SOLUTIONS FROM OUR TECH TEAM
Power, The Ultimate Aphrodisiac?
This is your column, devoted to the technical challenges of boat ownership. Our top
tech team is standing by to help with the answers to your questions
I’m on a trawler with a microwave oven powered by an
inverter. My house bank is two Trojan golf-cart batteries. I notice
that my battery monitor drops way low when I heat a cup of coffee
or warm a rotisserie chicken — below 12 volts. Then it jumps right
back up. Is this wrecking my batteries by making them think it’s a
full cycle? I know batteries only have so many cycles.
— Mac McCarthy
Ocean City, NJ
When all boat systems
are go, it’s a good idea
to run your engine at the
same time. Read about
current draw here, from
Don Casey: A 700-watt microwave seems like it should draw
just under 6 amps at 120 volts, but due to reactance inherent
in the microwave circuitry, the actual draw is closer to 10 amps.
Microwave ovens also operate about 30-percent less efficiently on
the square-wave power most small inverters supply, so now we’re
up to 13 amps. We calculate current draw by dividing watts by
voltage, so when the supply voltage is 12 rather than 120, the current draw is 130 amps rather than 13. Add to that the inefficiencies
of the inverter and your microwave is drawing close to 150 amps
from the batteries when it’s running.
Your two-Trojan bank is only rated at 225 amp-hours total,
based on a 20-hour discharge rate — that is, a constant load of
around 11 amps. It’s no wonder that when you draw 150 amps
from this bank, the voltage takes a dramatic dive. In answer to your
question, yes, this is probably shortening the life of the batteries,
not because of cycling, per se, but because these are thick-plate,
deep-cycle batteries and not designed to provide high bursts of
current. While your microwave is running, the current draw is
similar to holding down the starter button on a non-starting engine
for that long. About 45 minutes of microwave operation, less as the
batteries age, will completely flatten this bank.
One solution is to run the engine while running the microwave so that your charging system at least partially absorbs the
load. A second option is a larger battery bank. Dividing this load
over four or six batteries, rather than two, reduces the demand on
all of the batteries. A third possibility is to power your inverter/
He’s been one of
the most consulted
experts on boat care
and upgrading your
boat for 30 years,
and he has been on
our BoatU.S. “Ask The
Experts” website for
the past decade. He
and his wife cruise aboard their 30-footer part
of the year in the eastern Caribbean. His books
include Don Casey’s Complete Illustrated Sailboat
Maintenance Manual, and the recently updated
This Old Boat, the bible for do-it-yourself boaters.
The VP/Technical Director
for the American Boat
& Yacht Council (ABYC),
John grew up boating.
He’s been in the indus-
try since 1990, with
diverse experience from
a yacht brokerage and
boatyard to owning a
marine supply store. He and his family sail their
classic 1976 Irwin ketch, a boat he completely
restored, including the hands-on installation of
all systems. John is a trusted source for techni-
cal information for industry professionals.
He’s maintained, lived
aboard, and cruised long
distance on boats with
his wife and family for
most of his adult life.
He can take apart and
fix almost every system
aboard a boat, has writ-
ten two books, filmed
a two-set DVD on East Coast cruising, served in
numerous editorial and columnist positions for
top marine magazines, and has won seven first-
place awards from Boating Writers International
and many awards for his technical writing.