SuperCap Charging and Delivery System using Solar Panels

[Farukh Khan]

Hello Guys,

I am trying to come up with a power system where I have 3 super capacitors, one attiny45 and 3 solar panels. I want the supercaps to get charged accordingly from the solar panels with over charging protection and some other safety measures and then deliver a constant clean 5V to some modules.

The things I have in hand now / The things I want to utilize for this project:

1. https://www.arrow.com/en/products/xv3550-2r7307-r/eaton - 3 Pieces
2. https://my.element14.com/microchip/...tiny-20mhz-dip-8/dp/1288352?ost=ATTINY45-20PU - 1 Piece
3. https://www.aliexpress.com/item/32861874199.html - 1 Piece
4. 5V 250mA 1.25W Generic Solar Panels - 3 Pieces

Do I need some sort of Buck/Boost convertor to get a constant 5V from those 2.7V supercaps? If discharged completely they might have almost 0V in them. What is the proper way to do this setup?

Help me out guys.

Thank You.

 

[Audioguru]

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.

 

[dave9]

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."

 

[Farukh Khan]

Sorry for the late reply.

So, I will answer @dave9 accordingly.

I have settled for 3 solar panels and 3 supercaps of the models I provided, due to the space constraints of my project. I am actually trying to develop a very small form factor self-sustainable AI system for weather observations. So, I must need to use the 3 panels and the 3 supercaps for highest possible efficiency based on my form factor. I want to use the microcontroller and voltage monitor for 3 different things. To monitor, battery, panel and usage voltage and amps. And to somehow limit the supercap charge and discharge cycles so that it doesn't overcharge or it doesn't discharge to very less voltages where my system gets unstable. And Lastly, as a feedback controller to my main SBC for shutting down things properly, if something goes wrong with the power system.

I did not choose battery because the usage from this power delivery system would be very low most of the time in my project. Sometimes it might spike to high power usage and then most of the time it will be a low powered system. And I want the reliability of supercaps instead of battery because this project might have some underwater usage in the sea area, and I am not suppose to put chemical batteries in the sea.

I am willing to achieve the best efficiency possible in small form factor from the supercaps for power delivery. So, really want to get as much juice as possible out of the caps when needed.

Hope I have cleared the project goal to some extent. Please do suggest me how I can achieve the goal.

 

[dave9]

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.

 

[Farukh Khan]

Does using three 2.7V 300F caps in series to achieve 8.1V 100F reduces the overall energy? Suppose if I use the three 2.7V 300F caps in parallel to give me an effective 900F and use a boost converter to run a constant 5V 1A load, and then do the same with 8.1V 100F with a buck converter, will it yield the same amount of time I would be able to use the constant load?

 

[Farukh Khan]

And I was also wondering, if anyone could propose me a 3 power source solution and a single regulated 5V output. The 3 power sources would be a battery, the 3 supercaps and the 3 solar panels working together for an efficient constant 5V output.

 

[Audioguru]

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

 

[Farukh Khan]

So can I go with 2.7V 300F 3 caps in parallel and then use th 5V 250mA three solar panels also in parallel to charge them using some sort of MPPT? And maybe then use a boost module for constant 5V output?

 

[Audioguru]

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.

 

[BobK]

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

 

[Farukh Khan]

@BobK continuous current draw at 5V is around 300mA. In high draw moments max current draw would be around 700mA for 3-7 seconds max and then fallback to normal. and burst spikes of 1A is possible for some amount of milliseconds in a certain action trigger. But won't go above 1A by any means. Project deployment will be in a very sunny area, so around 10 hours of proper sunlight per day can be expected atleast. Number of days without sufficient sunlight it would need not need to be operating. But if it is possible to get some runtime, why not?

 

[BobK]

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

 

[Farukh Khan]

@BobK the 300mA I mentioned is the maximum continuous power draw. But there will also be times when the CPU will have nothing to do at all, where the current draw can get as low as 50mA. So, it seems to me that the solar panels would work. What do you think? Because the burst current is only for 3-7 seconds during log download. Otherwise mostly the CPU will sit idle, unless we command it to collect a sample of data for a t period of time which will have continuous current draw of max 300mA. And this time for continuous collection of data may vary from couple of seconds to couple of minutes and maybe an hour at max. And how do I implement a battery in this setup? and use 3 power sources - battery, supercap and solar panels together?

 

[Farukh Khan]

@dave9 @BobK I am going to the lipo batter route for this project. Now, I just need an highly efficient way to use the 3 (1.25W) solar panels to get a stable regulated 5V@1A rated output for charging the lipo. Please do let me know which way I can achieve this. Should I go for a parallel solar panel connection or a series solar panel connection? Then should I use a simple boost converter or a buck converter for this? or any sort of MPPT or PWM module to achieve high efficiency or most possible power from the panels?

 

[Alec_t]

I just need an highly efficient way to use the 3 (1.25W) solar panels to get a stable regulated 5V@1A rated output

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

 

[Farukh Khan]

@Alec_t I said rated for 5V@1A not that it needs to provide 5V@1A. I know, the max it can provide at 100 percent efficiency(which is not possible) would be 5V@750mA not 1A.

 

[Audioguru]

I am going to the lipo batter route for this project. Now, I just need an highly efficient way to use the 3 (1.25W) solar panels to get a stable regulated 5V@1A rated output for charging the lipo.

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.

 

[Farukh Khan]

@Audioguru I myself know all the things you just mentioned. I got some redundant information rather than the exact information I needed which is how to use the solar panels to efficiently produce 5V output. I already have a charging and discharging circuit for the battery which will take in 5V and charge the batteries at their rated 4.2V max and 3.1V min. And btw, am not using the supercaps anymore in the system.

 

[Farukh Khan]

Looking for something like: https://www.dfrobot.com/product-1712.html or https://www.dfrobot.com/product-1714.html

But with higher efficiency and without the lipo or any other battery charging circuitry. Only the solar panel in, MPPT and 5V regulated USB output.