Clearly, your boat batteries are a vital part of your 12 volt electrical circuit. But in a way they're only the middleman; they just store the electricity which is generated elsewhere and meter it out as required to your 12 volt systems and appliances.
These three elements - requirement, storage and supply - must be properly matched or the whole system will be compromised.
Too little storage capacity (the boat batteries being of insufficient amperage), then they'll need constant recharging.
And if your charging equipment doesn't provide enough power then your boat batteries will never be fully charged, a situation leading to their early demise.
Getting the balance right is important on any boat, but probably more so on sailboats than powerboats because:~
Modern cruising yachts demand increasing amounts of its energy to power their ever more complex systems, so adequate means of both storing and generating it are of vital importance. Fundamentally - if disappointment is to be avoided - the relationship between supply and demand must be properly managed, ie:
Energy In = Energy Out
which means you can only take out what you put in. Rather like a well-managed deposit account - hence 'battery banks', possibly?
So to properly manage our energy equation, we need to evaluate both sides of it:
It's important to match the battery bank capacity to the current requirement. Too low a capacity and the charge times will be long and frequent; too large and it will be difficult to fully charge it through an engine driven alternator.
Before we go any further, we should have a look at battery amp-hour ratings. These refer to the available current over a nominal period until a specified voltage is reached. Rates are normally specified as either a 10 hour rate or, almost invariably these days, at a 20 hour rate.
This means that a battery rated at 100Ah at a 10 hour rate with a final voltage per cell of 1.7 volts is capable of delivering 10 amps for 10 hours, when a cell voltage of 1.7 volts is attained. Thus a 6 cell 12 volt battery at this stage would show a residual voltage of 10.2 volts - or flat, not a good shape for a battery to be in.
But don't be fooled into thinking that a battery rated at 20 hours is 'bigger' than one rated at 10 hours for the same capacity. It isn't, it's the other way round. If you were to choose a 100Ah battery rated at 20 hours, it would have 10% to 15% less capacity than a 100Ah battery rated at 10 hours.
So, a battery's capacity depends on how fast you discharge it. It's very important to understand that the faster you drain your batteries, the less capacity you will have at your disposal. The good news is that the converse is also the case. Discharge your boat batteries at a slower rate than the one specified and you will find that you have more capacity than you thought you had.