The stock batteries in DSMs are not reputed to last very long. Many people experience failures within the first year.
Most people recommend replacing the stock battery with another brand, such as an Optima, Diehard or other aftermarket make.
It should be noted that poor contact (corrosion) on the battery terminals can cause the battery to behave as if it were dead, even though it may be ok. Also, persistently dead batteries may be the fault of the alternator or voltage regulator, not the battery. Poor idle and other problems can be caused by a defective battery.
Another little-known and highly annoying fact is that auto batteries, with few exceptions, are not designed to be run dead and then recharged, making them very different from most other rechargeable batteries. Recharging the battery too quickly will result in large amounts of internal heat, causing battery damage. Typical recharging methods (auto battery chargers, or running the car for a while) may damage even "bulletproof" batteries. Charge dead batteries gently to avoid this problem.
Those caught in the dead-battery trap will want this refresher course in how to properly jumpstart a car. A few notes on these techniques: the negative is connected away from the dead battery (onto the frame) to minimize the chance of creating a spark that might ignite hydrogen gas leaking from the dead battery. This is what can cause a battery to explode, not the 'parallel' nature of the batteries, as described in DDD#7. Fortunately, few modern automotive batteries are prone to leaking flammable gases, but better safe than sorry.
For lots of info on batteries in general, read Alex's Electronic Resource Library
The Last Word: Some DSMs just seem to eat batteries. The proliferation of stock and aftermarket electrical accessories such as headlights, fog lights, in-car entertainment and navigation systems can strain a 12V system. That's why some car makers have changed to the 42V system. Storing a car for the winter with the battery connected can also draw down the cells owing to current draws from accessories in standby modes.
Battery-eating monster cars may also eat a corresponding number of alternators, making things extra hard on the battery. Investing a hardy deep-cycle battery like an Optima yellow or blue top might prolong your battery life - or, possibly, just give you enough to limp home the next time your alternator fails. Remember, the less times you discharge your battery, the better - even one discharge to 9V or less can be quite hard on it.