Lithium Ion batteries pack a lot of power by weight compared to other types. There are 2 things that need to be handled differently than nicad on NiMH. They cannot be used as a direct substitute (even if they look like other AA's) since they run at about 3.6 (or so) volts. They cannot be charged in the same way as nicad or NiMH.
Normally, you put in a specified current (about 0.2C) until 4.2 volts is reached. Then, you keep the voltage tightly regulated at 4.2 volts until the charge rate drops to 10 percent (about 0.02C).
This design uses a regulated supply for both parts of the charging. Only the charging current is monitored since the supply voltage is maintained at 4.2 volts. The charging current is controlled using variable pulse width of the voltage feed.
Please Note: This type of battery is known to Explode if charged in the wrong way. It's up to you to decide if making this type of charger is within your technical abilites.