How can you tell circuit board on 18650?

How can you tell there is a circuit board that regulates overcharge and over-discharge on the battery 18650?

Aside from physical differences (the board on the negative terminal, and a strip running along the side of the cell), you can't really be sure you have a working protection circuit unless you test it. Drain the cell to 2.4v (that should be safe enough), and if it doesn't cut out before that, you probably don't have a working circuit on it. Similarly, you could try over-charging with a charger that goes to 4.35v, but I wouldn't recommend that. The over-charge protection probably kicks in around 4.25v.

The search function is your friend.....
http://www.lygte-info.dk/info/battery protection UK.html
P

peter yetman said:

The search function is your friend.....
http://www.lygte-info.dk/info/battery protection UK.html

Click to expand...

The link does not answer the OP's question (unless you expect the OP to teardown the cell to compare against photos there).

Well that was constructive,
If you can't say something nice.......
I'm out.
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peter yetman said:

Well that was constructive ...

Click to expand...

Actually it was meant to be constructive (not a critique). Namely, while no doubt there is helpful info on the linked page, explicitly pointing out that there is no definitive answer there may save folks the trouble of reading (and rereading) that long page searching for the answer. And knowing that the question hasn't been completely resolved by the link may encourage others to add further answers.

There are some protected cells where it may not be so easy to discern the underlying circuitry without unwrapping them, so I think is it good to have further discussion on this, esp. since it could be very dangerous if someone wrongly concludes that a cell is protected when in fact it is not.