snip:  We have been on shorepower for 18 months & hope to disconnect for
trip to Bahamas. Wondering what effect this may have had on our
batteries. M/V Mello Moon

You didn't mention the type of batteries, their age, or the charger, all
important to this discussion.

For brevity and comprehension, I'm going to simplify terms and
descriptions, so hopefully the list experts won't jump me if I've left
out details or grossly over-simplified something.  I'll assume he has
flooded batteries for this discussion.

Your concern is: what is the likely remaining life in your batteries.
As previously stated, typical batteries in typical use typically last
four to five years - with lots of exceptions.

There are a number of things that cause batteries to die.  Two of the
more common ones are failure to hold an adequate charge and a shorted
cell.  Failure to hold a charge is usually due to diminished plate area
due to sulfation.  When a battery is fully charged, the sulfuric acid
electrolyte's concentration is strong and its specific gravity is high.
As the battery is discharged, the sulfur transfers out of solution in
the electrolyte and deposits onto the plates.  In turn, the acid becomes
weaker and its specific gravity decreases. Recharging the battery
reverses this action and the sulfur is driven off of the plates and back
into solution.  The problem that develops is when the battery is not
timely recharged and the sulfur remains on the plates for more that a
couple of days.  Then the sulfur turns into a hard sulfate that is no
longer easily driven back into solution.  This sulfate diminishes the
total capacity of the battery and is likely one of the most common
causes of premature battery failure.  Incidentally, charging  the
battery to the point of gassing (bubbling) can help to break loose some
of the sulfate crystals.  Equalization is a more serious form of
charging into the gaseous state.

It should be noted that achieving gassing is also important to overcome
another problem that batteries experience.  That is stratification.
Stratification refers to the acid separating out into various strengths
or densities.  The heavier acid will settle to the bottom of the cell
with the original density in the middle and the least dense on the top.
Problem here is that the lead material of the plates can be damaged
(read sloughing) by the stronger acid at the bottom and also by the
watery electrolyte at the top.  Charging to a gassing state stirs the
electrolyte and prevents stratification.  It might be noted that Trojan
specifies regular charge that finishes with a relatively high voltage
which, I suspect, is intended to both help drive off sulfate and to
destratify the battery.  If a battery is simply left on a standby charge
and unused, it should be brought up to a gassing phase routinely, say
every month or so.

Besides sulfation, the other typical cause of battery failure is a
shorted cell.  The plates can be shorted due to warping from to high a
charge or overheating, but the typical short is due to the build-up of
sloughed plate material in the bottom of the cell reaching the bottom of
the plates and electrically short circuiting the plates.  Battery
manufacturers purposefully leave an open space at the bottom of the cell
below the plates to allow for this.  One manufacturer I personally know
called this space the "trash pit". How much they leave is strictly up to
the manufacturer and is likely determined by a number of trade-offs.  In
order to attain a greater amp-hour rating, they might opt for larger
plates and less room in the trash pit trading off electrical storage
capacity for a reduced overall life.

How fast the trash pit fills is a function of how much material is
sloughed off the plates.  Each time the battery is recharged some
material is shed.  Higher charge rates and greater amounts of discharge
cause the plates to slough more.  Charts that graph battery life with
depth of discharge show that battery life in discharge cycles is
inversely and exponentially related to the average depth of discharge.
This is one of the primary reasons I advocate using larger battery banks
where possible to minimize the average depth of discharge.

So we see that the charger and its application are crucial to achieving
long battery life.  Most chargers made in the last century were dumb
chargers.  That is they charged to a single compromise voltage and
remained on at that voltage indefinitely.  Probably more batteries have
died while connected to that type of charger than ever died of old age.
Most newer chargers are smart three of four stage devices.  They do a
bulk, then acceptance charge bringing the battery up to a gassing level
for a few hours then drop back to a true float voltage.  Four stage
chargers actually turn off the charge entirely after a period on float
if little current is being drawn.  Batteries kept properly watered and
charged with a smart charger can easily last more than ten years as long
as they aren't overheated or abused in other ways.  Good Luck!

Tom Collins - Misty Sea

Tom,
Wow!  As excellent an explanation of battery behavior as I have ever seen. Thanks for sharing it with us.

Mark Richter, Ortona, FL
Mark's Mobile Marine
Electrical System Specialist