# Solar Panel and Super Cap

Hi.

Now I have been browsing google for a while and can not get a straight forward answer...maybe my google skills suck, but I have seen loads of schematics for a "solar charger for super caps" but not an answer to the basic question...

Can I attatch a 5.5V super cap to a 5V solar panel without killing it?
Also a diode on the + line to stop current feeding back to the panel once the panel voltage dips below the cap voltage.

Does the cap keep pulling current and slowly increase in voltage till it pops?
Or will the cap stop drawing power and the solar panel will just waste any light it is getting though heat on its own internal resistance?

What is a 5V solar panel ? Is that a solar panel with electronics to keep the output voltage at 5V ?

A super cap of 5.5V is two super cap cells of 2.7V.
A higher voltage will damage the super cap for sure.
Here is a video : Let's BLOW UP some SUPER CAPACITORS! - YouTube

A diode can be used to prevent current going backward into the solar panel. It is better if a circuit is used without the need of a diode.
A super cap also leaks. It is not the same as a chargeable battery.

As in "in full sun/100,000lux ish , this panel will produce 5V".

So not over 5V in the good old UK.

Does charging the caps at a voltage lower than their rating protect them? Do they suddenly stop charging and the energy is given off as heat via any resistance before it?

Or will it continue to charge and go bang at some point?

Whats the open circuit voltage of the Solar Panel?
Most Solar Panels have a voltage rating that dependant on the power output.

A solar panel rated at 5 volts, will probably put out close to 7 volts.

Your super caps, are rated at the highest voltage that they can be trusted to survive (any more is russian roulette) .

Use a solar controller to keep the voltage in a safe range.

I have solar panels charging my super caps (12 volt system [13.9v max]), but I depend on the voltage controller to keep it is a safe range.

If the output voltage of a solar panel is indeed limited, the voltage will not get higher. Then the capacitor will not continue to charge further and the energy of the solar panel is just not used. It is not turned into heat, it is not used. However, the voltage is not (hard) limited.

The 5V could be for a certain current, and it could be 7V open voltage (with no load). I think you have to protect the super capacitor and you can't connect it to an 5V solar panel.
To directly charge a capacitor, I would use a diode and normal capacitor of 16V.

(while I was typing this, mauried already wrote about the open voltage and jack wp already mentioned the 7V. Well, I'm going to post this anyway. We all three agree on this.)

Are you trying to use the super capacitors as a battery substitute? Or what is your goal?

If you have two super caps rated at 5v (actually that adds up to 4 cells of 2.5v each cell), I would feel safe hooking them in series (equals 10V), and use a 5v solar panel. I would first use a volt meter tho, to make sure that the solar panels do not exceed 10v at high noon. But if you add a voltage regulator, you will get more bang for the buck of solar light.

Easiest way is a zener load regulator across the panel, e.g. 5.6volt/1watt.
Or a 5.1volt zener and power transistor if the panel is a bit bigger.
The backflow diode drops another ~.3volt.
Don't use the (leaky) zener across the cap.
Leo..

Wawa:
Easiest way is a zener load regulator across the panel, e.g. 5.6volt/1watt.
Or a 5.1volt zener and power transistor if the panel is a bit bigger.
The backflow diode drops another ~.3volt.
Don't use the (leaky) zener across the cap.
Leo..

Thanks! The zener diode looks like a nice easy bet.
Just ordered some 5.1V diodes.

Johnny010:
Just ordered some 5.1V diodes.

So you're going to use a power transistor as well.
A 5.1volt zener across a small panel limits it to ~5.1volt, but you also have to use a backflow diode between panel and cap.
That (schottky) diode drops another ~0.35volt.

You can always increase the zener voltage by adding a normal diode in series.
Leo..

Ah damn it!

Ok, well I want a zener to place across the panel in parallel to limit the voltage. A Schottky in series along the + line after the zener to stop back current flow?

As the Schottky drops 0.3V ish, id want a zener with a breakdown of 5.6V ish to take me to 5.3V to the load?

EDIT: Realised my comment below is dumb. I need to limit the current over the diode ofc. otherwise it will become more like a bulb filament.

There appears to be a use of a current limiting resistor in series a lot of schematics before the zener...I assume I will need this?

It depends on how many watts your panel puts out. If your panel only puts out .5 watts, then a 5.6v 1w zener should not have a problem.
What is the rating of your solar panel?

Looks like I wont need it then. Thinking AA batteries are more what I need now. I kinda wanted to try the caps but will save them for another project.

I want to top up 4AAs with 300mAh a day. Assuming Id have 33% max power from the solar panel due to living in the UK, it would need to be putting out aprox 1Ah (300*(100/33)) over the 8 hours of light in winter = 125mAh

The panel produces 180mA 5V 1W apparently (WN 11 99x69).
I do not know where the 1W comes from, as P=IV = 0.9W (maybe the just say "1W" to round it up?).

So I am technically safe I assume to dodge the current limiting resistor as the diode is happy to the 0.9W the panel produces on paper?

I think your calculations are pretty accurate.
If you are ok with 98% safety, then that should work.

To properly charge 4xAA batteries you will need a 6V - 8V panel. 4xAAs at full charge is around 5.6V. For solar charging AAs I'd go 2V charge per battery,these cells work great.

I think our use of the term "AA" is not correct. Since AA is a physical size, rather than a voltage. So, we really need to discuss the voltage of each cell. is it 1.2 v, 1.5 v, 3.8v. Is it nicad, nimh, li/fes, li-ion ?
Even tho they are all AA size, their voltage, and characteristics are all different.