Charging Caps with solar

[tjones9163]

Hello, I currently have a 2S 2P solar configuration or (2 connected in series and then those are connected to another 2 in parallel. Each solar panel is 5v. I will have pictures below. But pretty much I am able to get around 8v with good light in my 2S 2P configuration, and then I use that voltage to charge a 1000uf 50v electrolytic capacitor to around 6v (checking with my multimeter) then I try to connect the output of the capacitor to a small toy 6v motor( the type you get in basic Arduino kits). With that setup I am only able to see a very small movement of the motor then the capacitor is discharged, then I added another 1000uf capacitor in parallel and charged them both to 6v and then connected the output to the motor and was able to get a little more movement but still barely any.

I am trying to learn more about electronics and what I am trying to understand is - without the capacitors and just the panel and the motor and I am not able to get any movement because I am limited by the very small 2 ma output from my solar configuration(which I checked with my multimeter) so that is nowhere near enough to drive a motor even connected in parallel as I have it. My question is when I am using my capacitor i let it charge to 6v. So does it have 1000uf capacitance at 6v or how does that work? I know that capacitors store charge on separate plates, but how does capacitance relate to built up current? or does it not work like that? I also tested with a timer and saw that with the same voltage it take way longer to charge a 1000uf capacitor then it does to charge a 10uf capacitor. so should the capacitor be thought of as energy stored? or what would be a good analogy?

I know this is a lot of questions but I am just trying to wrap my head around this. Thanks

[slipstick]

You're seeing what you should expect. If you're charging the capacitor at @ 2mA and then the motor tries to draw out say 200mA the capacitor is going to be flat in about 1/100th the time you had it on charge and well before then the voltage will have dropped to almost nothing.

In fact some modellers do use capacitors to run very small DC motors but the ones they use are supercapacitors with values of 10-50 FARADS. So at least 10,000 times the capacity of the ones you're using.

Try your capacitor driving an LED instead of a motor and you might get that to light up for a short while.

Steve

[Grumpy_Mike]

should the capacitor be thought of as energy stored? or what would be a good analogy

Yes.

Think of a capacitor as a glass and the charge as the water.

A capacitor stores charge which is measured in coulombs normally given the symbol Q, the formula linking this to voltage and capacitance is
Q = V * C
Where V is in Volts and C is in Farads.

Current is the flow of charge and a flow of one coulomb per second is the definition of an Amp.

So you see with a 1000uF capacitor charged to 6V you don’t get enough to turn a motor because the start current for motors is normally over an Amp. But you can light an LED with a resistor for quite some time because you only have a load current of 20mA.

Fun fact
It takes 6.2415 × 10 ¹⁸ electrons to get a charge of one Coulomb.

That is 6241500000000000000 electrons.

[system]

Looks like you have 4 solar cells in series, can't make out that drawing.

[ReverseEMF]

Here's some crude math [sans Calculus, which I barely remember much of, anyway ];

IT = CV [the form that's easiest for me to remember]
T = CV/I [after whipping a little algebra on the thing]

What that means is, for a constant current I, it will take T amount of time to charge the capacitor, C, to a voltage V.

So, in your case, the math would be:

T = 1000µF(6V)/2mA = 3 Seconds

Now, if your motor requires 200mA to run, and requires at least 4V to keep running, the formula becomes:

T = 1000µF(6V-4V)/200mA = 1000µF(2V)/200mA = 10mS

Now, that's a HUGE over simplification, because, as Grumpy_Mike pointed out, the motor is going to require a large current to get it started, AND, the current will likely drop, as the Capacitor voltage sags and the motor turns slower and slower until it stops at 4 volts [totally made up scenario, of course ]. Typically what happens is, the current spikes as the motor spins up and then settles at some sustaining current. And, because this is a capacitor, the voltage will not remain steady, but will gradually fall, thus making that "sustaining" current change with time. So, to really do the math right, Calculus is required. But, my cheesy little formula can get you in the ballpark.

For instance, if we try this with a 100F SuperCap:

T = 100F(2V)/200mA = 1000S

And, if we take a stab at calculating the impact the start-up current has:

V = IT/C = 1A(200mS)/100F = 2mV [The amount the voltage on the SuperCap drops while starting the motor, assuming a .2S start up time, and a 1A average current -- insignificant -- but real-world might be different! For instance, say it's more like 2A, and 1S:

V = 2A(1S)/100F = 20mV -- still insignificant -- so, I'm betting it really is!

Also, here's how long it would take to charge that SuperCap, from totally discharged:

T = 100F(6V)/2mA = 300,000 Seconds = 83.33 Hours!

And, here's how long it will take to charge the SuperCap if it has a starting voltage of 4V ['cuz, the motor stopped turning, so we disconnected the Cap]:

T = 100F(6V-4V)/2mA = 100,000 Seconds = 27.8 Hours!

Probably should invest in a larger solar array!

[Conservator2]

Also IF you are going to buy super capacitors, then please note that most of them are rated at only 2.7V. So you would need to put two of those in series, creating a 5.4V capacitor with half the original capacitance. And you would need to connect all your solar cells in parallel to not over voltage the capacitors. Also make sure the capacitors have an equal load before connecting them in series. (By connecting them in parallel for just a short moment)
Succes!

[MarkT]

If you want to power a motor, only a supercap/dual-layer cap has a chance of being useful, motors are greedy for power, even electrolytics only store a tiny amount of energy.

Supercaps tend to have energy densities about 50 times that of electrolytics and batteries have energy densities 10-100 times that of supercaps...

The formula for stored energy in a capacitor is 0.5 x C x V^2, units being joules, farads and volts.

So a 10,000µF cap at 10V only stores 0.5J (only enough to twitch a motor for a second). A typical AA rechargable cell stores about 10,000J which can run a 1A motor for a couple of hours.

As is often the case, a back-of-the-envelope calculation will be a useful guide to what is possible.

[tjones9163]

MarkT:
If you want to power a motor, only a supercap/dual-layer cap has a chance of being useful, motors are greedy for power, even electrolytics only store a tiny amount of energy.

Supercaps tend to have energy densities about 50 times that of electrolytics and batteries have energy densities 10-100 times that of supercaps...

The formula for stored energy in a capacitor is 0.5 x C x V^2, units being joules, farads and volts.

So a 10,000µF cap at 10V only stores 0.5J (only enough to twitch a motor for a second). A typical AA rechargable cell stores about 10,000J which can run a 1A motor for a couple of hours.

As is often the case, a back-of-the-envelope calculation will be a useful guide to what is possible.

Awesome explanation everyone. I will buy some dual-layer supercaps and give it a go.
Also isnt the technology used with supercaps used in things like solar gadening lights, that slow charge a super cap during the day and discharges it at night?
Thanks again.

[MarkT]

Also isnt the technology used with supercaps used in things like solar gadening lights, that slow charge a super cap during the day and discharges it at night?

No, they use rechargable batteries, which can store energy for longer (supercaps leak on short timescales)
in a much smaller/cheaper package. NiMH/LiPo typically.

[ReverseEMF]

Conservator2:
Also IF you are going to buy super capacitors, then please note that most of them are rated at only 2.7V. So you would need to put two of those in series, creating a 5.4V capacitor with half the original capacitance. And you would need to connect all your solar cells in parallel to not over voltage the capacitors. Also make sure the capacitors have an equal load before connecting them in series. (By connecting them in parallel for just a short moment)
Succes!

Also, Super Caps are quite over-voltage intolerant! And, when they explode, it's loud [and scary] -- personal experience
Also, probably a wise idea to use a charge balancing circuit on any SuperCaps in series. Otherwise, the charge distribution can get unbalanced, and more explosions!
Also, probably not good to charge a 2.7V SuberCap, above around 2.5V.
I had a conversation with an engineer at Maxwell, and he suggested I "form" new Caps by charging then to 2.5V, and keeping them at that voltage for 72hours.