It is off topic, but as long my answer does not trigger off a longer discussion (and it might help the OP to decide what to do for his project), I’ll answer this.
Especially Lithium batteries don’t like to be/stay fully charged over a long periode. They also dislike beeing stored away fully charged or discharged.
They are built for charge/discharge cycles and cope best when used the intended way.
If you have an intelligent charger which keeps the battery between 15/20% and 80/85% you can keep the batteries plugged in (if you need to).
Have a read here
http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries