Ford F-350 Battery Charge

2003 - Ford F350, 6.0l Diesel, Lariat, Dully, Fully Loaded: Battery Light Goes On And Off Randomly During All Types Of Driving And...

Is It True That When The Battery Is Discharged(in Any Amount) That The Alternator Holds Whats Left In The Battery Or Does It Completly...

I Charge My Batteries And The Truck Starts Fine. When The Truck Sits For 1 1/2 Days Or So The Batteries Are Dead Again. I Put On All New...