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Topic: Power for a few more seconds after disconnect (Read 2 times) previous topic - next topic


Rob Gray aka the GRAYnomad www.robgray.com

Nick Gammon


Bloody heck, why not just use a truck battery :)
Rob Gray aka the GRAYnomad www.robgray.com


Apr 22, 2011, 04:05 pm Last Edit: Apr 22, 2011, 04:10 pm by retrolefty Reason: 1

Just for interest, 5 hours after I started the sketch is still running. The power is down to 3.9V.

So as a proof of concept, it appears that a couple of ultracapacitors can keep your sketch running for 5 hours. This particular sketch spends all its time lighting up LEDs, which is probably a bit of a torture test, comparing to monitoring temperatures etc.

The two capacitors I used cost me $US 32 (for both).

Whilst they take a while to charge, I think this is acceptable. After all, you put charged batteries in as a battery backup don't you, not discharged ones. Either you could organize a more sophisticated charging circuit, or just pre-charge them.

Something like this could keep a security system running for hours, even during a power outage. And if you double the number of capacitors, you double the time they will keep running. Coupled with the suggested power board, with its DC to DC converter, you could probably squeeze something like 8 or more hours of performance out of them (or 16 if you use 4 capacitors).

The question I would have is how does this compare to a more traditional back-up using batteries as far as initial costs, volume (size) used, and recharge time, etc. Most batteries have a pretty flat voltage discharge curve while the super cap would need some kind of boost voltage regulator to match battery voltage Vs time performance, which might be critical to some applications such as using analog input values where a changing reference voltage would hurt calibration accuracy.

So while super caps are interesting I haven't yet seem an application where a properly sized battery doesn't perform as well or better for less costs and space requirements. Super caps have been around for quite a while now, but I've seen few pratical applications for them, and I don't count the auto stereo installations where I think the visual appeal is more important then what they actual add to the systems performance. But then maybe I'm being too pessimistic? Show me the math.  ;)



Capacitors are nice, I love them.
They have no substitute for supply a high current in short time-frame. 
It's like a taser gun, or inductive cannon they try to build in NAVY.
Laser cannon is also based on capacitors discharge.
Unfortunately all this stuff is military game oriented.

Nick Gammon

Bloody heck, why not just use a truck battery :)

I don't want to use a battery because of the extra cost and complexity ...

Personally I have found that backup batteries (particularly big lead-acid ones) work fine the day you install them, but then three years later when they are needed their performance is sub-optimal. Also batteries have memory effects, suffer if over-discharged etc.

Now these capacitors might be bulky for the intended purpose, but I suppose it depends what that is. And as for the RC time constant, a way around the flattening off of the curve would be to have more capacity. Eg. have enough capacity to hold 10V but charge them up to 5V, which is on the fast part of the charge curve. This could bring the charge time in my example down to around a day, and you could halve that with a smaller resistor if the power supply could deliver 100 mA.

Nick Gammon

Not me, I got the idea from Afrotechmods.


Nick, you caught that he didn't want to use batteries, yes but you missed this part...

In the first version this will only take a second or two but later I might need up to a minute.

He doesn't need 7 hours of run time. A 2600F cap is way overkill.
10 PRINT CHR$(7)
20 GOTO 10

Nick Gammon

Agreed, absolutely. But since I had them to hand I wanted to see how far I could push them.

BTW, I disconnected the Arduino before going to bed, and this morning reconnecting it, the sketch fired back into life. The caps are still at 3.6V, so the self-discharge rate must be pretty low.

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