As a ABYC electrical tech I have tested installed and designed countless battery systems. One of the biggest complaints I get from my customers is my batteries don’t last when I am at anchor. In most cases I have found the boat owner just doesn’t really understand how batteries really work.
To start lets go back to electricity basics. Think of Voltage as pressure pushing electricity in and out of our battery system. Second is current flow or Amps, the speed at witch the battery is being depleted. The amp hour rating is the maximum Amp capacity a battery is capable of delivering at given rate of discharge.