If you are putting it in storage, you have to leave them at 40-60% charge.
@raulvolador has a thread that explains this somewhere in these forums.

I think the final word is not out on this one. I have seen some pretty scientific test that have not shown significant difference in degradation with regard to SOC used for storage.
In this report the result was that there is no significant difference if stored at 40% or 100%. the only thing batteries don't like is being discharged too deep. the longer they stay under minimum voltage the worse.
So if you fly until force auto landing, let them cool down and recharge at least part asap.
For long term storage I'd store the battery separately. There might be a tiny battery drain otherwise that could drain the battery below minimum voltage over a few weeks. Even better: Fly
Tello frequently and don't bother!