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SHERMAN^(OPPOSIT
E,^2 )When pondering replacement batteries, options abound, but take a
system’s approach to determining where you’ll get the biggest bang for
your battery bucks. BY ED SHERMAN3 POWER ME UP
SYSTEMSI
t’s time: Your engine barely
turns over to start and
your cabin lights are getting
dimmer faster, both signs that
you might have a shortage of
available electrical power. That
means you’ll either need to
replace your existing batteries
or fi gure out how to improve
on your current power supply.
What are your options? A
trip to the West Marine web-
site is telling. Currently, it lists
56 different battery options.
Of those, 17 are fl ooded-lead
acid; 36 use absorbed glass mat
(AGM) technology; there are
seven gel-cell batteries; and
three lithium choices that are
proprietary and dedicated
replacements for Torqeedo
products. If you have any faithin market-driven inventory
control, then the choice is
clear: AGM is the way to go
today. If the buyers at West
Marine select twice as many
AGM battery options as com-
pared with traditional fl ooded
cell devices, that speaks to
me. Further, the only lithium
choices West Marine currently
offers are dedicated to a specif-
ic product that has always used
high-quality lithium technology
as its power source.
So let’s look at where we are
with each of these technology
choices and what you need to
consider if you are thinking
about shifting from one to
another.IT’S A SYSTEM
To really dig into this whole
matter and get it right todayrequires thinking about batter-
ies as part of an onboard sys-
tem. This system includes the
batteries, an alternator, voltage
regulator, a shore-power-driven
battery charger, maybe an
onboard AC generator and,
for the cruising sailor, water,
wind and/or solar chargers. The
system also includes both the
AC and DC wiring needed to
properly interconnect all the
other equipment. The prudent
sailor will probably want to add
a battery-monitoring system
as well to act as a fuel gauge for
the electrical power supply.
Let’s begin this analysis by
answering the question: What
are we trying to achieve?
Value for your battery dollar
is the most likely answer.
Historically, I’ve always used
cost-per-amp-hour of power asmy guide. This is determined
by knowing the average cycle
life and using the battery’s
amp-hour rating and either a
50 percent or 80 percent dis-
charge, depending on the rec-
ommended levels of discharge
per cycle. Next, multiply the
amp hours by the number of
cycles, and then divide the
battery price by the number of
amps derived earlier, and you’ll
have a value for the cost per
amp hour. The problem with
this approach is that it relies
heavily on battery-cycle life,
which is controlled in large
part by all the elements in the
system that I described above.
When I wrote the fi rst edi-
tion of my Power Boater’s Guide
to Electricity back in 2000, the
numbers looked like this:
Flooded-cell lead-acid:
0.00065 cents per amp hour
Gel cell: 0.0027 cents per
amp hour
AGM: 0.0013 cents per
amp hour
Keep in mind that these
fi gures were based on manu-
facturers’ estimated cycle life
for their respective products,
so there may have been some
“optimism” in their advertised
cycle-life expectations. Also,
because we really didn’t know
any better at the time, I
assumed an average 50 percent
discharge level for each cycle
for all three technologies;
deeper discharge levels hadn’t
been recommended.
Today’s numbers look a bit
different because they take
into account current pricing
and 80 percent discharge lev-
els for both gel-cell and AGM
technologies. Here’s how
today’s cost-per-amp-hour
values add up:
Flooded-cell lead acid:
0.015 cents per amp hour
Gel cell: 0.010 cents per
amp hour
AGM: 0.016 cents per amp
hour
Clearly, prices have changed
for all three types of batteries,
but it appears that large
disparities between them have
been reduced considerably.
(Cycle-life data for this
analysis came from both West
Marine and civicsolar.com.)
Add to these prices the cost