In Crysis 2 when the maximum nanosuit perk ends, you exit armor mode completely. My suggestion would be, when it ends, just start reducing energy than right out completely exiting it.

Not a bad suggestion.

what about a warning when there is about 10 seconds max nano left?

what about a warning when there is about 10 seconds max nano left?
Nice suggestion, a visual representation of when it will end, so you can take cover or quickly activate armor mode.