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Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462
Posted: 03:29pm 20 Aug 2012
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Read some more information about Supercapacitors and got bit worried about true level of self discharge.
Some publications say 1% per 24 hours, which sounded great.
Came past article on 2 types of Balancing one bleeding and the other one more expensive that did not use power (negligible level ), but what struck me was comment that self discharge was 1% per hour.
Another article talking about different types of electricity storage systems, that talked from lead acid batteries through NiCad, Li-Ion, LiFePo, Supercapacitors,it clearly stated that Supercapacitors are for “Short term storage”.
This would rather confirm 1% per hour losses than 1% per 24 hours.
For some reason I did not save pages, to check again, but it should not be too hard to bump into those pages again.
Overnight got thinking along the lines, that maybe it depends on year of manufacture.
Possibly earlier produced Suercapacitors were prone to higher self discharge than ones made in last few years.
Supercapacitors are not that new technology and could be improved with time.
Could somebody help me here?
I understand that charging without balancing say 10 Supercapacitors 2.5V connected in series, some of them will be destroyed while others will have hardly any charge.
Despite fact that for example 18V solar panel with 21.7V short circuit voltage will be well under 25V maximum allowable for the lot.
George
Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333
Posted: 05:37pm 20 Aug 2012
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Im not sure on the exact details of self discharge, but from your comment i wonder if its 1% per 24h, and with a bleed resistor it drops to 1% per 1h.
To be honest i dont feel super caps are worth adding to a system and the money better spent else where.
Super caps are great for picking up the slack in high power drain requirements with a instant result, like in the use of high current amps that will cause the power rail to sag briefly before the power supply will catch up.
This is not the case with the average battery system we run so of little benefit, although they could be good for a system like Trev runs in his electric car where when he stands on the throttle huge power is demanded for a short instance which is where the super caps would work best.
Pete.Sometimes it just works
Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904
Posted: 03:51am 21 Aug 2012
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I assume you meant "21.7V *open* circuit voltage Klaus
Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904
Posted: 04:18am 21 Aug 2012
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Pete, I just looked up the specs for the 3000F Maxwell ultra caps I have, they do not quote any self discharge as a percentage. Rather a leakage figure of 5.2mA max is given. Since their short circuit current is over 9000 Amps it could take some time before its fully discharged.
I might find out how that relates to terminal voltage as I do have a spare capacitor on hand. That would involve charging it up to 2.5V and then measure the terminal voltage at intervals. IT may take a while as I'm busy with other projects but I'll report here once I have some figures.
You are right as of their best use, in my case its to provide the big 'kick' when the fridge compressor kicks in. Or it can also absorb the extra solar if the sun peeks out from a cloud suddenly faster than the battery bank.Klaus
Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462
Posted: 07:06pm 21 Aug 2012
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Sorry, it should be "21.7V *open* circuit voltage"
(mixed up with "short circuit amperage")
Looking through more info and it definitely says that up to 2 supercapacitors can be safely used without any type of balancing, above that it is essential.
Supercapacitors are portrayed as something with almost endless life and are compared to lead acid batteries with relatively shorter life.
(This makes somebody who has little idea like me, believe that supercapacitors can be used as alternative to Lead Acid battery.)
George
Tinker Guru Joined: 07/11/2007 Location: AustraliaPosts: 1904
Posted: 04:19am 22 Aug 2012
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On my lithium battery balancer I use 2 smaller supercapacitors (100F each) in series without any balancing.
You are right with their life span, measured in millions of cycles and about 20 years.
However, capacitors store energy *very* differently from a battery and you really cannot compare the two.
In a battery electrical energy is the product of an electro - chemical reaction. This can be maintained at a steady output voltage for a considerable time until the process requires reversing in the form of recharging.
Capacitors store energy as a charge potential that is great for instant energy kicks but they don't last. The higher the current drawn the quicker the terminal voltage drops.
You may power a LED light for some time but it will get dimmer and dimmer all along.
If you try to power a motor or other high power device the stored charge will drop much sooner than being useful to power such a device.
You can find small super capacitors inside computers to maintain the memory chips and that works well for a long time as the power draw is in the nano Amp range.
You'll never find super caps in laptops to replace the batteries, it just does not work.
Klaus
Georgen Guru Joined: 13/09/2011 Location: AustraliaPosts: 462
Posted: 06:02pm 22 Aug 2012
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Thanks for reply.
For experiments I got ‘Step-Up Step-Down Converter’ 2A
(In 2.8 – 32 V, Out 1.25 – 28 V),
that I could put on 2 supercapacitors connected in series.
Converter was quite happy to lit my 12V LED from 3 AA rechargeable batteries in series, as well as from 18V 40W solar panel.
By the way read another reputable article saying it is OK to get up to 3 Supercapacitors in series without need for balancing.
So could use one Converter to load SuperCaps from 18V Solar Panel not to exceed total allowable terminal voltage.
And use another Converter to drain power from variable voltage source to get constant voltage.
This gives me 7.4V for 2.5V SuperCaps or 8.0V for 2.7V SuperCaps.
Not great, but allows use of around 60% of total available capacity.
Gadgets gobbles up 10% each, making just about 40% available.
With 2A limit, unless I can source similar converters that can handle higher currents.
Most probably some kind of restrictors should be used to tame the explosive power availability from the SuperCaps.
Of course improvement can be made by converter which could use lower “IN” voltage or lots more SuperCaps, but Non Draining Balancing is beyond my level.
(Is there information how to make one for say 10 SuperCaps 2.5V ? )George
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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