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Super Capacitors for RTC battery?


The FEZ boards do not contain a holder for a coin battery to power the real-time clock when power goes away. An external holder and two 1.5 volt batteries could be used but that would be overkill in terms of required capacity and size.

I have been reading up about super capacitors. There are large capacitors in terms of the charge they will hold and are often used as backup power supplies. I think it would be easy to use them for the RTC. Small and simple to wire.

I know the voltage rating on the capacitor must be greater than 3.3V. 5V seems to be the next higher voltage rating.

I assume that I could attach a super capacitor between the RTC pin and ground, watching out for the polarity of the capacitor. When the board was plugged into power, the capacitor would charge,

Would this work?


Yes, a super capacitor can replace the battery. You will need to add a diode from 3.3V to the capacitor to charge when FEZ is powered and then the capacitor will power the internal RTC when FEZ is not powered.


I used a volt meter this morning on a Cobra that was powered via USB and found that there was 2.98V on the RTC pin. If a diode to 3.3V is used will there be a voltage mismatch issue?


No ti is okay. This is why you have a diode.

I will try to draw it!

VCC ---->|-------+------ VBAT


OK I understand.

I checked the LP24xx(?) chip manual and could not find the required current for the RTC off battery. Any idea what it is?


not sure. I think this is in teh datasheet not user manual


Just out of curiously, why was an RTC battery omitted o nthe Cobra? That’s one feature that I really wanted to see.


Chris. I suspect the holder takes a bit of real estate.


That would be my guess, too, but there are a bunch of different mounting options, like a vertical battery holder that wouldn’t take up so much space, at least not on the board. I guess it would make the board a lot taller though.


The VBAT pin is there is someone needs this feature. This is not used by everyone.


I have ordered a batch of six super capacitors on ebay and willl try one and report back.


I would expect the cap to keep the RTC running for a month…so let us know please.

I also found out that the super cap can be connected directly. No need for a diode! We have the diode already on the EMX module! I still need to double check this info


Part of me really wants to connect one of those supercaps backwards to see what happens :whistle:


hahahaha! I was just thinking the same! :whistle:


Have fun :wink:


[quote]Have fun Wink

Here’s another good one from our friend Dave, from the EEVBlog:


I have been working with electronics since I was 15 years old. In all that time I have never felt the urge to explode a component. That is not to say I have done it by mistake.

If you must explode a component, a super capacitor appears to be the best to use. Most of them seem to be made of organic material.

Explode Green!


Done some capacitor shootout at school :smiley:
Teachers always freaked out and asked what the blast was ::slight_smile:


I had a few minutes to play with a super capacitor and a Domino. Here are my results:

I used a .47F 5.5V super capacitor.

I had general concern about inrush current. This is the initial current draw that happens when the uncharged capacitor is connected to a voltage. To limit this intial current I used a 100 ohm resistor.
At the low current draw of the RTC, under battery conditions, the voltage drop across the resistor is very small. During intial charging, with higher currents, the resistor will ease the load on the 3.3V supply.

For my testing, I was able to use an analog input to monitor the voltage at the capacitor.

With nothing connected, using the analog input, I measured a voltage on the VBat pin of the Domino. it was ~3.3V.

I then connected the minus of the capacitor to ground and the plus to a 100 ohm resistor and the other end of the resistor to VBat.

This did not work… The volltage a VBat was dragged to under a 100 mv and was increasing very very slowly. Putting in a large resistor increased the charge rate, but it was still very slow. I was concerned about the effect of dragging down the voltage source that was power VBat for a long time.

I then tried a modified version of schematic that Gus suggested above. I modified it by adding an inrush protection resistor in addition to a diode.

I connected the capacitor minus to ground. I connected the plus side of the capacitor to VBat. I then connected a diode in series with a 100 ohm resistor between the 3.3V pin and the plus side of the capacitor. (Anode of diode to 3.3V)

This works.

Within a minute the voltage at the plus side of the capacitor is about 2.7 volts. The diode I used has a ~.7V drop across it, so 2.7V makes sense. To reduce this drop, a Shockley diode could be used.

The NXP data sheet says that the RTC backup fails at about 1.6V, so 2.7V is fine.

I have not made tests to see how long the capacitor keeps the RTC going. But at 2 microamps, it should be a while. If the Panda has a VBat pin, I will buy one and do a long term test.

It turns out the super capacitor is a lot bigger than I thought. it is about the size of eight cr2032 stacked coin batteries. A coin battery in a holder is smaller and neater. Less parts.

I will be repeating these tests on a Cobra. The Cobra might not need the diode.


Why?! Look at this picture. The supper cap we sued is in the lower left corner, the blue circle thingy.