Portable battery charger: how can I recharge the batteries without direct connection to my home electricity?
You can make battery powered system. Use 12v battery and buck converter to 5v to save power(instead of using linear voltage regulator which cost much power).
your idea is to make automated watering system&gt;&gt;&gt;&gt; so i suggest you to place water tank in height may be 2 to 3 feet above the pot water level. and use solenoid valve instead of motor pump. and use tip122 to switch ON whenever you want to put water to plants.
are you using any sensor to read water level ? or if your board has more waiting time i suggest you to put your arduino in sleep to save power!!!! use this library here.
In power saving mode using Arduino board does not help you much. you may need to use barebone.
##################
if you find difficulties, do one thing
if your power plug and balcony is in one digit meter distance, place arduino where you get power. get lengthy wire connect it to switching transistor and valve... Simple...