3.7.5 Reforming
To IEC 60384-4, aluminum electrolytic capacitors are to be subjected to a reforming process before acceptance testing. The purpose of this preconditioning is to ensure that the same initial conditions are maintained when comparing and assessing different products.
For this purpose, the rated voltage is applied to the capacitors via a series resistance of approximately 100 W for VR £100 V DC, or 1000 W for VR >100 V DC, for a period of one hour.
Subsequently, the capacitors are stored under no-voltage conditions for 12 to 48 hours at a temperature between 15 and 35 °C. The leakage current must then be measured, at the latest after 48 hours.
If the capacitors meet the leakage current requirements without preconditioning, this procedure can be omitted.
3.7.6 Leakage current behavior with no voltage applied (voltage-free storage)
The oxide layer may deteriorate when aluminum electrolytic capacitors are stored without an externally applied voltage, especially at higher temperatures. Since there is no leakage current to transport oxygen ions to the anode in this case, the oxide layer is not regenerated. The result is that a higher than normal leakage current will flow when a voltage is applied after prolonged storage. As the oxide layer is regenerated in use, however, the leakage current will gradually decrease to its normal level.
Aluminum electrolytic capacitors can be stored voltage-free for at least 2 years, and capacitors of the SIKOREL series for as long as 15 years without any loss of reliability. Provided that these storage periods have not been exceeded, the capacitors can be operated at rated voltage directly after being taken out of storage. In this case, reforming as described under chapter "General technical information, 3.7.5 Reforming" is not required.
When designing application circuits, attention must be paid to the fact that the leakage current may be up to 100 times higher than normal during the first minutes following the application of power.
When the capacitors have been stored for more than two years, it is decisive whether the circuit will tolerate high initial leakage currents. A circuit that has been stored for more than two years with the capacitors incorporated, should be operated trouble-free for one hour. This will usually regenerate the capacitors so far that storage can be continued.