Does anyone else keep a battery charger in the back of their smart? I drive it every day so, don't worry about the battery failing due to non-use. My 08 got the upgrade just before I bought it, however, I still suffer flashbacks to it not starting in the dealer lot when it had the old battery and, would just feel safer having a battery charger w/me at all times. If anyone else does this, can you recommend any brands/types and, will it lower the gas mileage sitting back there (those things are heavy!)? THANKS!