Charging laptop computer batteries used to be easy in the good old days of Nickel Cadmium technology and lead acid cells. All you had to do was build a circuit that would adjust the voltage in such a way that the charging current did not exceed 50% of the laptop battery's milliAmp hour (mAh) capacity. A couple of hours later and presto - you had a charged laptop computer battery. A few milliAmps above or below the maximum capacity were easily tolerated, with only a slight degradation in the overall life of laptop computer batteries.
Today, Lithium Ion laptop computer batteries are the most advanced technology in terms of energy generated per pound. These laptop computer batteries are unfortunately very unforgiving, when it comes to charging. The charging current and voltage must follow very strict curves over the 180 minutes recommended for a full charging cycle. Lithium Ion laptop computer batteries will not tolerate lower levels of current like the car battery can when it is close to full charge. Instead, laptop computer batteries require a special circuit that will control that current and limit it as the laptop batteries approach a fully charged state.
For this reason, all laptop computer batteries are equipped with a rather sophisticated charging circuit inside the laptop battery that regulates the charge and discharge current and also communicates with the computer about remaining capacity. This communication occurs over a specific data bus that is tied to power management software installed on every system. (Many users are familiar with the related Control Panel application that estimates remaining battery life and controls settings that minimize power consumption.)
This same charging circuit has a temperature protection feature that ensures the laptop battery doesn't over-charge and over-heat. Some laptop computer batteries achieve this functionality via a thermal fuse that opens when the temperature exceeds 50-60 degrees Celsius. Other laptop battery designs use a thermistor (thermal sensitive resistor) that communicates the laptop battery temperature at all times to the charging circuit and stops the charging process in case of overheating. Temperature management is so important in Lithium Ion laptop computer batteries that the newest laptop battery designs employ both methods and components.
An interesting fact regarding laptop computer batteries is that most of the laptop battery's charging takes place in the first 80 minutes of charging. This is the case in laptop computer batteries, despite the fact that a full charge takes generally 180 minutes. This is a little known fact that can save us time while waiting for that well-known amber light to turn into green. The graph above illustrates this fact. Notice how the battery's charge (on the Y axis) levels of dramatically after an initial period of time (X axis).
Please note! Despite this graph, new laptop computer batteries should be fully charged for at least the first three or four cycles to ensure that the laptop battery is properly "broken in".