Do laptop's power bricks provide constant voltage?
So far as I know, power supply "bricks" for laptop/notebook computers are not really "chargers" in that they do not attempt to impose a suitable regime for charging a bettery of specific chemistry according to it's current measured state. They are simply power supplies. 

Any charging circuitry is, I believe, usually incorporated within the computer housing the battery.

The name "charger" is often a misnomer.

You can therefore expect these power "bricks" to provide a near-constant voltage when connected to a load that draws a current within the range provided for by the designers of the system.

If the load is too small or too high, the voltage may deviate from the stated value considerably.