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I can use Arrhenius’s Law to determine the liftime with regards to temprature. However what about DC operating point. Say you have a 25 V cap at 16 V. I remember hearing that that also improves life time.
There's conflicting evidence (and mostly anecdotal at that, unfortunately) on how much longer an elko will last for a given percentage of voltage derating. Basically, there seems to be little benefit from operating at less than 75% of the WVDC rating, and if you need really a long (decades long) working life then you'd probably be better off with film capacitors.
edit - added missing "need" in last sentence.