Well... respectfully... I don't think I'm wrong . I said there's no such thing as a 4 or 5 AH 18650, and that's true - there isn't! Just because you can combine many of them in series or parallel doesn't make that untrue. When speaking of 18650's (or AA batteries, or whatever), it's customary to talk about the capacity for just one. You don't say "An AA battery can be 28 AH!" just because I can combine 10 AA's in parallel each with 2.8 AH. The false claims of those knockoff vendors are also talking about one 18650. They claim some fairly absurd capacities, like 4 or even 5 AH per individual 18650, which is a good sign to stay away from that brand! The best 18650's I'm aware of at the moment are 3400 mAH, and those are new and fairly expensive.

True, but you might be surprised how bogus some of the claims are! Lights that run on high for 2 hours on a good, name brand 3.1AH 18650 have been tested to get 20 or 30 minutes of runtime on some of the knockoff batteries, which is in line with 10% of their "rated" (i.e, totally bogus) capacity. Obviously some do better than 10% of course, but you never know what you're going to get. Some of the less scrupulous vendors sell take-offs from other products with a new label stuck on. Li-ion batteries lose overall capacity with time (as a function of temperature and state of charge), so if the cell is already a few years old and has been stored in a high state of charge in warm temperatures, it can easily be in pretty bad shape. A few years of shelf life under those conditions can put it at 50% of its original capacity, which may have been 2.1AH, so it may be effectively a 1 AH battery being sold as a 5 AH battery. The worst cases seen are indeed in the 10% range.

Generally, it's better to get a high quality battery of lower marked capacity, than one of the ones making utterly impossible claims like 5AH per 18650.