I have never had a problem on Transatlantic or domestic USA flights.
Carefully follow the guidance on your airline’s website and also the national guidance.
I individually insulated the cells and then packed them in a 4-cell storage case. I also carried a 26650 cell inside a light.
No issues at LHR, LAX, SLC or FCA. My bag was hand searched at Heathrow, possibly because of the batteries. There wasn’t much of anything else in my carry on beside clothes and some earphones.
TSA:
Battery terminals (usually the ends) must be protected from short circuit (i.e., the terminals must not come in contact with other metal). Methods include: leaving the batteries in their retail packaging, covering battery terminals with tape, using a battery case, using a battery sleeve in a camera bag, or putting them snugly in a plastic bag or protective pouch.
Loose lithium batteries are not allowed in checked bags.
https://www.delta.com/en_US/traveling-with-us/baggage/before-your-trip/restricted-items
Delta:
Tips to properly transport spare lithium batteries:
US Federal Guidance
Spare (uninstalled) lithium ion and lithium metal batteries, including power banks and cell phone battery charging cases, must be carried in carry-on baggage only. Lithium metal (non-rechargeable) batteries are limited to 2 grams of lithium per...
Power Banks, cell phone battery charging cases, rechargeable and non-rechargeable lithium batteries, cell phone batteries, laptop batteries, power banks, external batteries, portable rechargers
UK CAA guidance:
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