Is draining a Li-Ion to 2.5 V harmful to a Li-ion cell?
The minimum discharge voltage varies between various sites, datasheets, etc. but 3.0 V - 2.7 V is an empirical value. For redundancy and to reach max. battery life, your device has to track voltage and shut down before the over-discharge protection will. but not to maximize battery lifetime. Just as lithium chargers have to stop at 4.2