Battle Hardened permanently increases your Life meter by one pip, allowing you to withstand a small amount of additional damage.
Battle Hardened gives one extra pip of health upon spawning. However, if a player is damaged, the health added by Battle Hardened will not regenerate. A Medic can restore it using a Health Syringe.
The effect stacks with similar bonuses provided by Medic Buffs and from capturing a Command Post. The pips will grant 30, 25, 20, 15 and 10 health bonuses respectively; totalling a maximum of 100 health bonus.