Renewable Energy Questions/Discussion > Renewable Energy Q&A

Super Capacitors

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Pete:
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

lighthunter:
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!

Pete:
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

rossw:

--- Quote from: Pete on June 05, 2021, 06:12:15 pm ---I have ordered them

--- End quote ---

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!

Pete:
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

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