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| [GMCnet] Inverter selection [message #180824] |
Sun, 19 August 2012 08:19 |
k2gkk
Messages: 4452
Registered: November 2009
Karma:
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Senior Member |
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The following was posted on one of my "boating" info lists.
I believe it applies greatly to our GMC coaches and deserves
consideration by our members. Our safety is no less worthy
than those venturing out on the water!
I hope it's not so long that it gets rejected.
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
~~ ~ D C "Mac" Macdonald ~ ~~
~ ~ Amateur Radio - K2GKK ~ ~
~ ~ USAF and FAA, Retired ~ ~
~ ~ ~ Oklahoma City, OK ~ ~ ~
~~ ~ ~ "The Money Pit" ~ ~ ~~
~ ~ ~ '76 ex-Palm Beach ~ ~ ~
~ www.gmcmhphotos.com/okclb ~
~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
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There are several issues with inverter selection, only one
of which is the inverter device. Other significant issues
include the battery bank that will power the inverter, and
any alternate means you have/plan to have to charge the
batteries (generator).
In general, about size. Small size means small components;
i.e., small transformer, small fans, small heat sinks for
diodes, etc. Heat is an extreme enemy of these devices.
That includes internally developed heat and ambient air
temps of their surroundings. Small componentry increases
the chance that the device will experience heat related
problems. Also, low cost inverters are designed for use
as stand-alone devices, not as components of a larger
electrical system. You'll need to assure that the device
you buy meets ABYC codes for use on boats. Most do not.
For example, there should be a relay inside the device
that switches the safety ground onto the inverter when
it is operating in 'invert' mode, and off the inverter
when its operating in "passthru" mode. Harbor Freight
type devices are not generally compliant with marine
safety codes, and it is your job to verify that whatever
you buy carries a "UL Marine" or equivalent testing
certification and to understand the standards to which
the device was tested. The Magnum unit that others have
mentioned is a superb unit, and does meet the marine
safety certifications. It can be mounted in any
orientation, so I have mine bolted to the overhead in
the engine room, mounted up-side-down, where it is
out of the way of everything else, but also near the
batteries.
I'm sure you understand that the 120VAC power the inverter
puts out comes from energy stored in the boat's battery
bank. A 1500 watt space heater cube will draw 158 amps
from the batteries all by itself, so even run for a very
short time, will chew up the amp hour capacity of the
battery bank. Likewise, with a microwave oven. A small
900 watt microwave oven will draw 95 amps DC from the
batteries. Furthermore, because of something called
Peukert's Law, high levels of discharge current
significantly reduces the total amp hour capacity of a
battery bank. These devices (space heater cube, hair
dryer, microwave) can easily use as much amp hour capacity
by themselves in 15 minutes that the whole rest of the boat
would use over 8 hours, so you need a fairly robust battery
bank to support them. A 500 aHr bank would be marginal;
a 750 aHr bank would be desirable. Bigger is better. Do not
discharge deep cycle batteries past the 50% level. Doing that
shortens battery life. Actually, 40% max state-of-charge is
better.
Aboard sanctuary, we have stopped using the inverter alone
for these purposes. When it's time to run a hair dryer, or
the microwave, or to make space heat on a chilly morning/
evening, then I run the genset for that period of time.
It's much easier on the batteries.
You need to also plan for how - and how quickly - your
battery bank can be recharged. On boats, batteries typically
recharge from either the on-engine alternator, from an
onboard genset, or from shore power. With 15 amps from a
genset or shore power, the Magnum MS2012 will charge batteries
at a rate of 100 amps; the MS2812 will charge at a rate of
125 amps. It'll take 2 - 3 hours of genset time to replace
the amp hours of a 50% discharged battery bank of 500 aHr.
Sanctuary is fit with a 110 amp HO Balmar alternator on the
main engine. It charges the batteries at the same rate as
the MS2012, and likewise takes 3 hrs to charge a 50%
discharged 500 aHr battery bank.
Finally, Sanctuary has a single battery bank that serves
for both house and engine starting. My engine starts fine
with a 50% discharged bank. But as Bill Donovan said, it
is essential to have a way to monitor bank State-of-Charge
(SOC). Magnum offers an add-on for that purpose that's
only about $200. If you have the DIY skills to install this
system yourself (not hard, by the way) the cost of a Magnum
platform will be 2 boat units. If not, double that to include
the electrician's labor and markup on installation materials.
So here are the messages:
1. Make sure the inverter you choose meets the ABYC Marine
safety standards for use on boats;
2. Make sure your battery bank and re-charging systems can
support the AC loads that will run on the inverter; and
3. Think of this as an integrated system, not just a component.
Do it right the first time, or be disappointed, experience a
breakdown or two, and do it over.
Hope this is useful.
Jim
Peg and Jim Healy aboard Sanctuary
Currently lying Ipswich, MA
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