It does not make a difference if it is dead or not. It should take max 3-6 hours to charge to full power even if you still have battery left. There is a big difference in how long it will last. There is a standby mode when you are not using it which most phones last up to a few days 3 max. But if you use it a lot the battery will die a lot faster. When I go on long trips I have a charger that I can put in the car so I do not have to worry about it. I also use the hands free kit which works very good to.
Most batteries today are Lithium Ion batteries. They do not suffer memory loss like the old nickel hydride batteries did. This happened when you would charge the battery when it was only 1/2 down and after awhile, it believed that 1/2, was empty. With Lithium Ion batteries, we generally advise that you charge them for 8-12 hours before use, but this is purely precautionary. I would definitely take the phone in to the retail store and advise them that you believe the battery is bad. I would guess they can either test and / or replace your battery.
All the phones now come with lithium ion batteries and charging should not be an issue … although it's best not to charge them over 24 hours..Overnight every night is fine … they all do have a life though leaving it off when you do not need it can extend that. Although you would miss calls.
Cell phone chargers as well as any other ac / dc power cube use a small amount of power even if the device it powers is not plugged in. The amount of power used is only a couple of watts but if you leave your charger plugged in all the time that power is being used 24/7. Over time it adds up. And if you multiply that usage by all the other power cubes you may have plugged in it can be significant. In addition, any device that has a remote control (TV, VCR, DVD, air conditioner, etc) also is using power when turned off. A good solution is to plug these devices into a power strip. Then you can just turn off the power strip to eliminate the "parasitic" power usage.
There is nothing at all wrong with purchasing an aftermarket battery for your phone. I have used many in years of cell phone usage. I would recommend that you purchase a Lithium Ion battery instead of Nickel Hydride. The charge current depends upon the technology and capacity of the battery being charged. For example, the current that should be applied to recharge a 12 V car battery will be very different to the current for a mobile phone battery.
Battery chargers for mobile phones are notable in that they come in a wide variety of connector-styles and voltages, most of which are not compatible with other manufactures' phones or even different models of phones from a single manufacturer.