Let's talk about the battery first. A 4.8V battery won't read 4.8 when it's fully charged since 4.8 is nominal voltage. A full battery will read around 5.4V. When the battery discharges to 4.7, below nominal voltage, it's going to be dead soon and needs a recharge.

Okay, charging. At first, a battery should undergo a series of slow charges at a rate of one tenth its capacity. Therefore, your 1200mAh battery should be slowly charged at around 120mA. After a while you could "push" it at the field to 1C, or 1.2A; that's as fast as I'd go. It will complete the fast charge in an hour. Remember, this fast charging should not be done with a new battery. I usually don't fast charge my batteries just to play it safe. I don't want to lose 1K+ worth of aircraft just because I'm impatient.

Checking the battery. Just hooking it up to a voltmeter tells you very little about the state of the battery. Without a load, an almost dead battery can appear almost full. Therefore, you must load the battery when testing. Products like the Hobbico Digital Voltmeter will load the battery if used properly. This will insure an accurate reading. Remember, a full battery will be around 5.4 volts and a "dead" battery will be below 4.8. Please note that voltage as a function of capacity is not a linear relationship. Basically, this means that if you had a battery whose voltage was exactly between 5.4 and 4.8 it would not be half used. It would actually be less than half used since the voltage tends to drop off fast to begin with and hover around 4.8 for a while. After the voltage hovers below 4.8 a bit it will suddenly drop off.