when thinking of the battery bank aboard a cruising yacht, I like to think of a bank account (seems a very convenient analogy)
when your battery bank is FULLY CHARGED, you are comfortable & can use equipment at will ~ not unlike the great feeling of that full bank account
as you make withdrawals from this bank, you need to start considering how you will make deposits back ~ that same uncomfortable feeling associated with a dropping bank account comes to mind (see how relative this analogy is?)
* the alternator on the engine is one way to bring the batteries back up
* the shore power charger is another way
* some boats use solar & wind generators
unlike my cel phone, which seems to still work when the gauge says 2%, the battery bank on the boat shouldn't go below 50%
conversely, it is very hard to get right to 100%, especially when out at anchor or motoring
every boat is a little different, but you might come up with a zone between 50 - 90% - meaning you make sure you start recharging as you approach 50% & you keep that charging going until you reach 90% (knowing below 50% will hurt most batteries and above 90% might just be too hard unless you are alongside with great shore power)
BUT HOW DO YOU KNOW?
some people talk about measuring voltage, but that's going to throw you bad information if you've just been charging or drawing (making deposits or withdrawals) - so we're working to add BATTERY MONITORS to our fleet - these 'battery fuel gauges' are the only sure fire way to know where in fact the bank is
don't ROB the BANK ~ related MYSTERIES & MISCONCEPTIONS
~ some people over the years think that when you plug the boat in, DC appliances magically change over to AC appliances because the boat is plugged in ~ this is not the case ~ but you may be making withdrawals & deposits at the same rate and holding your bank balance 'even'
~ be aware of multiple chargers when they exist and make sure you have engaged all charging devices you need
~ also be aware that a big invertor/charger may take everything the dock can throw at it - so not the best time to also have hot water tanks & other high load items pulling too