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[moved] SUPERCAPACITORS: trusting the label...

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LAN_403

Signification

Wed Feb 17 2016, 10:39AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I recently ordered some 500F supercapacitors from ebay--they were labeled 500F @ 2.7VDC. Since I have handled a lot of these--this particular "500F" seemed very small and lightweight. If you are like me, you can't just go to your bench, hook up the supercap and read 100's of Farads. However, this time I really had a real gut suspicion about the 500F labeled value. Since I don't have a "supercapacitor meter"...never really heard of one...but I DO have a current/voltage regulated power supply, I decided to actually test this one.
I used the formula:

(1) I=C(dv/dt).

The idea was to simplify the process by setting the numerical amplitudes of "C" and "t" the same. This way allows me to write eq. (1) above as I=V:(numerically speaking).
Here is the process used to find "F": The -labeled- C=500F @ V=2.7V, would be charged @, say, I=2A, that is, a regulated constant current of i=2A. now we suppose that "C=t" or 500 'seconds' = 500 'Farads'. In other words, If the capacitor is actually 500F and is charged at a constant 2A for 500s then it's capacitance is 500F. Since I suspected the cap was under 500F, I watched the volt-meter display and timed the charge time with a stopwatch--just to be safe. Since "500" sec. equals 8:20, I expected the voltage to reach 2V in 8 min 20 sec, However, it was at 2V in only 80 seconds, which says the cap under test was only 80F--I tested seven more with the same results +/- 10s.

To verify things, I tested a "KNOWN GOOD" 360F @ 2.7V (A brand-name supercap "D" cell) Charging at a regulated 2A should take 360 sec (6 minutes) to fully charge to 2V--It was spot on! These 500F supercaps were FAKE!--the ebay seller knew--and did not hesitate to refund my money. OK, this works, and is convenient since more people have "regulated bench supplies" as opposed to "supercapacitor meters"--I looked for such meters with no luck.

"I THINK THEY ARE REALLY STARTING TO PUSH THESE FAKES--THE PRICE, PHYSICAL SIZE AND WEIGHT ARE TEMPTING!"

Ebay sells some nice low-cost digital readout Volt - Current power regulators which I plan to use to charge very large supercaps (to 9kF). Another feature of these boards with significant application in this process, is that the supply (controlled by push-buttons and a 4-digit LED module) also reads out the amount of CAPACITY=AMP*HOURS (fits nicely with Q=CV) expended in the charging process (note, the decimal may need mental adjustment for hr's vs sec's here. This 'running sum' continues until power is removed! Hopefully, constant attention is not required either, by using the voltage AND current limit function!

For instance, the fake 80F cap charged to 2V @ 2A would indicate:q=CV=80F*2V=160Q. The real 360F indicates (@2V): q=720Q. Note the difference!...also, the first thing I noticed when charging the 360F cap (@2A) was the much --slower--charging rate--even though I was charging a 360F cap compared to a supposedly 500F cap!

Also, if anyone is familiar with this, I was wondering if it is foolproof to leave the supercap connected since BOTH current and voltage are limited (the cc / cv LEDs would simply alternate? haven't gotten to this just yet.). Also, in the past (ten years ago), I had fallen victim to such a device that was destroyed when unconnected while the charge-holding device (actually a small battery that time) was still connected--My guess says: the battery current flowed backward through the device. My gut says: the newer ones, i.e. B3603 have been protected from such mishap as, discussed above.

My current device is the Minghe B3603 (36V @ 3A) BUCK dc-dc converter. I have just ordered the B3008 (30V @ 8A BUCK).

Dr. Slack

Wed Feb 17 2016, 02:47PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Yes, you can set CV to 2.7v or less and simply leave it, they are capacitors (and not LiIon batteries for instance).

Talking of which, and on the topic of trusting eBay, there are some wonderful 18650 LiIon batts on there. This size is the standard for (re)building laptop battery packs. A good capacity for that size is 2.5Ah. A constant battle of one-upmanship has led to cells with 5.4, 7.2 and even 10Ah being offered at increasingly lower prices. Avoid.

klugesmith

Wed Feb 17 2016, 08:44PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Good catch on the fake capacitors.
How soon will the fakers include some ballast, to give the product a respectable heft?

It's too bad most fraudulent vendors depend on the attitude that
"immediate money back to dissatisfied customers"
means
"There is no justification for negative feedback".

Dr Slack mentioned battery cells. Other nerd-bait items often overrated or counterfeited include laser pointers, wallplug power supplies, and flash memory devices.

Kizmo

Thu Feb 18 2016, 11:17AM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Nice find but I think this thread serves its purpose better under General Science and Electronics

Conundrum

Fri Feb 19 2016, 08:32AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

I've seen the ballast trick used, the latest scam is to use non ferrous materials (eg lead, byebye RoHS!) to get around buyers checking with a magnet which caught the earlier iron ballast fakes.

EDIT: Re. my earlier comments about fake GM tubes, the sellers have now changed this to "faulty".. people still buy them hoping to get a bargain even though some are up to air and basically worthless without a vacuum pump and suitable gas.
Unless you know how to tell or are willing to buy a job lot and throw out all the duds, save your money and buy tested ones.

I've been trying to explain to SO why buying used phone batteries on ebay is a bad BAD idea but she didn't listen. Fortunately I caught it at the bulging stage before it did any damage.

Signification

Sun Feb 21 2016, 12:58PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

OK guys,
I'm lost. What is the iron ballast trick?? ...fake weight? It sure was too light.

I ran into this tear-down video of EXACTLY the same 500F cap I spoke of.

Throughout, the guy had no clue that is is a fake

Conundrum

Sun Feb 21 2016, 05:30PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Basically they wrap the inferior cheap (typically 50F) capacitor in an iron tube, or they fill the empty space with iron beads.

Signification

Fri Mar 11 2016, 07:45AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

I used one of those cheap Chinese power supplies with the 4 LED digits and 4 push-buttons (...love those things, but you must get the hang of using them). Along with the equation t=CV/I. Rewriting the equation as t=(V/I)C and setting CV and CC to the same value i.e. V=2.5v and I=2.5A, yields t=C. So, make sure the cap is discharged first, then start the power supply (cap charger) and a stopwatch at the same time--for each second that elapses ('till the CV LED goes on and the CC LED off), each second is in 1:1 ratio to the Farads. When done, the seconds are numerically equal to the Farads. C tolerances are well within limits to know what C you have. Ratios can be changed--for example, if V=1v and I=10A, t=(1/10)C, for a x10 faster reading!--watch out the supercap's low "V" limitations though.

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