Charging 7.4V Li-Ion battery
The answer to your question is: 8.4 volts.  This will give you an idea of the voltages you will be working with.

However, first read and please understand the comments about safety of these li-ion cells.  They are an excellent source of power, but also a good way to hurt yourself and start a fire.  The li-ion cells from the laptop are VERY sensitive to overcharge, over-discharge, over heating, and physical damage.  As a result you must built or incorporate a battery management system into your charger that will deal with these issues.

To get you started:  Li-ion cells are charged via a constant current/constant voltage system.  You start by charging them at a constant, controlled current (less than 1C) until the voltage of the cell becomes 4.2 (or 8.4 with 2 cells in series), then you hold the voltage constant at 4.2 and allow the current to drop as the li-ion battery fills up.  Charging stops when the current drops to a pre-determined rate (for example 1/10 of the starting current).

Li-ion batteries do not need a trickle charge, and in fact a trickle charge is bad for the battery.

Also, when charging more than one Li-ion in series, after many charges, the batteries will start to become unbalanced. This means that that each individual battery in the series will not be the exact same voltage. This could result in over-discharge in one or more cells during use, or over charge in one or more cells when being charged.  The end result is a high likely hood of venting, fire and bad things.  So you need to add a Balancing charger if you intend to charge the two batteries in series.

So..... a little more than just set a voltage!  Hopefully this will give you an idea of where to get started, and give you a healthy respect for the potential danger of this technology.