2 years ago
Last edited at 5:25PM on 1/4/2012
With most batteries (even with most that claim not to overcharge) will overcharge. It won't ruin it, but if you do it every single day for a year and a half, you'll start to notice a slow, gradually accelerating drop in the battery life. You'll probably be able to get a new phone by the time it would become problematic. Best bet: don't fully drain it, charge it at the right time; between 15% and 5% charge, and try to avoid overcharging it for TOO long (i.e. the whole day). Oh, forgot to mention, you should also drain it completely once every 3 to 5 weeks, but don't drain it constantly.
The circuitry in the provided charger should stop the charging once the battery is full, however, leaving it plugged in keeps it sort of heated up. Heat is bad for batteries and electronics. It doesn't get THAT hot, so overnight shouldn't be too bad. I have a digital lamp timer, so I set it to only charge for 2-3 hours and finish just before I wake up in the morning. Works well and it's always 100% charged when I wake up.
The iPhone series of phones, like most smartphones, use a lithium ion battery. A lithium ion battery is reliable so accidentally leaving it on charge for a full 24 hours is alright, but try not to. Don't permanently keep it at full charge, drain the battery to under the 20% warning at least once a month. This is called a battery cycle, this should be the first thing you should do whenever you get a new phone, regardless of make or model. A cycle will show your phone where to shut down automatically at low battery to save just enough power to keep it's clock running. If it's charged to full or over 80% all the time, over time the average battery life time will deplete massively and the phone will automatically shut down at 40% instead at around 10%.
Why are so many people saying no? It does harm your battery in a way. When leaving it on overnight, it makes the battery life last shorter. It's happened to mine very recently. I left it on overnight every day for about a week and the battery went from full day to half a day.