Never had a problem with any single lithium ion on cells. I’ve carried up to 10-12 in carry on at a time. This is in past 2 years on flights to Europe from the U.S. I did however just have a 166wh battery for a battery operated chain-saw get taken away 2 weeks ago; which was totally my fault as I thought a single contained battery limit was 300wh.
Glad to know that, was about to limit my cells when travelling.
Not per cell, per unit.
For example you are allowed to carry 10x 3500mAh = 35000 mAh as separate cells. => 10 units
But if those same cells are inside a powerbank then it counts as 1 unit of 35000 mAh, which is above the allowed limit.
The rule is, what the most outside label says. When in a PB they won’t look into the PB.
It’s quite similar to bottles of water. Even if the bottle clearly has way below the limit in it, but the label says otherwise it’s forbidden.
You can take 90 ml in a 100 ml bottle but not 5ml in a 200ml bottle.
BTW the rules on water are changing. At my local airport they upgraded their scanners over a year a go and big bottles of water as no longer banned.
According to delta, spares can be brought in carry-on as long as they are in a proper container, like the box or plastic carrier they came in.
No, Your thinking is reversed—That is exactly what you are NOT ALLOWED TO DO.
If there is a battery fire in the passenger compartment, then it can be detected and counter measures taken to put it out;
If there is a battery fire in the luggage compartment, then it would not be detected and could destroy the plane.
Ya I flew last weekend domestically within the United States and I had several flashlights and a bunch of 18500, 18650, and 21700s because I didn’t want to deal with bringing a charger. I had two power banks - both 20,000 mah. No issues whatsoever
Yes, I can confirm this.
I had a few Li-Ion (they were 18650, I think) without the capacity printed on them.
The chinese security officer did not let me keep them.
Same for powerbanks, please ensure they always have a label with the capacity on them - if needed, put one on by yourself. 
I’m thinking, maybe it’s better to put in a DIY powerbank (eg. 4x18650) which has a “capacity label” marked on that DIY powerbank and then place those 18650 batteries inside. I wonder if this will get throught the China security inspection (even if the internal 18650 batteries inside may not have a capacity label). I’ve noted some DIY powerbanks that have capacity labels (below say '20000mAh") (even when you need to place your own batteries inside, which can be different capacity)
Yes, pretty much that approach worked for me on my last trip through China.
The (DIY) powerbank had a capacity label and the officers were pleased. They didnt try to open the powerbank.
For obvious reasons, there are a lot of battery wraps with brand and low capacity marks on the Chinese market 
.
On other boards it seems some airlines have had issues with Li-on batteries in overhead compartments, they are now saying they want the batteries stored in the carry-on that goes under the seat.
Fires are more advanced and harder to access in the overhead compartments.
I normally travel with my lights that way anyways so to me it makes sense. Stopped carrying extra batteries as most of my travel lights have onboard charging. The lights are always mechanically locked out as well.
Technically, the batteries inside flashlights, as well as flashlights with built-in batteries, still need to be checked, they just don’t usually do so.