Electrolytic capacitors and the effect on lifetime of time spent in storeage?

[treez]

I once worked for a company who made capacitor chargers. They were charging up 500V, 470uF Elec capacitors to 400V and discharging them once per second.

Whenever they had to do a production run, they had to time the ordering of the capacitors so that they were manufactured immediately before the production run. This seems a little odd. Are we seriously saying that any product that contains electrolytic capacitors has to have them made right before the production run?

I understand that leakage current is higher in old electrolytics, but after 10 minutes with voltage on it, this reduces right down, and modern capacitors have very low leakage even after years in storage.

So why is it that electrolytics have to be made right before the production run?....i mean, how would anyone know if a product was made with old or new electrolytics? So are we saying that a “3000hr 105degc” El capacitor is not that after say three years in storage? They are not date coded, and even if they were, you couldn’t see it once mounted amongst other components.

 

[BigBoss]

Since those capacitors are stored accordingly with manufacturers' recommendations, lifetime does really not start because the main problem with elco caps. is drying of the internal liquid. This occurs just under loaded condition due to rising heat.(If ESR was 0 Ohm, there won't be such problem at all)
Therefore manufacturing date is less important than working hours.But it doesn't meant that a capacitor that has been stored more than 10 years can be used. Those components should carefully be selected as recently manufactured.
Also brand is more important than all those.A high quality electrolytic capacitor may work almost more than 20 years depending on avarage working hours.

 

[FvM]

So why is it that electrolytics have to be made right before the production run?

.
Nonsense. You should however refer to manufacturer application notes about long time storage, e.g. from Epcos/TDK:

7.3.2 Storage conditions
Capacitors are often stored voltage-free either in original packing or already mounted in a circuit (final application or spare part assembly).

Our aluminum electrolytic capacitors can be stored without damage at ambient temperatures ranging from typical 55 °C up to the upper category temperature. However, it must be taken into account that storage at elevated temperatures will affect leakage current, useful life and solderability. In order to avoid impairing these properties, capacitors should be stored between +5 °C and +35 °C and at a relative humidity of ≤ 75%. Long-term storage at high temperatures and in an environment with high humidity should be avoided. Likewise, one should avoid storing the capacitors in environments that contain halogenated gases (and other hazardous gases), sprinkling water or oil as well as exposure of the capacitors to direct sunlight, ultraviolet rays or other radiation.

If not otherwise specified, our aluminum electrolytic capacitors can be stored voltage-free at above stated conditions (from +5 °C to +35 °C, relative humidity ≤ 75%) for at least two years; capacitors of the SIKOREL series can be stored for as long as 15 years under these conditions. Within these storage periods the capacitors can be operated at their rated voltage directly after being taken out of storage. It is recommended to mount the capacitors in the application within one year of delivery in order to prevent any problems with solderability of capacitors on PCBs.

If the capacitors have been stored at temperatures above +35 °C or for longer than abovementioned periods, it is necessary to consider whether the application circuit is able to tolerate high initial leakage currents. If this is not the case, one should perform the special reforming procedure (see also chapter "General technical information, 3.7.6 Reforming"on page 17) in order to enable normal operation in the application or to continue the storage. Capacitors that have been stored in their original packing should be reformed according to the standards by applying rated voltage via a series resistance of approximately 100 Ω (for VR ≤ 100 V DC) or 1000 Ω (for VR > 100 V DC) for a period of one hour. Finished or semi-finished goods that have been stored with mounted capacitors should be connected to an appropriate input voltage source for one hour. If the circuits contain capacitors connected in series, it should be considered to increase the voltage incrementally.

 

[treez]

As a distributor, with a load of caps about to go out of date (> 2 years in store), are you going to expensively put them all through "voltage exposure procedure", or just change the date on the storage card to say they are new? The caps arent marked with the date, so no one would know

Its difficult to put numbers on actual leakage currents that would occur after x years in storeage at y degrees C.

Ive looked through all the attached app notes, but none really shed any light on what actual leakage current would occur.

I suspect many el caps have been stored for ages…and when they are first plugged in when in a product, they conduct really high leakage, which results in serious electrolyte damage, which in turn results in a vastly reduced life time.

 

[Easy peasy]

you can re-form them with a variac and a bridge rectifier and a DC current meter ( or AC power meter ) - they will be good for another 2-3 years then - p.s. leave them charged.

Bring up the variac slowly and note the power or DC current and keep it to reasonable levels until you get to rated volts - hold for 5 mins - then rinse and repeat until all caps re-formed ...

works on 5, 10 year old caps too - just bring up the volts slowly ...

 

[treez]

Thanks, imagine trying to do that to a huge batch. Imagine that many manufacturers simply won't bother. The fact that such procedures, aswell as the one using a dc source and 100r/1k resistor, are recomended, shows what a serious issue this leakage current is.

 

[Easy peasy]

leakage current is pretty low for a quality electrolytic - witness the fact they stay charged for days - many days - with no load attached ....

--- Updated Jul 7, 2020 ---

you could do 10 or 20 at a time - in //.

 

[FvM]

Dedicated reforming procedure is suggested if the capacitors are exposed to high charging currents, e.g. VFD input filter capacitors. In many applications, the capacitors won't suffer damage if regularly powered after five or ten years of storage.

 

[dick_freebird]

This may just be "tribal knowledge" that persists long
after any problem it addressed, has been solved
upstream. People come and go but irrelevant
procedures accumulate.

 

[Easy peasy]

We've had to reform plenty of 470uF 450VDC caps that were supposedly only a year old from the manufacturer - nippon-chemicon ...

we modified the run-up procedure in the factory to allow for this ...