Yhyjyk

in about 3 months I will be flying from Estonia (Tallinn) to New York (Newark) via Stockholm Arlanda by SAS. I have an Aukey 30000 mAh (~111 Wh) battery bank which I'd like to bring on board with me. I did some research and discovered that batteries over 100 Wh require approval by the airline, and on my previous flight from Tallinn on British Airways I wasn't allowed to bring in the battery bank with me, and had to hand it over for storage.

I've contacted SAS customer service and they have given me approval (verbally, over the phone) to bring the battery bank on board in my carry on. They also say it's written somewhere in the booking notes, which the check-in staff would see. However, how would the staff at airport security in Tallinn and Newark know that I'm permitted to carry it? Can they look up my booking and verify it there, or should I request a written approval from SAS to show security?

In short, they don't know, but they also (usually) don't care. There are three different things at play here:

1. 
   Airport security, run by the TSA and its equivalents in other countries. The TSA is fine with power banks. In some airports, you're required to take power banks out of your bag, but this is just to ensure that they're not bombs or something. (Of course, large lithium batteries actually make pretty impressive incendiary devices, but apparently no terrorist has figured this out yet...)


2. 
   Aviation authority rules, set by the FAA and its equivalents. This one is cut and dry: FAA regulations prohibit carrying portable batteries over 100 Wh. Your airline does not have the power to waive this!


3. 
   Airline rules, which each airline can set for themselves. In this case, you've received permission so you're good.

In practice, what's going to happen is that your power pack will (most likely) be waved through TSA security and allowed on board by the airline. You are technically violating FAA regulations, but it's unlikely that anybody will notice or stop you, or that the extra 10 Wh poses a meaningful extra risk to anybody if they don't.

FWIW, I always travel with a 20000 mAh power pack and I've had somebody inspect the label exactly once, and that was in Beijing, which is rather infamous for really strict and often pointless security rules. I'm somewhat surprised to hear Tallinn did the same to you, but in my experience this really is the exception rather than the rule.

Since I'm now I cannot comment directly on jpatokal's post. It is not the case that the FAA bans all battery packs above 100 Wh. As their own link states, up to two batteries above 100 Wh and up to 160 Wh are permitted onboard, with airline approval. Sometimes the airline approval is just provided carte blanche; sometimes it's provided with advance permission, as in your case.

Many countries's security are lax about this and don't check the capacity of the battery. China is one of very few places where I've had a security officer examine the rating on the battery.