This is directly from the Awaytravel website https://www.awaytravel.com/battery

The policy for the airlines are spelled out and the FA/GA are only enforcing policy that has been established to keep us safe. The FA/GA usually have enough challenges with travelers, that we shouldn't attempt to 'one up them' when the policy is established in black and white.

Why would I need to remove the battery?

• If you're checking your Carry-On
  If you check or gate-check your Carry-On, you are required to remove the battery and bring it into the cabin with you (where you can keep using it to charge your devices).
• If you’re flying out of Asia
  We recommend removing the battery before you go through security in Asia—they have different airport security regulations, so this will make your trip a little smoother. You can replace it once you’re through.
• If you’re carrying on a Delta or United Airlines flight
  If you are carrying onto a Delta or United Airlines flight, the gate agent may ask you to demonstrate that the battery is removable. You can easily remove the battery from our Carry-Ons and bring it into the cabin with you (where you can keep using it to charge your devices).

What if the airline asks me about the battery?

Since these policies are still new, everyone at the airlines (or in line at security) might not be as up-to-date as you. If an employee ever has a question about your bag (or the rules), you can find the policies of the major US airlines below.

Delta
"Customers checking a smart bag with a removable lithium-ion battery must remove it and take it with them into the cabin. Customers carrying-on a smart bag containing a removable lithium-ion battery must remove it from the designed enclosure prior to boarding the aircraft. This is consistent with Delta's policy today requiring customers to place spare lithium-ion batteries in their carry-on luggage."

"Pack spare batteries in carry-on baggage."
From Delta Airline's official statement and from their policy page as of February 13, 2018.