When thinking of your house battery bank it is useful to view it as a bank account.  Whenever you operate DC powered equipment, you are making withdrawals from the bank that will eventually have to be balanced with deposits.   The deposits can come from the alternator when the engine is running, a generator, shore power, solar, or wind generator.  

Unlike your cell phone battery which can run down to 0%, the battery bank on the boat must remain above 50%.  Going below 50% can seriously damage the battery.   Battery technology is very complicated, many would say akin to black magic, but there are some useful rules of thumb.   The state of charge, SOC, can be reasonably well estimate by examining the voltage.   A full battery is around 13.2 volts and at 50% would be approximately 12.2 to 12.5.  These measures work best when the battery has rested, neither drawing or charging, for two hours.   But are indicative even under load.  You can visit the Battery University at https://batteryuniversity.com/learn/article/how_to_measure_state_of_charge to learn more.

Some of our boat come with a battery monitor which provides a much more accurate state of charge indication than the voltage rule of thumb.   These monitors allow you to see charging rates, discharge rates, and current state of charge.  

It is important to manage your electrical usage while cruising:

• Turning off lights and electronics when not in use,
• Set refrigeration to the warmest level that preserves the food,
• Reduce opening and closing refrigerator,
• If boat is equipped with an inverter, limit its use to essential items.

The banking account analogy breaks down a little for making the deposits.   Unlike your bank, the battery has a maximum charge.   Adding to the battery after that point can damage the battery.   Your cruising yacht has equipment to prevent this from happening.   As state of charge of the battery increases the rate of charge decreases.  After 85% SOC, the rate of charge will change from bulk to absorption.  In practice, it is difficult to get the charge above 90% while you are cruising.  Again you can find more at https://batteryuniversity.com/learn/article/charging_the_lead_acid_battery.

When you plug into shore power a battery charger can be turned on to start adding power to the batteries.  The DC electronics on the boat are still taking power from the battery and are not directly run from the shore power.   The batteries will be filling at the net of the shore power minus the DC power consumption.

While you can spend a lifetime understanding all the details a few simple rules of thumb, will make your cruising more enjoyable:  

• Pay attention to the state of charge of the battery and don’t allow it to get to low;
• Don’t waste battery power running unnecessary lights and electronics; and
• Charge the batteries when needed.