Super capacitor?

[najrao]

I picked this up along with two 4S,10Ah lithium batteries. They all had the same blue heat shrink, and I thought this was just another lithium battery (of different rating). Only this has M6 screw terminals, the others had flying leads.
Skinning the heat shrink off, I see a sealed metal can, carefully sleeved over by thin adhesive paper  (?) board. There are NO markings even after peeling this paper. One terminal is welded directly to the can's top, the other on an insulating stand-off.
I fed 200mA for an hour or so, taking it that the insulated post is positive. The voltage rose to 2.75V. If this is indeed a single super cap, I would be destroying it if I allowed a higher voltage charge, so I stopped. It holds 2.69V after 24hrs. By more careful monitoring of charge input, I can find its effective capacitance, but ----
How do I prove my theory that it is a super cap? How do i find its limiting voltage rating without stressing it by overcharge? What good if any is 2.5V single super cap? It may very well be a lithium battery, but unlikely.

[Rerouter]

A super capacitors charge curve will resemble a straight line, where a lithium will look like a 2 slop curve, shooting up rapidly to about 2.2V then curving off as it starts charging up its usable capacity,

[najrao]

Thank you, Rerouter. I had thought as much.
Discharging by 10.0R to 37% of initial voltage of 2.44V took some 9000s, and showed an effective capacitance of some 900F. Later, shorting further from 0.65V remaining into the 10A range of multimeter gave 0.5A, holding for 1min. with NO dropping at all. This from adsorbed charge.
One way to get at the voltage rating seems to be to charge with a very low current, only a little more than expected self leakage. I consider 10mA, or even 100, as harmless to a 900F cap at avalanche. Problem is: it would take TWO WEEKS to get to 15V if fed at 10mA. Reaching breakdown at much higher currents (amps) could destroy the cap pretty suddenly, so speeding up by this method is not advisable.
Comparing volumes with another super cap I have suggests that this can should house 900F at at more than 2.5V,  possibly up to 10! There is, alas, no way to get in and see how many unit caps of 2.5V in series there are inside.
I appeal to the infinite wisdom of eev membership to devise a method.

[Rerouter]

Discharge it down, Ideally with a constant current sink, If it drops the same voltage in the same time step, its a supercap, if it suddenly falls off like a rock under ~ 2V, its a battery,

[Gyro]

Thank you, Rerouter. I had thought as much.
Discharging by 10.0R to 37% of initial voltage of 2.44V took some 9000s, and showed an effective capacitance of some 900F. Later, shorting further from 0.65V remaining into the 10A range of multimeter gave 0.5A, holding for 1min. with NO dropping at all. This from adsorbed charge.
One way to get at the voltage rating seems to be to charge with a very low current, only a little more than expected self leakage. I consider 10mA, or even 100, as harmless to a 900F cap at avalanche. Problem is: it would take TWO WEEKS to get to 15V if fed at 10mA. Reaching breakdown at much higher currents (amps) could destroy the cap pretty suddenly, so speeding up by this method is not advisable.
Comparing volumes with another super cap I have suggests that this can should house 900F at at more than 2.5V,  possibly up to 10! There is, alas, no way to get in and see how many unit caps of 2.5V in series there are inside.
I appeal to the infinite wisdom of eev membership to devise a method.

Surely, no matter how little charge current you use, once it reaches breakdown voltage it will be immediately destroyed by its own internal stored energy.

[najrao]

Absolutely, Gyro. I was afraid of that too. Only, I don't know what else I can do. Hoping someone knows better and will tell me.

[digsys]

.... more than 2.5V,  possibly up to 10! There is, alas, no way to get in and see how many unit caps of 2.5V in series there are inside.

If there is 2 or more in series, there has to be voltage equalization / leveling resistors on each cap. It is usually 10-30mA or even higher. You would see that
with no load connected. BUT it may have an active circuit only turns on when the voltage rises too high (similar to some cheap LiIon BMS systems).
In that case you could use a thermal camera on the pack while under heavy charge / discharge, and hope to catch a "hot cell .
Last idea - if you have a DMM with 1 very sharp fine tip, just puncture the wrapping at each cell spot, and see if the voltage increases. I've had to do similar on
many occasions - the tiny hole mostly seals ok.

[najrao]

Digsys, thank you.
The sealed case appears to be stainless steel all over its six faces. There is absolutely no access to cells. Not even by x-ray, let alone thermal camera.
Any voltage balancing resistors would draw a current however small  even at 2.5V applied, but detecting it as apart from any charge/discharge currents will need an absolutely *constant* source. We can try a 317 fed by a large battery to hold the 2.5V constant within a fraction of a millivolt over minutes on end. Worth trying, do you think? Very skeptical I am.
If there is a steady current at constant voltage, it still does not tell how many cells.
The self leakage observed was: 2.75V dropping to 2.69 after 24 hrs. This may be shunt resistor/s, or intrinsic capacitor leakage.
Yours, baffled.

[Gyro]

Just a thought from scanning web images, lifepo4? ...

http://sell.lulusoso.com/selling-leads/1972447/3-2V-60Ah-lifepo4-battery-cell-for-E-Bike-hybrid-supercapacitor-battery-.html

[Siwastaja]

Just a note, there is a Chinese trend to call lithium ion batteries "super capacitor lithium ion batteries". It's similar to putting every related and non-related term to the title on Ebay to increase search result visibility.

If it's a battery and at 0V, it's dead. Just charge carefully to 1V or so and draw the curve. If linear, a cap. If highly nonlinear (in any way), a dead battery.

[najrao]

Gyro and Siwastaja, Thank you.
From the looks of it and comparing to the China e-bike super cap/battery, it would seem to be one such; I will search more thoroughly to glean any more available info.
I had to keep the terminals shorted directly- no ammeter- by a 14awg×<50mm long copper wire for over an hour to bring down the (shorted) voltage from 7mV to under 1. Removing the short, I find that the (open) voltage rises once again gradually, and has reached >500mV in just 3min., and still climbing. I suspect the internal connection to the much distributed electrodes constitutes enough (internal) resistance to prevent evacuating the whole of the charge by external terminals alone. And then every electrolytic capacitor behaves similarly, more or less.
I am quite convinced that it is a super cap and not a voltaic battery, though the similarities in construction retain vestiges of voltaic action in what is really a super cap, especially near voltage zero.
The China e-bike "lithium" super cap battery seems to be always 3.6V, and if this applies, perhaps there are no series connected cells after all. 3.6V may also be exaggeration, more like 3.0. Still, I continue to be bugged to find a way of setting a decent rated voltage for it.

Edit: I hate it when an OP drops his subject without a logical end. Not me, and so:
Picking up from Siwastaja's sugestion, I tried to discharge the "super cap" well as could, but even an hour of bolted short would not reduce the subsequent open circuit voltage to zero. It picked up again to 0.65V in half an hour. My inference of voltaic action in super cap may have been, to say the most charitable, far fetched. And so, I tried recharging from 0.65V with a controlled 1.0A input, hopefully to reach 1.65V in a long time. Lo and behold! It took less than 5 MINUTES to reach 2.6V! And so I learnt to treat it as a Lipo, and continued charging at the same 1A. 6.5 hours later, it reached 3.5V; charging was enhanced to 2.0A and another two hours on, it is still 3.55V.
The cells of all the other batteries I picked along with this are LiFePO4, and so may be this unit. I am limiting charge to 3.65V just in case: I expect over 20Ah to go in!
Needless to say, the charging battery is kept outdoors and well clear of inflammable stuff. So far, it has stayed cool to touch.