I know it all varies and depends on the airline, country, weather and what not, but in general it is OK to have a flashlight in carry-on with a battery inside, right? How about spares - is there a generally accepted rule for that?
According to this thread ‘lose spares’ are verboten but ‘properly stored’ ones are ok? What’s properly stored? Is e.g. a battery wrapped in electrical tape to prevent accidental shorting OK?
I’ve carried more than 5 18650 flashlights, each with a spare battery, on international flights in the past in my carry-on or on me. I never had even so much as a question at any of the airports . The reason for the number of flashlights were that on some vacations we were trekking through the Amazon and Africa, and my family members didn’t see the need for flashlights. Until it got dark and they realized that we weren’t in San Francisco where there weren’t street lights everywhere 
Batteries were in the flashlights and spare ones were in plastic battery cases.
It’s mandatory it’s in your carry-on and not checked bag. I always have at least one light, often 3 in my carry-on, can’t say I’ve had any issues (thus far).
For storage; plastic boxes look more official than tape.
Nope - the TSA folks do not like things they do not understand.
As in - what are you hiding under the tape?
Yep - if it can be easily inspected. I would avoid batts. with really oddball logos or graphics.
All the Best,
Jeff
So, cells branded for a certain vape shop demo.
Not sure about other countries but any flights originating from or terminating in the USA are subjected to FAA rules. Per FAA website, “Spare (uninstalled) lithium metal batteries and lithium ion batteries, portable rechargers, electronic cigarettes and vaping devices are prohibited in checked baggage. They must be carried with the passenger in carry-on baggage. Smoke and fire incidents involving lithium batteries can be mitigated by the cabin crew and passengers inside the aircraft cabin.”
More details are here. Lithium batteries in baggage
One caveat is when you have to gate check your hand carry and they will ask you to remove the batteries.
I’ve never had TSA or any other airport inspector pull batteries out of my carry-on to look at them, whether they’re in my lights or battery cases. So my experience logos or even tape on them are non-issues.
In my experience the TSA bag check experience is a bit variable. Agents within a given airport will be relatively consistent, but the process another airport - with a different set of peers and management - can differ considerably more.
Sometimes I got asked what this “large pack of cylindrical objects” is, but after telling them that it’s a box of spare Li-ion batteries everything was fine.
Btw, this is a recurring topic and you should be able to find a lot of information in older threads.
...it would ruin my trip if I was obligated to throw away a light at security...
Yeah, trust me, nothing would be “thrown away”.
The one “confiscating” it would surely give it a good home… even if it’s perfectly legal for you to carry it on.
That said, best is to do it the way it’s done in shipping, ie, by “storing” it in a POS throwaway light.
Somewhat related: in Kanchanaburi (the home of the bridge over the River Kwai, kind of…) there is a weekly flee market. One stand features a variety of Swiss Army knifes, mostly the tiny Classic keychain ones. On some probing the vendor mentioned the security check at the BKK airport as the origin of his stock…
