My Friend and his VW Golf Battery

My buddy has a VW Golf (other manufacturers are available) that died while on the way home.  The AA determined that a new alternator should be fitted but the battery is still not performing correctly.  After leaving the car for 24 hours on the drive it failed to start this weekend and the battery was flat.It is highly likely that the battery went completely flat as the proverbial pancake when the alternator failed during driving.  The alternator converts engine energy into electrical energy in order to recharge the battery.  Batteries do not like to be hammered and run down, fully drained. This fully drained state can cause irreversible damage.  When we say that a battery is discharged, often it still has about 1/3 charge remaining and can recover with a gentle overnight charge. Once seriously damaged from discharge a battery cannot usually be recharged either by a mains charger or from the on board alternator.We have advanced testers, as used by auto electricians, and also advanced chargers that can be used to check and recharge the battery very slowly over a day or two.  This approach might keep you on the road for a while but in all honestly the path of least resistance and probably the best course is to buy a new one.