I have a 1999 Formula 41 PC. I have purchased three new group 31 deep cell batteries 105 amp hours each. After fully charged to 12.8 volts, it takes only 3 hours out on the water before they drop to 11.2 volts. While out on the water, I only have the stereo on, water pump, fish finder and head pump running. I don't understand how three batteries only last that long? I guess i need to buy a Victron battery management system to figure it out, but wanted to know if this is common among boaters? It seems freaking insane to me. Any recommendations would also be greatly appreciated. THank you.