Super Cap leakage current
Super Cap leakage current
(OP)
I'm examining a data sheet for a supercap for a memory hold-up application.
Normally Nichicon has great products and this supercap may be another except they've got some cockamamie brief statement of what the ALL IMPORTANT leakage current is. No where do they define it clearly - to me anyway.
In the following data sheet they state:
Leakage Current | 0.5C(mA) [C:Rated Capacitance(F)]
http://nichicon-us.com/english/products/pdf/evercap_en_uw.pdf
General Nichicon SuperCap data sheet.
http://nichicon-us.com/english/products/pdf/evercap_en.pdf
Could they really mean that a 1Farad offering is going to have a 500uA leakage current??!! That's 50 times more leakage than the next company's offerings.
Normally Nichicon has great products and this supercap may be another except they've got some cockamamie brief statement of what the ALL IMPORTANT leakage current is. No where do they define it clearly - to me anyway.
In the following data sheet they state:
Leakage Current | 0.5C(mA) [C:Rated Capacitance(F)]
http://nichicon-us.com/english/products/pdf/evercap_en_uw.pdf
General Nichicon SuperCap data sheet.
http://nichicon-us.com/english/products/pdf/evercap_en.pdf
Could they really mean that a 1Farad offering is going to have a 500uA leakage current??!! That's 50 times more leakage than the next company's offerings.
Keith Cress
kcress - http://www.flaminsystems.com





RE: Super Cap leakage current
"0.5C (mA)", where C is value in Farads.
.: 1.0 F would be 0.5 * 1 in mA, or 0.5mA, or 500uA.
There's a one in a million chance that they intend mA = microamp (basically a newbie typo), but extremely unlikely.
Or this might be one of those worst case things, but no guarantee if it's that bad.
I'd 'make friends' with an Application Engineer there if possible. Emails, possibly leading to a long telephone call. Typically worth a fortune, but free.
RE: Super Cap leakage current
Thanks much for the confirmation VE1BLL. I'll have to change to a 76 cent cap instead of this 42 cent cap.
I've got a perfectly good hold-up using an Li 2032 coin cell that provides months of backup for about 32 cents for the battery and the holder but due to bureaucrats you cannot ship these puny coin cells on passenger aircraft period or more than 4 mounted inside products with the USPService. So I have to dump a multi-month 32 cent solution in lue of a 90 cent solution that holds up the memory for a delightful 20 hrs.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Super Cap leakage current
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RE: Super Cap leakage current
The reforming, that is said to be necessary if a VFD has stayed inactive for a prolonged time, and the discussion that we had some time ago has worried me somewhat. So, I found a few old electrolytics that has been around, and never in operation, for at least ten years and did some tests on them. They were 2200 uF/16V and I connected them in series with one 10k resistor each to be able to watch the leakage current over time. I ran them at 15 V, well-regulated and without ripple.
It developed into a small test in my ongoing "The Automation Engineer and the Reality" and the conclusion is that you do not have to worry much about small capacitors at low voltage. There's simply not enough energy available to start the leakage current->high temperature->increased leakage->higher temperature avalanche that causes large capacitors to explode.
I haven't finished the text yet, but the results were interesting, I think. At least, they were not alarming.
All five capacitors had an initial leakage current that was very similar to the other's and with a mean value close to 6.0 uA.
The mean leakage current after 1 h operation was 4.2 uA and 20 h later all leakage currents had dropped to 1.1 uA.
This was at room temperature, around +22C and I haven't done any tests at elevated temperature yet.
The most interesting fact here is that:
1. these capacitors behaved quite well after more than ten years of sleep.
2. they all had very similar leakage currents
I then reversed polarity (yes, a NO-NO!) and watched leakage current when voltage was increased from zero up to 5 V. It grew überproportional and there was a tendency for an avalanche when I interrupted the experiment. Leakage current (reverse) was then all over the place with 130 uA as the highest value. What was really interesting is that the capacitors behaved quite well as long as reverse voltage was kept below around two volts.
NOW - listen carefully - I then restarted the leakage current measurements and after around 1 h, the leakage current was back to a low 1.7 uA and sinking.
These measurements take long time. I logged it all with a recorder - so I wasn't tied up for 20 hours, but still. I have gained some insight and will be doing the same test with a few other capacitors. If there's any thoughts about what to look for - please let me know.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Super Cap leakage current
The ultimate cap I guess I'll have to use is this one:
http://www.cooperindustries.com/content/dam/public/bussmann/Electronics/Resources/product-datasheets/Bus_Elx_DS_4376_HV_Series.pdf
You'll note its leakage is only 10uA @ 1.0Farads. So a lot better than the scaled version of the Kemet and way better than the O.P. one that bleeds copiously at 500uA.
Gunnar: Very interesting... What you're seeing jives closely with all the supercaps I'm looking at. They all make a big fuss about the leakage current value being viable only after some fixed time under bias. Usually at least 30 minutes. I'm very pleased to see that after reverse biasing your caps seemed to take it in stride and return to low leakage performance.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Super Cap leakage current
RE: Super Cap leakage current
Keith Cress
kcress - http://www.flaminsystems.com
RE: Super Cap leakage current
Also, triple-check the bureaucrats. Usually the specs themselves are better written than that, and would provide explicit exceptions for tiny cells.
On the other hand, I've noticed that some of the batteries used in avionics, can't be shipped by air. I guess they're trying to preclude shipping an entire box of them by air, but still...
RE: Super Cap leakage current
RE: Super Cap leakage current
So, I agree with VE1BLL - if there is room, AA or AAA works very well - but may need to be changed every third or fourth year.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Super Cap leakage current
I'd rethink the 50% assumption at the 500nA load point.
RE: Super Cap leakage current
RE: Super Cap leakage current
I seem to have missed some numbers along the way. 500nA * 1month/ 1farad = 1.3V ?? Just what is the allowable droop for your circuit? Seems to me that even the 10uA leakage is too much; I seem to get a droop of 26V after 1 month.
TTFN

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RE: Super Cap leakage current
The point: There are alternatives to lithium. If lithium is the issue.
RE: Super Cap leakage current
Battery cost = $0.36 x 2 = $0.72
Holder cost = $0.21 x 2 = $0.42
Total Solution = $0.72 + $0.42 = $1.14
(140mAHr/500nA)/24Hr/day = 11,666days. Hmmm. That could be enough. :)
Interesting idea iop995. This thing only has about 31V available on regular electrolytics. And the biggest problem has been the switcher it uses to isolate and drop the voltage, so at the moment another switcher is not something I'm too eager to embrace. :)
Gunnar; I actually switched to an AAA which fit and was about a $0.50 solution before re-reading the clock chip data sheet and seeing a minimum battery voltage of 2.0V is required. Can't fit two AAAs.
iop995; It appears transportation doesn't have any interest in non-Li batteries.
Hi IR.
I don't think I'm following you. My clock circuit needs 500nA to stay happy. Super caps only really come in 2.7V or 5.5V versions for this app. 2.7V doesn't give any headroom since it has to be charged thru a 0.6V drop of a diode. So the next cheapest supercap solution appears to be 0.33F @ 5V. $1.14 total solution.
Charged to 5V - 0.6V = 4.4V
2V is the dropout voltage. 4.4V - 2V = 2.4V of cap droop.
Equivalent resistance for 500nA = 4Mohm
-4M x 0.33F ln(2.0/4.4) =>>> 12Days (based on the assumption of constant drain which is probably wrong and means the time might actually be 1/2 that.
The supercap solution has an additional problem. The caps are rated with horrid lifetimes. Standard is 1000Hrs at 70C. Pathetic. I have to have another AL cap in this design and I can easily use a 7000hr @ 105C cap for that.
I think I'm going to opt for the two A76s. After choosing and using the Li solution, then changing to the AAA design and laying it out only to recognize it was unusable. Then designing in and laying out the 5.5V supercap.. I am now going to opt for the A76 Alkali solution.
VE1BLL; Cheapest rechargable I can find is 1.78 whoops that's Li based too. A lot of rechargeables are NiCad (horror) or Li (shipping horror) Seems a no go.
Thanks you guys for the dialog. Much appreciated.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Super Cap leakage current
TTFN

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RE: Super Cap leakage current
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