I say it has to do with the charger itself. I have always used OEM chargers on my phones (razr, treo, Q) and other than the Q, the battery life I have not seen a difference. The Q just had horrible battery to begin with.
On the otherhand, my brother bought a car charger from best buy and his battery took a dump, I'm not sure if it was OEM.
Car chargers now claim they have a special chip that prevents overcharging.
CIGARETTE LIGHTER CHARGER - PMNHG
Specially designed circuitry protects your Blackberry from overcharging and short-circuiting