The lifetime rating is (usually) quoted as "so many" hours at a certain temperature and that is the crucial thing to be aware of. For instance, consider a capacitor like this one: -
Some in the range are only rated for 5,000 hours at 105 °C but, if they were run at an average temperature of 95 °C the lifetime/endurance figure will double to 10,000 hours. Should the average temperature be 85 °C then the lifetime doubles again to 20,000 hours.
So, if you were considering this capacitor for a design and the ambient internal working temperature remained no higher than (say) 45 °C, you can expect that the endurance figure would be 64 x 5,000 hours = 320,000 hours = 36.5 years.
But, the lifetime/endurance figure is also extended by running at a lower voltage. As an example, a 16 volt capacitor running at 8 volts can double the endurance figure. If the 5,000 hour @ 105 °C device were run at 45 °C, you could expect a duration figure of 73 years!
Try using this calculator and note the following : -
- $L1$ is the 5,000 hours stated by the capacitor manufacturer and
- $T_M$ is the temperature rating of capacitor (105 °C).
But, also remember that although this figure is an endurance number, it doesn't mean that the capacitor in question hasn't degraded over that period in time so, if you are looking for an electrolytic capacitor that has long life, read the data sheet to find out what the "end of life" endurance performance values are: -
Important end of life parameters to consider: -
- Capacitance change (within 25% of original value) - this may sound good but the original value could be +/- 20% and so the final value might be 75% of 80% of the initial nominal value i.e. a 100 uF capacitor might have degraded to 60 uF. Design with care in other words.
- $\tan \delta$ (losses or dissipation factor) less than 200% or the initial value.
Note also that the lifetime endurance test is usually performed at the rated ripple current so, if you read the small-print on your prospective capacitor and it doesn't mention this then maybe you should reconsider that choice.
As always, read the data sheets carefully and make comparisons with similar components from known reputable manufacturers. Here's an extract of another data sheet that shows the effect of ripple current and temperature on endurance figures: -
This device is rated for 10,000 hours at 105 °C and the graphs show endurance doubling for every 10 °C reduction in operating temperature. The Y axis shows the degrading effect of sustained ripple current. Note that "1.0" on the Y axis is the nominal ripple current that it is rated for.
Choose your electrolytic capacitor with care and note that good devices will always have this type of information in their data sheets. If you pick one that seems too good to be true then don't be surprised if it isn't.