2013-07-04

Supercapacitors info (incomplete)

There are tons of information about supercaps on the internet.
That is why I decided to put everything, that I find practical and interesting here!

The Wikipedia link: http://en.wikipedia.org/wiki/Supercapacitor

The supercapacitors are capacitors that can hold nearly 1000 times more energy than a normal capacitor. In many cases they can replace rechargeable batteries, or lead-acid batteries.
However there are a lot of differences, that are important to know.

I will make a table of comparisson and descriptions. Please leave a comment to correct my data.
Red color means worse, Green means - better.


Supercaps NiMH batteries
1 Energy density 5-10 Wh/kg 60-120 Wh/kg
2 Charge time seconds few hours
3 Discharge voltage drop down to 0V, constant Almost stable 1.2-0.9V
4 Charging cycles ~infinite 500-1000
5 Price expensive cheap
6 Element voltage 2.7V 1.2V
7 Self discharge fast slow
* Working Temperature
* Charging circuit
* Shelf life


Explanations:

  1. Even 1000 times more than biggest capacitors is not enough to hold the energy of a normal 1.2V NiMH battery. A 3000Farad 2.7V supercapacitor has almost the same energy of a single AA 2500mAh 1.2V NiMH battery.


    The calculations are very simple:
    AA Battery = Ah*V*3600 = 2.5*1.2*3600 = 10800 Joules
    Supercap = 0.5*F*V*V = 0.5*3000*2.7*2.7 = 10935 Joules

    Lets compare also the size and weght:
    AA Battery = length - 50.5mm,   diameter - 14mm
    Maxwell 3000F/2.7V supercap = length 138mm,    diameter - 60mm (~12x bigger)

    AA Battery = 31g (NiMH)
    Maxwell 3000F/2.7V supercap = 510g (~17x heavier)

    Some more calculations:
    AA Battery = 15.28 Joules/mm2 ,   348.39 Joules/g
    Maxwell 3000F/2.7V supercap = 1.32 Joules/mm2 ,   21.44 Joules/g

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  2. Everybody knows you have to leave NiMH to charge overnight. Supercaps charge in seconds, if you can supply enough current for them. Like every capacitor, they charge on a curve. At first maximum current flows, and after that it lowers, as the supercap charges. In the end no current flows any more.
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  3. While discharging the supercap linearly drops to 0 Volts. This means that you can use all the stored energy. However most devices are not happy if the supply voltage changes in so big range 2.7V-0V. On the other hand NiMH batteries keep almost stable around 1.2V while discharging. When they drop under certain voltage, they can die and become useless. To solve this problem a DC-DC converter can be used for the supercap.
    ----
  4. The supercaps have the advantage that they can be recharged many many times, and their capacity does not change much in time. NiMH batteries have a limited number of charging cycles, and their capacity deplete in time. Specialist also reccomend that you fully charge and discharge them (discharge to ~0.9V) to keep them in good health. No such problem with supercaps
    ----
  5. Yes, the supercaps are expensive, but it seems the technology is not so complex, and they will get cheaper in time. These days (July.2013) one of the best offers I found is - Maxwell 350F 2.7V for 11$.
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  6.  You know one NiMH battery can be charged to 1.2V or even slighly higher. The supercaps are not like ordinary capacitors, where you can buy 500V capacitor. It does not matter if the capacitor is big or small - it is limited to 2.7V (some supercaps like LiOn supercaps can reach 4V). If you need higher voltage you can stack several supercapacitors, like batteries.
    ----
  7. Both NiMHs and supercaps discharge while they stay idle. However modern NiMH can keep up to 80% of their charge after 1 year of staying on the shelf. The supercaps can discharge completeley for a few weeks. However LiOn supercaps voltage drops about 5% for 3 months, which is quite good.

* not finished, updated almost every day, little by little :)


References:
http://electronicsbus.com/batteries-vs-edlc-ultracapacitor-supercapacitor/

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