Self-Healing Capacitors

Even though I am having problems with test accuracy, some of my test results are consistent with industry knowledge. The technique discussed on this page is applicable not only to the example aluminum electrolytic capacitor, but also to other capacitor chemistries (with varying degrees of success).

1 microfarad aluminum electrolytic capacitor

Aluminum electrolytics have a well-documented change in leakage when stored and when exercised. I tested a 1 µF 50 V Nichicon aluminum electrolytic capacitor that has sat around at room temperature for 12 years without being used.

It measures 1.08 µF at the beginning and the ending of the test. So, if you were to go by capacitance alone, this capacitor seems fine.

The capacitor is charged to 5 V for two minutes, and then allowed to discharge through the measuring circuit until the voltage reaches 2.75 V. This process is repeated many times.

Old stock aluminum electrolytic capacitor self healing

Old stock aluminum electrolytic capacitor self healing.

As you can see, the capacitor gets better at retaining charge with each iteration. The leaky areas inside an aluminum electrolytic capacitor are converted to aluminum oxide (an electrical insulator) when a charge is applied. The capacitor is repairing itself.

The rate of improvement tapers off as the quantity and severity of leaks decline. There are wider gaps between the first couple of cycles. The improvement between the first and third tests is double. The improvement between the eighth and tenth test is 14 percent.

I also tested another old 1 µF capacitor from the same batch. This time, I charged it for less than a second, measured the results, and then charged it for 22 minutes. The initial charge produced a line similar to the first capacitor. So, I know there were in a similar initial state of leakiness.

The 22 minute charge produced a line in the middle of the test results. So, the capacitor will heal by simply being in a charged state. Multiple charge-discharge cycles are not necessary in order to repair the insulator.

Even when discharging, some healing occurs. Otherwise a 22 minute charge would produce identical results to 11 two-minute charges. Therefore, the capacitor heals so long as some (unknown) level of voltage is applied or stored.

A week later, the capacitors were retested. Their performance had not declined measurably. Therefore, the repairs are lasting.

Dealing with Old Capacitors

If you buy old stock capacitors from eBay or surplus retailers, or if you have some capacitors that you bought years ago, the capacitors will perform worse than newly manufactured ones.

If you make a solar robot, it may take longer to charge up for the first few cycles, as a portion of the solar power is simply being leaked. Also, the charge may not power the motors for as long (or propel a solar roller robot as far), because additional current is being consumed by the leaky capacitor. However, after a while, the performance may improve. So, charge that old capacitor overnight before a contest or public demonstration to achieve superior results.

You want to be especially careful with old capacitors that are going to be charged up to a high voltage, such as in a camera flash or computer power supply. The high voltage causes a greater current to flow across the leaks, and the energy is released in the form of heat. This can dry out the capacitor, or destroy it with excess gas or a permanent short circuit in the insulator. If possible, charge the capacitor to a lower voltage for a while to repair the insulator.

Now let’s take a look at ultracapacitors.