# Internal Resistance of a Supercapacitor

I am attempting to replace a wall plug power supply with a supercapacitor. From analysing the behaviour of the wall plug I can see that the current drawn is .25A with 800ms spikes of 1.5A (as determined by an arduino board).

However when I try to use the supercapacitor (15F, 7VDC), the voltage appears fine, but not enough current is being drawn. The highest current I have seen drawn on the multimeter is .11A, so the system in question does not work.

I think this may be down to the internal resistance of the supercapacitor. So the next step I will try is to increase the number of supercapacitors and place them in parallel, thereby decreasing the resistance.

However as supercapacitors are advertised to be capable of charging in a matter of seconds, surely this means they can handle a high current (at least 1.5A anyway). Or will supercapacitors draw more current during charging than they would be capable of supplying during discharge?

Any help here is greatly appreciated.

• The internal resistance (ESR) comes in affect when you draw current from the capacitors and will drop voltage (V=IR) depending on the current and the ESR. When you say the voltage appears fine, how are you observing at it? Mar 29, 2018 at 11:38
• What is the output voltage of this "wall plug power supply"? Mar 29, 2018 at 11:42
• In the datasheet of the capacitor, you should find the ESR specified. For a Vishay MAL219691205E3 which has similar specs than yours, the ESR at DC is 12.5Ohm. Supercaps have high ESR, and if (as I suspect) you bought a cheap one on ebay, it is probably much worse than this (but you most likely can't tell, since there's no datasheet provided at all, is there?). So that is probably your problem, indeed.
– dim
Mar 29, 2018 at 11:45
• @R.Joshi The voltage across the cap and at the load where the current is needed appears as it should. The output voltage of the wall plug is 5.5VDC, so I wish for the supercapacitor to replace that. Mar 29, 2018 at 12:04
• @dim, you may have found my answer. The supercapacitor is a Vishay and is indeed similar to the one you have found. The ESR of the one I have is 12.5ohm, so at a low voltage this makes sense that the current is limited. Mar 29, 2018 at 12:07