Weak alternator, dead cell in the battery, or something is on pulling power from the battery when the cars is not running.
Probably alternater if the battery is less than 5 years old, may be the battery, may be both, may be neither.
The alternator does not hold a charge. It charges the battery. If it will not charge the battery then either the alternator is defective, battery is defective, or the connections are corroded or loose. You should read 13.5 to 15.5 volts at the battery with the engine idling. Check it with a DC volt meter. If you get less the alternator is defective. If you get much more the voltage regulator is defective.
If the battery is less than 2 years but doesn't seem to hold a charge
Kinda-sorta. With the right kind of charger, regular batteries can be recharged to some degree. They won't hold as much charge and may overheat. And they'll hold less and less for each charge. Not many people find it worthwhile to do.
Apart form normal use of the battery, it is possible that the battery is getting old. Most types of rechargeable battery become less able to hold a charge as they age or may indeed have a finite number of times that they can be recharged. This will happen to phone batteries and to car batteries. When this happens you need a new battery and make sure that you dispose of the old battery properly (they contain chemicals that harm the environment).
12.6 volts 100% charge 12.4 volts 75% charge 12.2 volts 50% charge 12.0 volts 25% charge Any less and the battery is essentially dead.
Less than $1
Your battery will be on for a short time (i'm talking about, like, 70% less charge then it used to).
Either the battery is bad and won't accept a charge or the alternator is not putting a charge into the battery. Have the battery tested. You can hook up an inexpensive volt meter to the battery while the car is running. You should see from 13.5 to 14.5 volts if the alternator is working properly. If it is less than 13.5 volts then the alternator is not charging.
It won't hold a charge. It discharges quickly or discharges without a load. The case sides are bulging. The battery gets hot in patches or overgasses. The plates look white rather than grey. If it is an opaque case battery, the lower half is whiter and less opaque. A post is loose.
Yes. But you will only have a battery with a little less than 4.5 volts capacity.
A battery float charger is intended only to keep a fully charged battery from losing its charge. It will NOT work to charge a battery that is discharged or damaged. Check the voltage of the battery to be charged. Do not use charger if the battery voltage is less than 9.6 volts.