Is it practical to use capacitors as backup power supply?
I saw some 4F 5 volt capacitors at my local electronics store (as audio components) pretty cheap (ish). I’m wondering how feasible to use them as backup power in place of heavy batteries? Sure it’ll be a bit more expensive, larger in size, but they should be lighter for the same power stored?
Its mostly of illumination uses, maybe a fan or to power a laptop. I plan on using LEDs and EL lighting. LEDs uses low voltage DC, perfect for the low voltage capacitors. EL require higher voltage, but will run on DC also. Fans I have are mostly 12V type. Some 5V and some 24V also. I haven’t figured out the laptop part yet.
The main reasons I think capacitors are better than batteries:
Lighter,
100k’s or 1m’s of charge/discharge cycles vs a couple years for lead-acid, most batteries will die quick from extremely deep discharges.
no acid. Caps are sealed electrolytic.
no maintenance, they are sealed. In storage they are good for 10-20 years in ideal conditions.
no charging limits- unlike batteries I can charge then as fast or slow as I want.
no discharge limits- batteries have internal resistance, they can get hot from heavy discharge enough to kill themselves. its also a bad thing too, sorta makes charged capacitors a serious handling hazard.
any opinions/reasons why it wouldn’t work?
-thanks
3 Responses
guru
09 May 2010
Darrell Tay J
09 May 2010
Hmm. 4 Farad capacitors? Then it should be possible, though the discharge time is very fast, therefore it might make your backup power supply impractical. I think capacitors have been used in uninterruptable power supplies, especially those high value capacitors.
billrussell42
09 May 2010
Go through the numbers.
E = ½CV² = 50 Joules (energy in the cap)
Now, I don’t know haw big these are, but compare it to a rechargable NiMH C cell, which is quite small. It holds 12 amp-hours of charge. multiply by 1.2 volts to get 1.4 watt-hours. Multiply bu 3600 sec/hour to get 5000 watt-seconds, which is a joule.
So we have your cap holding 50 Joules of charge, versus 5000 for a C cell. You can compare the sizes, I suspect the cap is larger.
So you get 1/100 th the storage in the cap versus the cell, and the cell holds it’s voltage as it is used. The cap has a voltage that immediately starts falling, and your DC-DC converter design has to work with a dropping voltage.
I’d stick with the batteries.
.
capacitors have been in use for very many years
the one you mention is relatively new, ~20 years old
they were first used in computer memory back-up
they are much larger now and begins to appear (are currently being tested) on trucks for battery alternative
eventually will be on automobiles…
your question why it would not work
they have high ESR,
Guru