Just to weigh in here...Mooncatt is right. However, I've been charging my phone overnight every night for years (different phones, of course) and have not detected appreciable loss of capacity (or enough that caused me problems). But, keeping the phone on the charger all the time will kill a li-ion battery faster than it should. The chemistry in the battery actually creates pathways between the anode and cathode of the battery which could short out the battery.
Heat will kill phone batteries as well. Unfortunately, most users use a case so that when we drop our Precious, it will be somewhat protected. Cases are an insulator around the phone, causing the phone to get hotter than a naked one. This could cause a problem while charging, making the battery warmer.
All that said, a battery is not functioning correctly if it is dying 5 months into it's normal service life. Can't tell you if the problem is with the battery itself or something running on the phone. BTW, my phone is rarely lower than 50% at the end of the day but there has been days where it has to be charged mid-day. Typically, that is due to cellular signal levels.
My $.02