Never hook up a charger directly to the electrical system of any vehicle!! The safest way to charge the batteries is to remove them, top off with distilled water, test them if needed, then trickle charge overnight. Wal-mart sells some decent trickle chargers for around $20.
2006 Ford F-250 FX4 CCSB
Go: 6" Fabtech suspension lift, 20" Procomp Series 6001 wheels with 325/60-R20 Nitto Terra Grapplers, SRL Tune, Edge Insight CS
Show: Leer 700 Tonneau Cover, Pioneer X920BT Navigation, 8" JL Audio sub, Viper alarm/remote start