Charging ultracapacitors with high voltage

[Hobbyer]

charging ultracapacitors

Can a capacitor be charged, without damage, with a voltage higher than it's specified voltage, if the charging is stopped when the capacitor has reached it's specified voltage?

Here is the situation:

I have an ultracapacitor:
140 Farad, 2,5 Volt, ESR 7,2 mOhm, Leakage current 0,1 mA, Short circuit current 500 A

Then I have a solar cell matrix:
20 Volts in full sunlight, 5 volts with just my room light. About 2 Watts power in sunlight and in room light in the milliwatt range.

Goal: I want to charge the capacitor with the solar matrix with very little wasted power.

I could use a linear regulator, but that would waste power and my linear regulator did not get enough power in roomlight to even start regulating anyway.

Another way would be to use a switching regulator, but I did not have enough current for the one I had also.

Yet another way would be to construct a sort of charger, that would charge a smaller 35V capacitor to 2.5 volts and then pump it into the Ultracapacitor, but I was thinking if I could do it simpler way if it was possible:

Thing is that the capacitor can drain more power than the solar cell matrix can provide, so the voltage across capacitor will show the capacitor voltage or slightly above it (only tested with a 3300 uF, 35V capacitor). Therefore I thought that it _might_ be safe to charge with 20 Volt solar matrix, until the voltage across capacitor has reached 2.5 Volts.

This way I could also charge the cap faster, because the capacitor charging curve would be based on the higher charge voltage.

Problem is can this be done without destroying the capacitor (and maybe causing severe collatelar damage) assuming that I can reliably stop charging at 2.5 Volts.

I've looked for an answer for a long time, but I could not find any. Also I did not want to try out and risk destroying the capacitor, since I only have a handful.

 

[FvM]

ultracapacitors high voltage

There are two points.
1. The supercap maximum ratings have to be kept absolutely.

To avoid health and fire hazards, do not operate the capacitor beyond the voltage or temperature limits given in the data sheet. Any excess may also result in a reduction of lifetime.

I don't need to comment further on this issue, I think.

2. Operating a solar panel with maximum efficiency requires an electronic converter. The technique is designated Maximum-Power-Point-Tracking (MPPT), you'll find lots of posts on edaboard.

Cause the maximum-power-point will be always above 2.5V in your application, you clearly need a down (buck) converter.

 

[Hobbyer]

voltage current curve for ultracapacitor

Thank you for the answer.

I quess I need to find converter that works with currents in the microamperage range and good thing you mentioned MPPT. Did not even know about that issue with photovoltaics.

 

[abrahim.sabir]

ultracapacitor short circuit

first lets estimate the actual time required to capture 140farad worth of capacitive charge. Assuming a moderate output of 1watt from the solar panel.

20v x I = 1watt .
I = 1/20 = .05amp = 50miliamp @ 20volts


q= cv

q = 140 x 2.5 = 350coloumbs

q = i x t

350 = 50/1000 x t

t= 350000/50 = 7000 sec ~ 1.9 hrs


Now lets build a potential wall to come in series with the capacitor when you connect it to the solar cell directly. 2.5 volts accross the cap and .7volt per silicon diode in forward bias.

20 = (.7volt * 25 diodes) + 2.5 (volts on the Ucap)
20 = 17.5 + 2.5


25 diodes connected in series to give 17.5volts of potential hurdle and 2.5 volts left for the capacitor. this seems to be the simplest solution to me. as a further safeguard you can have a 2.5 zener connected across the Ucap to safeguard it against higher voltage.


Infact if you are thinking of using the solar panel only for charging the ultra cap then the best will be to connect it directly to the Ucap with a series resistance of about 100ohm and let the zener bleed away the superfluous charge which comes after 2.5volts have been achieved.

Please note that the silicon diode's .7 volt knee is not standard in all available pieces so practical measurements need to be made before using any such combination. the good old fashioned way of measuring the current flow as your increase the voltage beyond 17.5 should give you the exact characteristics of the diode pack.

Notwithstanding the above, as a precaution against tolerances it is advisable to use a 2.4volts zener in all of the above mentioned cases.

do let me know what you decided on.