I have four Air 2S batteries, all just over two years old. Three are 3500mAH, one is 3750mAH. All have just about 20 charges on them. I don't notice any swelling.
Are they safe to fly with? Is there anything I can check? I don't want to lose the drone in flight because of a battery failure.

Hi there. Thank you for reaching out. You may initiate a test flight using this battery as long as there is no error message on the DJI Fly app. Before the test flight, make sure to fully charge first the battery that you are using on the drone and always check the aircraft status on the upper left corner of the DJI Fly app. Perform the test flight in an open area away from any possible interference. Should you have other inquiries, feel free to contact us. Have a nice day ahead.