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Battery Life (and Death)
A car battery should last about six years, but like most car parts, that all depends on how you treat it. Multiple discharge/recharge cycles shorten any battery's life and using electronics in the car while the engine is the quickest route to a dead battery. Of course, a battery can maintain a charge while the engine is on, but once it's off electronics draw directly from the battery.
To avoid this recurring auto nightmare, always turn the headlights and interior lights off when you're done driving. Remember that leaving electronics like GPS or cell phones plugged into a car charger can drain the battery, too.
No matter how well you take care of it, eventually your battery will die and you'll need a replacement. Failing batteries usually display obvious symptoms that let you know it's on its way out. Slow cranking on startup indicates that the battery may not be able to provide enough power to fire up the engine, and an illuminated Battery Warning Light on the dashboard is clear indicator it needs attention. If vehicle electronics like remote locks or interior lights randomly stop working, a dying or dead battery could be why.
Also, batteries-alive or dead-are full of chemicals, so do nature a favor and dispose of dead ones properly. Don't just toss it in the trash because chances are your local mobile mechanic or auto supply store can recycle it for you.
Source : http://www.msn.com/en-us/autos/autosownership/the-most-important-facts-and-myths-about-your-car-battery/ar-BBBh2A9