I need some help in understanding what’s going on inside a super capacitor. I did some experiments using regular capacitors as a backup power supply to a real time clock chip. Mathematically, the amount of time an electrolytic capacitor (1500, 2200, and 4700 µF) would power the chip became predictable once I came up with a formula. However, when I connected a super capacitor, the math broke down.
The real time clock should have exhausted the stored power in the super capacitor after exactly three days. Instead, it is still maintaining the correct time after four months! Clearly, something is physically different about a super capacitor. It seems to be acting more like a battery than a capacitor.
Can someone explain to me how the chemistry of a super capacitor differs from electrolytic capacitors? Why does the amount of charge stored seem to far exceed the capacity indicated by its Farad value, under an extremely light load?
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Your calculations are probably correct, however you may have used the worst case current from the datasheet, if you use the nominal current it should be good. A couple of months is the expected retention at room temperature. Just be careful if you use schottky diodes in your circuit (to power the RTC from normal 5V) the leakage current in a poorly selected diode can exceed the standby current. (Schottky diodes are made with 3 doping levels, producing forward drops of 200, 300, 400mV, the 400mV has lowest leakage current, it will usually have a H in the part number). A supercapacitor is a electrolytic double layer capacitor (EDLC), and each electrode is coated in very fine carbon granules, the total surface area is about 1000 times higher than just aluminium foil, with a much thinner dielectric, hence the increase in capacitance (and drop in maximum operating voltage). There is no actual oxide layer for the dielectric, an EDLC has charged layers of ions a few molecules thick instead, and charge and discharge just move the ions back and forth across the layer.
“Super-caps” are, indeed, electrolytic capacitors. They’re construction typically uses tantalum (not aluminum) plates to obtain a large capacitance value in a relatively small package compared to standard aluminum electrolytics. Plus, their electrolyte formulation differs enough from aluminum capacitors to allow a larger charge vs. size capability.
Because of their large charge capability, they are ideal for use in short-term backup applications where low currents (typically, less than 100 microamperes) are required (i.e., real-time clock chips). However, they are not the same as a battery (i.e., lithium coin cell) as, like regular electrolytics, super-caps do self-discharge over time and they can not deliver a large supply current (i.e. >1 mA) for longer than a couple of seconds.