Author Topic: Super Capacitors  (Read 614 times)

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Offline Pete

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Super Capacitors
« on: June 03, 2021, 09:26:42 pm »
I am seriously thinking of adding a super capacitor bank to my off grid solar setup.
I have a 660 amp hour battery bank and 24 volt system. It does what I want but sometimes the startup current of things like my air compressor seem lacking.
The compressor runs fine but starting under load it sounds like the inrush current is a bit high for the batteries.
So I have come across a bank of 500 Farad super capacitors, that I am thinking of putting in parallel with the batteries to supply those large startup currents.
Does anyone have any experience of using super capacitors with their systems?
Pete

Offline lighthunter

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Re: Super Capacitors
« Reply #1 on: June 05, 2021, 09:48:10 am »
Hi Pete! my first thought was... the discharge curve of capacitors have a very steep initial slope and i was thinking no.
But, i did the math and assuming its correct, it would work very well. I tried to use practical numbers here for clarity.

If the 24v equivalent load is .1 ohm approximately 240Amps and you have a battery voltage of 24.5v and you start the compressor, 500F can provide a 5-6kw boost for just over 2 seconds and finish at 23.5v charge level. Thats not bad, the batteries would add some to the start as well. Pretty amazing really.

 There is something called effective series resistance with a capacitor and this was not in the calculation. May not hurt to check how much but either way its gonna reduce the sag.
 Might be fun to have a look at bat V with a scope or a min function on multimeter. Ive also had issues with compressors where the decompression valve isnt working and the motor has to start while compressing air. If this is working properly, the start with 90 psi in tank should be the same as an empty tank. 660AH @ 24v really shouldnt struggle with an air compressor start but i could see caps making a big improvement when charge level is 50%

All the best!
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LH

Offline Pete

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Re: Super Capacitors
« Reply #2 on: June 05, 2021, 06:12:15 pm »
Thanks LH, from what I read the capacitor banks are supposed to be able to supply 1900 amps when starting train motors.
Your calculations are much appreciated.
I had heard of supercapacitors being used in electric cars but so far never heard of people using them in solar setups.
I have ordered them and will post again here when I get them and see what difference they make.
Thanks for your help
Pete

Offline rossw

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Re: Super Capacitors
« Reply #3 on: June 05, 2021, 08:29:54 pm »
I have ordered them

Just make sure that you get ones rated for the voltage you'll be using them on.
There are lots that are quite low (2.7V for example) that will be an expensive but probably "exciting" experience when connected to 24V!

Offline Pete

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Re: Super Capacitors
« Reply #4 on: June 05, 2021, 09:34:53 pm »
Thanks Ross, I have ordered two 16 volt banks, I will put them in series, which I guess will mean I will only have 250 farads, but that will help the batteries a fair bit with starting currents.
The company I am getting them from said that they need a resistor in series, to charge them up to the battery voltage before they can be connected in parallel.
From what I have encountered with my inverters charging up their capacitor banks, I can well imagine the spark that would happen if they were just paralleled without a pre charge.
The capacitor banks are second hand from train starting applications, so hopefully they are changed regularly in that situation.
They are $198 per bank, which seemed like a reasonable price for experimenting.
Cheers
Pete

Offline rossw

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Re: Super Capacitors
« Reply #5 on: June 05, 2021, 09:46:50 pm »
I have ordered two 16 volt banks, I will put them in series, which I guess will mean I will only have 250 farads, but that will help the batteries a fair bit with starting currents.

Yep. Might think about some sort of balancing resistors, or if you're running them on 24V, perhaps some 14V zeners across them. It's all fine while they're balanced, but things don't always stay that way...

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The company I am getting them from said that they need a resistor in series, to charge them up to the battery voltage before they can be connected in parallel.

Absolutely!

All jokes aside, old car headlights are great for this (or on 24V you might need two, in series). When there's a lot of current flowing, the globes just light up and increase in resistance - a visual indicator that they're pulling current, and safely limiting the inrush.

As the current flow reduces, they cool off and resistance falls lower and lower. When they're no longer glowing, you can bypass them.

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The capacitor banks are second hand from train starting applications, so hopefully they are changed regularly in that situation.
They are $198 per bank, which seemed like a reasonable price for experimenting.

Sounds like a good deal actually.
They'd almost make a decent "jump pack" for starting a car.

Offline Pete

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Re: Super Capacitors
« Reply #6 on: June 06, 2021, 12:30:10 am »
Hi Ross, I have seen some balance boards on ebay for super capacitors. I may have to do some research on that.
Thanks for the ideas.
I will be running them across my 24 volt battery bank. Won't the regulators on the solar system prevent them overcharging?
Pete

Offline rossw

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Re: Super Capacitors
« Reply #7 on: June 06, 2021, 04:16:43 am »
Won't the regulators on the solar system prevent them overcharging?

Short answer: NO.

Longer answer: when charged, they are two very low current "things" in series.
If one of them is ever so slightly less resistance than the other, it will have a higher voltage across it.
Imagine if you will, two 1K resistors in series across your 24V line.
They'll have 12V across them, each. And that's true.
But what if one is 900 ohms and the other is 1100 ohms?
It won't be 12V any more, it'll be 13.2 across one and (24-13.2) = 10.8 across the other.
Now, what if it isn't 24V, but equalize voltage while you're charging... with lets say 29V across your bank, that should be a "safe" 14.5V across each, right??
With the same imbalance (only 10%) we're now up to 16V on one.... and 10% is unrealistically tight for tolerances in capacitors of that size!

edit: for clarity, when I say "resistance", I'm talking in general terms.
Lets say one of your capacitor banks is 450F and the other is 550F.
When they're charging, one will charge faster than the other, given it's a series circuit (two in series) so the one that's a lower capacitance will reach full charge before the other... and even if they charge equally (which they won't), one will likely have a higher leakage current... there are many ways in which an out-of-balance of two components will manifest itself.

Not saying it will, just saying it CAN. It depends how risk-averse you are!

Offline lighthunter

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Re: Super Capacitors
« Reply #8 on: June 06, 2021, 06:25:32 am »
Hi Pete,I was thinking of the balancing need as well however, since these are a "bank" they likely already have the components for balancing. If not, a few zener diodes and resistors would solve the problem. Newark sells 2.7v zeners for 40ยข each.

A little late to bring up now but i think similar results could be obtained by paralleling 8 40AH LFP cells (ive done and it works well),  guessing the caps would have more surge capability but lfp cells would add more storage.  6 of one or half dozen Ha! 
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Offline Pete

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Re: Super Capacitors
« Reply #9 on: June 06, 2021, 05:41:13 pm »
Thanks Ross and LH, I will see what the banks have when they come. I have looked them up online and it seems that they have balancing boards on them. I will have a look when they get here.
LH yes I can see what you say about the LifePo cells but they are very expensive here. Maybe one day when they get cheaper I will look at them. For now they are unobtanium for me. I have a fairly new bank of BAE gel batteries that I will look after as good as I can and hope that they will see me out, or at least new technology comes in that makes changing them it 10 or 15 years much more affordable.
My job now is to dig a 40 metre trench for an underground run from my new panels ( on a ground mount ). I have dreamt for years of the idea of making portable holes, still haven't managed to develop the technology, now I need a 40 metre long 500 mm deep trench. If anyone has a spare they dont' need it would be great.
Thanks for your help
Pete

Offline rossw

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Re: Super Capacitors
« Reply #10 on: June 06, 2021, 11:44:49 pm »
I have dreamt for years of the idea of making portable holes, still haven't managed to develop the technology

When I were a wee lad, we had a book "The bottersnikes and gumbles". They'd worked it out, perhaps look them up?

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now I need a 40 metre long 500 mm deep trench. If anyone has a spare they dont' need it would be great.

I can knock you up some 500mm deep holes that would be a good start if you put them close together.
When would you like to come get 'em?

Offline Pete

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Re: Super Capacitors
« Reply #11 on: June 07, 2021, 01:31:14 am »
Not sure about traveling to pick up holes with the pandemic. I may end up in the wrong place and not be able to come home to my island.
I went around today marking the trench out. I pushed a metal rod into the ground to find where the big rocks are. Then marked the route to get around them. Not too many bends but there are some.
We are in the middle of a wet patch at the moment so will wait until it is over and hopefully the ground will be nice and soft.
I will have to look up those bottersnikes and gumbles, they may have the answer.
Cheers
Pete

Offline Pete

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Re: Super Capacitors
« Reply #12 on: June 27, 2021, 12:25:19 am »
Just an update, I have finished installing the capacitor bank. The capacitors had balancer boards fitted so that was no problem.
I checked them out this morning and they are doing the job I wanted.
The air compressor starts very easily with no hesitation.
I checked the current being supplied by the capacitor banks when I started my saw bench and my cut off saw. The readings showed that the capacitors were supplying 55 amps when the saw bench started. So they are helping the battery bank heaps.
I have finished installing the solar panels too and we have got rid of our gas appliances.
I have a single induction cooktop ( which works great) and a benchtop oven which also runs really well.
So we are both happy to not support the gas industry now.
Thanks for the help
Pete

Offline lighthunter

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Re: Super Capacitors
« Reply #13 on: June 27, 2021, 11:21:53 am »
Thats good news! way to go Pete! Being completely off grid
as you are presents challenges that i know little about and is a very different situation as Ross pointed out. To "stiffen" the supply at any voltage is a clear win. Glad it worked out and getting rid of gas is awesome!
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LH