Electric double-layer capacitors

 

Hi sec!

I'm looking for an EDLC to power a uC and while not an issue here I'm
wondering about current limits. There are cases where current ratings
are not published or unbelievable as in [1].

I have specifications for Nesscap EDLCs such as ESHSR-0003C0-002R7:
3F
rated Voltage 2.7 V
DC ESR <79 mOhm
Max. Current 1 sec discharge rate to 1/2VR: 3.3 A

This cap should deliver 2.7 V / 79 mOhm = 34 A short circuit current,
about 10 times the rated maximum current.

How much would be a reasonable maximum current if it is not specified?

1) 1/10 the short circuit current? Would that be different for EDLCs
with equal capacity but different ESR?

2) As much as keeps the capacitor below its rated temperature? Calculate
the heat dissipated in the ESR and assume some heat capacity of the cap
or heat dissipation? Or just feel how hot it get's? Using the cap at the
maximum rated temperature appearently reduces it's life time to
something like 1000 h (42 days) so I guess if that's not what one wants
it is better to keep the cap cooler.

3) anything else?


Something different ... Is there a correlation between ESR and leakage
current?

Thanks, Bernhard


[1]
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=390046823966

    * Voltage: 2.7V
    * Capacitance: 1 Farad
    * Dimension (D x H): 8mm x 13mm
    * Weight: 1g
    * Small in volume, high capacitance in Farad level.
    * Large discharge current up to 1000C extreme long life up to
100,000 times.

1000C sounds like 1000 A. However, the smallest ESR for 2.5/2.7 V EDLCs
I could find at digikey.com was 90 mOhm. 2.7 V / 90 mOhm = 30 A. 2.7 V /
1000 A = 2.7 mOhm. Appearently there is something wrong here.

I asked the seller:
"Does that mean 1000 A discharge current? Can I have a data sheet? Who
is the manufacturer?"

He replied:
"It means that the capacitor may instantly release an large enough
discharge current. We don't provide data sheet. The capacitors are made
in China."