one solar panel left over), or 3X in series, then you need a charge controller limiting output to the capacitor bank 8.1V or lower.

Since using solar cells and capacitors is somewhat of an intermediate length charge/discharge cycle based on sun, vs night, vs use of the 5V to modules, I am wondering why you feel supercapacitors are the better choice than a smaller, more cost effective battery? I mean I could see the supercapacitor purpose for a very high charge and discharge rate but that doesn't seem to be the scenario with a few hundred mA of solar power.

Anyway you could then use a boost/buck rather than a buck/boost, or just a buck if you accept some capacity loss.

I recommend that you start over and define the task then the best way to accomplish it rather than "I have a spoon, three fiddles and a gnome, what's the proper way to play Beethoven."

 

You seem to be insisting upon something that is a bad design and should be scrapped in favor of a better design, using batteries.

If you insist on using what you have, I already answered that. Build it, test it, compare to requirements.

The other way around is the correct way to do it. Exhaustively determine requirements, determine components to meet them, built a bench mockup (or in this case outdoor mockup since you depend on a certain level of solar charging), test, build/buy enclosure that hoses them.

Please show us this requirement to not put chemical batteries in the sea. Are you aware there may be chemicals in a supercap, and that other equipment used in the sea uses batteries?

It doesn't even make a difference regarding chemicals in the sea, the enclosure is necessarily needing to be waterproof/sealed whether there are batteries, capacitors, or a short lived hamster on a wheel powering it.

 

Capacitors in series do not equally share the voltage so one of them will have too much voltage and be destroyed.

 

The voltage on a capacitor drops very quickly at the beginning of a discharge. Its voltage drops slowly when the charge is almost gone.
The voltage on a battery drops slowly at the beginning and during a discharge until its charge is almost gone, then its voltage drops quickly.

A voltage boost module wastes power all the time by making some heat.

 

Series or parallel, they will store the same energy.

Your explanation contains no numbers so I cannot evaluate whether it is workable or not. It sounds to me like the circuit will draw some low amount of power continuously and then bursts occasionally (when transmitting data perhaps?) If that is the case, we need to know:

- Current drawn at 5V continuously.
- Current drawn in high draw moments and how long and how often these occur
- Daily amount of sunlight available
- Number of days without sufficient sunlight it would need to keep operating

Contrary to what @Audioguru is saying, when drawing the same power from the caps, the voltage will drop slowest when fully charged and faster as the voltage goes down since you need proportionally more current for the same power at the lower voltage. If you are able to get enough burst power at 0.9V, your 2.7V you would be able to get 89% (8/9) of the energy from the caps before recharging. If you needed 1.35V you would get 75% (3/4) of the energy.

Bob

 

To get 300mA at 5V will require about 600 mA at 2.7V. At this rate, your 900F capacitor loses 1 V in 25 minutes. And as it loses voltage, the current needed increases. Which means they will not power your device for one hour at its continuous draw.

As you have been told, use a battery.

Worse yet, let’s see how much energy you need per day and how much your solar panels will produce.

3 panels at 5V and .25 A will be 37.5 Wh in 10 hours.

Ignoring the higher current bursts, your device needs 5V at 0.3 A for 24 hours or 36 Ah, if everything is perfectly efficient.

So, you cannot power this device from the chosen panels even with the most optimistic assumptions.

Bob

 

Input = 3 x 1.25W = 3.75W. Output = 5V x 1A = 5W. Do you see a problem there?

 

A single Li-PO cell will explode then catch on fire if you try to charge it from 5V. You need a Lithium battery charging circuit that can be powered from 5V. The charging circuit limits the current and voltage properly and detects a full charge then disconnects the charging.

A rechargeable Lithium battery is ruined and is dangerous to charge again if it is discharged below about 2.7V. Its life is shortened if it is discharged below about 3.1V so a low battery voltage disconnect circuit is needed. The single Li-PO cell will be 4.2V when fully charged then its voltage drops to 3.1V slowly. It will need its own voltage boost controller.

You need a circuit to separate the super caps from the Li-PO when charging then it connects them together when discharging. usually diodes are used but diodes produce a 0.7V voltage loss.