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Topic: Can I damage my supercap by charging it too fast? (Read 6212 times) previous topic - next topic

wyager

I just got my RTC working with a supercap as a backup, and it seems to work pretty well (after I figured out that Digikey posted the wrong specs for one of my crystals... ugh). However, I'm wondering if there's any danger to charging the supercap too fast. Basically, the only thing between the supercap and 5V is a Schottky diode, no resistors (although the cap does have a maximum 75 ohm ESR).

MarkT

You shouldn't damage the cap, you might damage the schottky diode if it can't take the current.

Sounds like there's enough serial resistance to protect the diode though - although that is I presume a maximum ESR spec.  If in doubt
add a resistor to limit the current to the pulse current rating of the diode.

If you had an application that charged and discharged the cap at a high rate then you'd have to worry about heating effects, but an RTC shouldn't be like that ;)
[ I won't respond to messages, use the forum please ]

wyager

OK, thanks. My diode is listed at 100mA on the digikey page and the listed datasheet says 200mA, so I'm sure it's fine (especially for the short charge durations it would experience).

Grumpy_Mike

I would disagree, you can damage a super cap by charging or discharging too fast. It is not justna heat problem but one of current density. You need to look at the data sheet of your cap.

smeezekitty

It depends how often you charge and discharge it.
If its charged occasionally and rarely discharged, I would not worry about it.
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

spookyjack

YOU CAN AND I WILL DAMAGE IT BY CHARGING/DISCHARGING IT TO FAST. the same is true with nearly any device

wyager

OK thanks. Does anyone have a link to a page with any info on the subject or any idea how to limit the charge speed?

smeezekitty

The esr, wires and power supply will limit it.
If caps are damaged from charging too fast, why does nearly every circuit that uses an electrolytic cap connect it directly to power?
Avoid throwing electronics out as you or someone else might need them for parts or use.
Solid state rectifiers are the only REAL rectifiers.
Resistors for LEDS!

wyager

Smeezekitty, I was thinking that maybe these supercaps are more sensitive to overcurrent or something...
Again, I'm not sure, I'm just keeping the possibility open.

retrolefty

Quote
I'm not sure, I'm just keeping the possibility open.


Then find the datasheet for your specific supercap. If it doesn't help you understand any limits or recommendations, then post the link here and we all can chew on it.

Lefty

wyager


Quote
I'm not sure, I'm just keeping the possibility open.


Then find the datasheet for your specific supercap. If it doesn't help you understand any limits or recommendations, then post the link here and we all can chew on it.

Lefty


I've already looked at all the datasheets, can't find anything about peak current...
Look at
http://industrial.panasonic.com/www-cgi/jvcr13pz.cgi?E+PZ+3+ABC0001+EECS0HD334H+7+WW
and
http://search.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&site=us&keywords=P11064-ND&x=0&y=0

retrolefty

Quote
I've already looked at all the datasheets, can't find anything about peak current...
Look at
http://industrial.panasonic.com/www-cgi/jvcr13pz.cgi?E+PZ+3+ABC0001+EECS0HD334H+7+WW


Well the front page shows a spec for internal resistance of the cap:
Internal resistance(max.) 75 (Ohm)

The  http://industrial.panasonic.com/www-data/pdf/ABC0000/ABC0000PE9.pdf link for their largest cap (.33F) shows graphs of internal resistance starting at 75 ohms and increasing as it ages over time.

So worst case charging or discharging current would be 5.5v / 75 ohms = .073 amps. That doesn't sound very dangerous for that specific cap to me?



retrolefty


Quote
I've already looked at all the datasheets, can't find anything about peak current...
Look at
http://industrial.panasonic.com/www-cgi/jvcr13pz.cgi?E+PZ+3+ABC0001+EECS0HD334H+7+WW


Well the front page shows a spec for internal resistance of the cap:
Internal resistance(max.) 75 (Ohm)

The  http://industrial.panasonic.com/www-data/pdf/ABC0000/ABC0000PE9.pdf link for their largest cap (.33F) shows graphs of internal resistance starting at 75 ohms and increasing as it ages over time.

So worst case charging or discharging current would be 5.5v / 75 ohms = .073 amps. That doesn't sound very dangerous for that specific cap to me? These kind of caps are designed to provide back-up voltage for ram chips and such, so they are not designed for high current density applications. Other 'supercaps' may be different and their datasheet should explain any special precautions.




wyager

That is, however, max internal resistance. It could be less. But again, since they neglect to mention overcharging, it is probably not a concern. One thing I find a little annoying though is that the cap appears to decay by about 5% every two thousand hours...

retrolefty

Quote
But again, since they neglect to mention overcharging,


What do you mean by 'overcharging' ? 

All caps will charge to match the applied voltage across their terminals and then current ceases to flow because cap and charging voltage are equal. If you try and 'overcharge' by increasing the charging voltage above the max working voltage rating then you will indeed destroy the capacitor. As long as the one stays within the voltage rating of the cap it's not possible to overcharge it. Now if you mean charging current rate density, then again we are back to the internal resistance of the cap that will limit the charging current rate per ohms law.

Lefty

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