Will the (overall) life of a battery be reduced if a car charger is used often? I've heard yes, no and maybe-- I am ready for a solid answer.

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Unless it is are really bad 3rd party unit no.. Lithium ION batteries do not have a memory effect, but they do have about a two year shelf life, so at about the two year point expect the battery life to start dropping off.. Sometimes rapidly.

They dont suffer from memory effects, but they do age (as Lazybones pointed out). Some good information here:
http://en.wikipedia.org/wiki/Lithium_ion_battery

The important thing is that your car charger stops trying to charge the battery when it reaches full power so that the battery doesnt overheat.

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MCSE: Messaging 2003; Windows Server MVP

Using a decent quality car charger is preferable to letting your battery run completely down while you're in the car. That WILL shorten battery life.

I think it's important to note that it's hard to get a deep discharge (0%) with most devices (including our phones) because when they get low they turn off.

I wouldnt worry too much about draining them completely; but I would worry about letting them sit in hot conditions for long periods of time (i.e. in the car under the sun or attached to a charger that is functioning incorrectly and causes it to overheat)

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MCSE: Messaging 2003; Windows Server MVP

As many have said, it will not negatively impact your life. Lithium ion batteries, from what I've read, actually do better with multiple short charges than they do with being fully drained/recharged.