Is charging your phone past full bad for the battery? In short, yes, charging your phone past 100% stresses the battery. This can cause damage that reduces the lifespan of basically any phone battery today.
Most phones contain lithium-ion batteries. Fortunately, a site called Battery University has a lot of information on how to protect these batteries. This is a quote from their article on charging lithium-ion batteries:
“Li-ion cannot absorb overcharge. When fully charged, the charge current must be cut off. A continuous trickle charge would cause plating of metallic lithium and compromise safety. To minimize stress, keep the lithium-ion battery at the peak cut-off as short as possible.”
Battery University
This confirms that excess charging is bad for the battery. It damages the battery and reduces its long-term functionality. There is no benefit to charging over 100% capacity, and trickle charge creates unnecessary stress.
It is also important to note that the battery should not raise more than 18ºF(10ºC) while charging as this can drastically reduce battery life. While this is hard to quantify for an average user, basic precautions should be taken such as not leaving the device on a bed or pillow, and not using very intensive applications while charging, such as games. Turning off the device while charging is actually preferable as it allows the battery to maintain a consistent charge rate and reduce chances of damaging the battery due to overheating or voltage stress, but this is not very practical for many users.
Do you need to charge your phone fully after buying it? The rumors that phone batteries need to be fully charged, activated, or otherwise woken up upon buying is completely false. A phone should never require an 8-12 hour charge, and charging for that long can actually be detrimental.
Lithium-ion batteries, the batteries contained in almost all phones, are actually “activated” long before shipment in mobile devices, and that process can be read about on Battery University here.
It is actually best to not charge the battery past 100% capacity at any point, as lithium-ion batteries cannot hold excess charge and are unnecessarily stressed from trickle charging.