Check the voltage on the LiPo with a voltmeter immediately after charging... it should close to 8.4V if it's fully charged.

LiPo chargers charge at a constant current until the max voltage is reached (4.2V per cell), and then they charge at that constant voltage (reducing the current to maintain the max voltage). The charger should end up reducing the charge current to zero (shutting off), leaving the 2S pack at 8.4V.

BTW... JR and Spektrum battery packs have all three leads on the connectors, and the "extra" one (the signal wire) is not connected to anything. JR and Spektrum use this extra lead on their switch harness and their batt input on the RX to allow for direct servo control (JR 72MHz) or RX/TX binding (Spektrum)...