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What factors affect the charge time of laptop batteries? Is it the laptop's use while charging? The age of the battery? The charger's wattage?

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yes All of the above. IF it is a Li-Ion battery or Li-Poly:

Normally the battery is faster charged to about 80% (capacity not display numbers), then it slowly finishes up at a slower rate till it reached the peak voltages that are set . On some laptops the peak voltage for li-ion charge can be set down a bit , to insure longer life.

These alogrythms for charging are CC (constant current) a specific amperage of charge, and CV (constant voltage) where the voltage is held at Say ~4.1 or ~4.2 Per cell. In CV (the second stage) Because the voltage Difference or voltage differential between the battery and the charge gets smaller , the charge rate (or amps) dwindles down until the voltages are very similar. At some low rate the charge is then terminated. (much more simple than it sounds, yet gets complicated many ways)

Basically though, to 80% capacity the Rate, or ammount of power that can go to charge the battery should not change much. so that is one clue to your puzzel.

This all gets very muddy, as various versions of the alogrythms are applied. Some alogrythms which simulate or attempt CC CV methods, and do act similar to the "proper" CC CV method, but are not using actual proper CC CV. :-) are you having fun yet?

IF the power supply AND the internal power capabilities (in the laptop) can handle it, you could charge at a full rate and use the laptop to its maximum potential also, and it could do both without effecting either. A bunch of Ifs, where you should remember that stuffing a larger power supply of the proper type would not insure that the laptop itself would charge any different. There are cooling conciderations for the charge curcuit itself, and the rest of the laptop that smartly might not allow so much heat going on, or any increased rate, while being used or not. How they cheapened it or designed it to cope with , heat, power, and specs.

PLUS the batteries dont really like to be heated by processors, which gets ignored too often. A User can usually treat ALL li-ion batteries better , in any li-ion powered products, by being more kind with the heat. Another clue to your puzzel. Keeping the heat down by not using the laptop when charging can get a battery to last much longer. Keeping the heat at "normal room temps" for these "normal room temperature" batteries treats them better in all situations, even storage.

An aged li-ion battery loses capacity over time and charge cycles. with proper li-ion curcuitry, this aged battery should actually take less time to charge. With the increased resitance, and lowered holding capacity. Generally there is less power that it can Hold.

That is where things get really muddy again, when it comes to explaining the reality, and the different methods that can be used. With Increased resistance the voltage can reach the second stage sooner CV and take longer to charge , even though less will ever be put in.

Confused yet?

At some point the age of the battery (depending totally on the quality of it) will be so Bad that the voltage will never be reached. Again how the curcuitry deals with this situation varies. That could again Increase the time it charged for, even though less charge will exist. Waste because the cells no longer "accept" the charge as well. Here it should be noted that the battery could be so bad, that you see things like the displayed charge rate drops quickly when taken off the charge, it could be a battery that is end of life.

now days they even have implemented a maximum age and number of charges into the curcuit , and the battery is deemed to be at end of life. varies , and some done very poorly again.

Refers only to Li-ion type chemisty, and varies because of many factors due to different implementations of charge alogrythms, and safety and info curcuits.

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Corrected again, I had the resistance going the wrong way. –  Psycogeek Sep 25 '11 at 14:56

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