I'm seeing wildly varying capacities that are seemingly fake on 18650 cells, such as these "9,800 mAh" ones on eBay, but are ~$1 / cell.
However, 'genuine' cells from Panasonic and similar name brands seem to top out around 3,500 mAh or so, but cost more like $8 / cell.
How can I tell true capacity apart from the lies?
Obviously when you already have one in your hands, you can measure its true capacity.
However lets for a moment think about where these fake cells might come from:
In the first case you can be sure that the real capacity is on a much lower level.
In the second case the capacity is at most at the level of the best competitors on the market.
In conclusion: anything that is above the well established brand names is a blatant lie. Anything that is at their level might be a lie or could be true, no way to figure out than to measure it.
If the cell is actually genuine (no guarantee about that) then it should follow the datasheet. Note that genuine and unmodified 18650 cells are not generally sold through legitimate distributors such as Digikey or Avnet, probably for liability reasons.
Most of the ones you see with unknown brand names (anything with 'fire' in it, IME) will have a fraction to a small fraction of the claimed capacity. Maybe 1000mAh rather than 4000 or 6000 or whatever they are claiming this week. The crap ones will be significantly lighter than the genuine cells, and probably made with inferior materials internally. If you're lucky (and they claim protection) they'll have short-circuit protection polyswitches to reduce the chances of drama. Undervoltage protection is also possible, but less common.
Many of the folks selling these online on eBay, Aliexpress etc. are shady criminals to begin with in that they're lying about the contents of the package and dropping it in airmail. Extremely dangerous practice.
There are websites (probably flashlight related) that do some testing and you will have a better chance if you follow their recommendations. You can also find some cells with protective circuits added from distribution.
I bought some 9800 mAh Li ion 18650 batteries on eBay and tested them with my genuine SkyRc charger/tester. maximum capacities were between 990 and 1080 mAh each with a sample set of four cells. Yes I was hoping for 2500 mAh, but for two dollars you get what you pay for. Where weight or size is not an issue, you are correct; The liars offer cheap energy capacity at even 10% of the advertised capacity. All the usual caveats apply with purchasing substandard products so good luck. :)
