Planing on taking the drone on an airplane with me and understand I should completely discharge the batteries before hand.

Can you really completely discharge them?

Will it harm them?

In the FAA part 107 training...they recommend putting the batteries individually in a plastic bag to prevent contacts from contacting anything metal.  My understanding is that fully discharging the batteries does hurt them.  At a minimum..it will shorten the life of the battery.

If you are talking about 0 volts, no that would be bad for your battery. If you are talking about only having a single digit percentage capacity remaining... yeah that would be ok.

Battery level should between 45% and 65% when the Intelligent Flight Battery is to be stored for a long period of time. If the Intelligent Flight Battery is to be stored for more than three months, it should be fully charged and discharged once every three months to prevent over discharging.

BeamMeUpScotty Posted at 2017-1-8 15:12
Battery level should between 45% and 65% when the Intelligent Flight Battery is to be stored for a long period of time. If the Intelligent Flight Battery is to be stored for more than three months, it should be fully charged and discharged once every three months to prevent over discharging.

Just a quote from the DJI FAQ

Batteries are usually rated for a certain number of charging cycles.  Deep discharges use that rating up quicker.

It's completely unnecessary
The FAA and airlines have not made that recommendation.
Airport security won't be checking and wouldn't know how to.
It's a myth spread in forums that's based on requirements for shipping bulk lots of batteries as freight.

Here's what the FAA say on the matter and pretty well all airlines base their battery rules on this:
https://www.faa.gov/about/office ... s_and_batteries.pdf

Check your airline's website and search for batteries to see their policy.

To attain the best life your Li cells should be be discharged below 20%  

Now more confused than every

This is from the Dji battery safety booklet

Travel Notice
1) Before carrying the Intelligent Flight Battery on an airline  ight, it must  rst be fully discharged. This can be done by using your aircraft until the battery is depleted. Only discharge the battery in a  reproof location.

Then they go on and say later that complete discharge can be harmful.