What is that exposed brown disk around the button on these batteries?

That's the protection circuit board, right? But why is it visible when it's usually (when I usually see it) under the outer plastic wrapper? And is it just a cosmetic difference or something more?

BTW that's from here: https://www.fasttech.com/products/1420/10001980/2204400

Looks like an insulator, or a washer.

Thanks.

I literally have no idea, I've never bought this kind of battery (18650) before. But I'm about to take the plunge.

You can read about which cells are good and what charger suits you here:
lygte-info.dk/review/batteries2012/Common18650IndividualTest%20UK.html
lygte-info.dk/info/indexBatteriesAndChargers%20UK.html
lygte-info.dk/info/isMyBatteryProtected%20UK.html

Awesome, thanks. And that leads to a page on battery anatomy, also helping a lot: The Anatomy of a Protected Battery

Here's what a protection circuit board looks like

I wonder if the cheap Chinese 18650 batteries that claim to be protected are truly protected? Between a cheap generic one labeled "protected" and a name brand that is not protected, which is more reliable?

"The PTC may not be present in high current cells and it may also be missing in cheap cells."
lygte-info.dk/info/isMyBatteryProtected%20UK.html

So it also depends on how you use it and whith which charger you charge it.
You can have luck with cheap cells.

When it's about sefety, i wouldn't take any risk.
Good protected cells are Keepower.

Thanks for the link--very helpful. It seems the generic ones I have are not protected.

Is it a bad idea (unsafe) to keep using the cheap generic batteries until they "go bad" (losing charges significantly)? I wonder if I should toss them up after a year or so even though they are still working well.

As long if they don't get warm during charge or use it's possibly safe.

Thanks, VegasTriker, the picture is filling in.

There are a lot of 18650 teardown videos on YouTube I found over the last couple days. Some show batteries clearly labeled "protected" which don't have a protection circuit board. I'm shocked! (Well, not actually that shocked, any more.)