I'm looking for a capacitor with the capability to last as long a time as possible. In the order of 25 years average, if possible.

• Maximum working voltage: 16V
• At least 10,000 uF capacitance
• Will hold 80% of its charge for at least 4 hours.
• About 50 charge/discharge cycles per day. (a total of about 500,000 cycles in 25 years)
• Max working temperature: 45°C
• Weight and density are not an issue.

The charge/discharge cycles will not be abrupt. That is, the cap will take about 10 minutes to fully charge, and one minute to discharge each cycle.

I already looked into supercapacitors, such as this one. But they seem to fall shot in the durability requirement.

Any other suggestions?

 

Panasonic shows some models rated to 10,000 hours up tp 105°C. (I'm not sure they claim to meet both specs at the same time?).

Is that not good enough?

It might be worth asking them.

 

Panasonic shows some models rated to 10,000 hours up tp 105°C. (I'm not sure they claim to meet both specs at the same time?).

Is that not good enough?

It might be worth asking them.

At that temp rating, they might as well last the hell of a lot longer at 45°C ... I'll look it up. Thanks!

 

My first thought is to use a computer grade capacitor -- If they are still made.

These are the big honkers with the screw terminals built in.

 

My first thought is to use a computer grade capacitor -- If they are still made.

These are the big honkers with the screw terminals built in.

You mean like these ones?:

https://www.tedss.com/LearnMore/Computer-Grade-Capacitors

 

How about "Conductive Polymer Hybrid Aluminum Electrolytic Capacitors" ? Would they work, better?

 

Going to need quite a few of those to get to 10,000 uf

 

I think the biggest problem will be finding something with leakage current low enough to meet the charge hold-up time requirement. Look very carefully at leakage specs. You may do better with a 25 volt cap than a 16 volt cap. Note the temperature at which leakage is specified and how it varies with temperature. Often electrolytics are spec'd to have leakage of "not more than ...", but actual leakage can be substantially less. I recommend getting some samples and testing them. Initial leakage is often considerably higher than leakage after voltage has been applied for several hours.

The 25 year life shouldn't be a problem, You are charging and discharging very slowly, so the "ripple current" in the cap is very very low and self-heating almost non-existent. If you have a capacitor rated at 10 000 hours at 105°C (this is very good rating, unless it is fudged, and Panasonic doesn't do that for anything I've looked at; usually specs for that type of capacitor are very detailed because they are used where capacitor lifetime and performance are extremely important) it should be good for for about 70 years. Cap life approximately doubles for every 10°C reduction in temperature from the rated temp. You would get even more predicted life time because the ripple current is so low. The 10 k hours life is probably spec'd at continuous RMS ripple current of several amps. Of course it won't last that long because the seals aren't likely to last that long and the thing will dry out.

Check the spec for minimum temperature. Some types like that don't do well at very low temperatures.

I've never looked, but you can probably find either a calculator or instructions for how to calculate the RMS ripple current on the web. I recommend assuming linear charging and discharging to simplify - it will be plenty good enough. But it really is nearly zero.

 

You mean like these ones?:

https://www.tedss.com/LearnMore/Computer-Grade-Capacitors

Yes, those are what I was thinking of. Hopefully the specifications reflect what I remember...

Back when computers were big enough to fill a room operators were very picky about down time. The price of a good capacitor was small compared to even a minute of lost computing time.

 

I recommend getting some samples and testing them.

That's definitely in my to-do list ... thanks for all the very relevant tips!

 

That's definitely in my to-do list ... thanks for all the very relevant tips!

I would start with the most expensive ones and work my way down.

Why? Because the manufacture of a better cap costs more. Of course, the manufacturer may just be upping the price to pay for the large number parts replaced under warranty because they don't meet the lofty specification for the high priced part.

 

That's definitely in my to-do list ... thanks for all the very relevant tips!

Still hunting? I just got an email about this.

 

I have my old faithful that comes out when a long RC time is needed. Rubycon, 10,000 uF @ 10 V, 85 deg.C, measured @ 21 meg. leakage.

 

I've found that supercapacitors have an unusually long life, as long as one does not exceed their rated voltage. Say, an 80% voltage keeps them well into their safe zone. Unfortunately, most of them go up only to about 6V, and I'd like to be able to work at 12V.

How complicated is it to adequately connect two or three caps in series? Would it be as simple as adding zeners to the circuit?