Renewable Energy Questions/Discussion > Renewable Energy Q&A

Super Capacitors

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rossw:

--- Quote from: Pete on June 05, 2021, 09:34:53 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.

--- End quote ---

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...


--- Quote ---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.

--- End quote ---

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.


--- Quote ---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.

--- End quote ---

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

Pete:
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

rossw:

--- Quote from: Pete on June 06, 2021, 12:30:10 am ---Won't the regulators on the solar system prevent them overcharging?

--- End quote ---

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!

lighthunter:
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 40¢ 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! 

Pete:
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

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