Li-po chargers have a series of phases they go through as they charge a connected Li cell. In a rather simplified manner it goes something like this, generally they first read the battery cell voltage to determine that one exists and needs charging. Then it applies a constant current while at the same time monitoring the slowly raising cell voltage. At about 4.0v (per cell) it switches to a constant voltage output until the charge is completed at a nominal 4.2vdc terminal voltage.
Lefty