Time Is On Our Side
The useful life span of an average automotive battery is four to seven years. With the auto companies' eternal quest to make cars and trucks more efficient, two things have conspired to shorten this life span: smaller batteries and higher underhood temperatures. Higher temperatures are so damaging that many cars have insulating blankets for the battery, or the battery is mounted in the trunk.
A single instance of deep-discharging a battery can reduce its life span by half, so avoid running a battery into deep discharge. If you do, then charge it properly as soon as possible before the plates inside build up a layer of sulfates. A properly charged battery won't freeze, but a discharged battery will freeze at temperatures well above zero. Any battery that's been frozen is finished.
How Can You Tell?
Is one cell of the battery visibly different than the others? Look through the side of a translucent-cased battery. If one cell has substantially less electrolyte than the rest, or if one cell's lead plates are much lighter in color, then that cell has gone bad and the battery is junk. Charge the battery for an appropriate time (depending on how powerful your battery charger is) and then use a hydrometer to measure the specific gravity of the electrolyte. The more charge on the battery, the more sulfuric acid there is in the electrolyte and the higher the hydrometer float will be on the scale. A battery that shows a low state of charge on the hydrometer after a long charge is dead. Don't rush to judgment, however, because a deeply discharged battery will take a charge very slowly at first.
Is there some parasitic drain in your vehicle that's discharging your battery? The simplest way to check is to temporarily install a battery cutoff switch in the ground side. With everything in the car shut off, put an ammeter across the switch, turn the switch off and measure the current drain. A few milliamperes (ma) is normal to keep the engine computer, radio presets and clock alive. Anything more than 200 ma will run your battery down in a few days.
This battery hydrometer shows a fully discharged cell.