Super Capacitor Questions 1

[controldude]

I have questions on the lifetime of EDLC's, or electric double layer capacitors. I've read several data sheets online from suppliers but don't have an accurate description of their life. Most give + or - some value after 1000 hours of operation. Does this just drop the capacitance if operated at the maximum voltage after this time? Is the life then permanent? How does the internal resistance effect this lifetime? I am trying to use this for a memory back up few charge/discharge cycles. Thank you!

 

[itsmoked]

controldude,

Yes capacitor specs are truely shocking with respect to lifetimes... Taken at face value one wonders how one's VCR could last more than a few months! The key is the temperature. The always quoted lifetimes are at the maximum temperature, however, the lifetime usually goes up exponentially with a drop in temperature. So a cap with a 1000hr life at 70C might have 10,000hrs at 60C and 100,000hrs at 40C. So you need to hunt around for the life-temperature curves. If you are looking for longer life, often paying the higher price for 85C rated caps will get you the longer life because you will be operating farther down the cap's life-temp curve.

It's all about temperature. If a cap has a higher internal resistance and you are feeding it with a bunch of ripple like a power supply filter cap, then it will experience a higher internal heating. It may be in a room temp environment but actually be really quite hot!

Yes there will be some capacitance loss as the cap ages.
Often they may just fail(bang).

Now in your case: memory backup. This sounds very benign as the cap should be getting filtered DC fed to it. It essentially sits under a fixed DC bias always. It doesn't get any easier for a cap than that. In most cases it is actually more benign than laying a bag in a stock room.