Battery charging circuit Q
If you want to deliver a full charge to LiPos, then you need something a little more sophisticated than 'switch it off when charged'. 

LiPOs use a 2 stage charge. 

In the first stage, the terminal voltage stays below 4.2v, and the charger limits the current to within the manufacturer's specifications for max charge rate. Depending on the quality of the cell, this phase need only take minutes, or with lower current you can drag it out as long as you like. 

The first stage finishes when the cell voltage reaches 4.2v. At this stage, the cell only has around 70% full charge. Note that some commercial 'fast chargers' call 'charged' at this point.

In the second stage the charger limits the voltage to 4.2v, and the cell takes an ever decreasing current. Do not exceed 4.2v, or you will destroy your cell. If you stop the first stage at 4.1v, you will improve the life of your cell somewhat. The second stage finishes when the current drawn by the cell has dropped to 0.03C. This stage may take an hour, and cannot be rushed without damage to cell.

You can obtain LiPo charger ICs that will do the control and threshholding for you.