when to charge life battery

Hi, regarding LIFE battey few questions please. 1. till what voltage Life battery can be used. For example 2S (3.3*2=6.6V) Life battery. is it ok to use till 3.0V ?. 2. After fully charge it shows in the charger 7.20V, but in the Transmitter (T16SZ) it shows 6.7V . Why is that ?