Nuts and Volts - January 2016

January 2016 65

least 300 mcd. The LEDs above 1CD typically use a clear lens, so can’t be seen off axis. As an alternate, if you only need it to run a couple of days, then just solder a L-36BSRD flashing LED inside a 9V battery snap. I make these up for cavers; they leave them behind like breadcrumbs to find their way back.

Bob Turner Salamander Bay, Australia

[#11151 - November 2015]

Super Capacitor Comparison

I need some help 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 acts 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?

#1 “Super caps” are, indeed, electrolytic capacitors. Their 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) since — 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.

Ken Simmons Auburn, WA

#2 For any given capacity rating, an electrolytic capacitor should behave very much like a super capacitor, at least in a low current circuit like a RTC. However, super capacitors provide much higher capacity in a given package size. So much so, that they are rated in millifarads (1 mF = 1,000 µF) or Farads (1F = 1,000,00 µF) rather than microfarads (µF).

Are you sure you took this into account when you did your calculation? If not, you may be off by a factor of 1,000 or one million. The other possibility is leakage current causing the electrolytic capacitor to drain faster than you expect. This presents itself as if there was a resistor connected across the capacitor, constantly draining away its charge.

The leakage current value is listed on the datasheet for the capacitor and must be taken into account when you’re using it for long storage applications like a real time clock.

Mark Lewus Denville, NJ

#3 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 three doping levels, producing forward drops of 200, 300, and 400 mV; the 400 mV has the lowest leakage current. It will usually have an H in the part number.) A super capacitor is a electrolytic double layer capacitor (EDLC), and each electrode is coated in very fine carbon granules. The total surface area is about 1,000 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 moves the ions back and forth across the layer.

Bob Turner

Salamander Bay, Australia

[#12153 - December 2015] LiPo Smart Charger

I’m looking for a smart charger for LiPo cells that can run from a solar panel. I’ve heard that the combination is incompatible because of fluctuations in output from the solar panel. Is this true? If so, is a workaround a larger panel?

There are a lot of solar to li-ion battery charger ICs. You might get one on a breakout board, like the following: www.ladyada.net/make/ solarlipo

I made a small solar powered solar tracker that has a similar IC, and charges two li-ion batteries to about 8.0 volts. It works well. It’s been in operation over a year now.