Let me second the thrust of Peter's remarks with different math.

Really good deep cycle lead acid batteries nominally store around 15
watt hours per pound (multiply 20 hour rate capacity by voltage and
divide by weight). However, if you discharge them all the way, they
will have a very short life. It is also hard to get them fully
charged, so a good estimate of actual usable capacity is around five
watt hours per pound, particularly in high drain applications where
the nominal 20 hour rate capacity is too high.

This means that if you want to use your proposed 1.5KW inverter for
one hour, you need 1.65KWH of usable battery capacity, which will
weigh around 330 pounds. This assumes the inverter has 90% efficiency,
which is probably close.

You can beat the weight with NiCd or NiMH, but the cost is so high
that I have seen NiCd batteries for house loads on only one boat, an
ex-Royal New Zealand Navy survey vessel. You can also cut it somewhat
by using cheap, light batteries, but their life will be much less than
a heavy deep cycle set (might be as few as 25-50 cycles).

It also means that you have to be able to put 3KWH back into the
battery (lead acid chemistry is only around 50% efficient). Since it
would be better not to charge it at the high rate you're discharging
it, you'll need to run your engine, with a new, big, alternator for at
least a couple of hours after each hour of inverter time.

As for cost -- the alternator and smart voltage regulator required
will be around US$1000. Good lead acid batteries cost around $2 per
pound, so it's about $600 for the batteries. Add wiring, fuses,
bracket for the alternator, new, heavy battery boxes, and the
inverter, and you're well above $2,500 if you do all the work
yourself.

One possibility is to put in a much smaller new battery set, but a
really big alternator -- say 200 amps -- and run your engine whenever
you want power. This will require careful management, as the battery
will not like being charged at that rate, but it will be a lot
lighter. Not much cheaper, though.

BTW, if you put in an alternator and battery set solely for the use of
the inverter, consider going to 24V. It's much more efficient and
requires much smaller wires.


Jim Woodward
www.mvFintry.com

Peter Bennett wrote in message rnews.com...
On Mon, 06 Oct 2003 04:41:29 GMT, "John Tretick"
wrote:

Hello Group,
+
I have a 24" Bayliner Cierra 2001 and I am interested in using AC when not
at port. No place to install a generator, so I'm thinking an invertor with
a batterstore. Possibly increasing my batteries to 3 and wiring in GFI
circiut to power mixers, inflators , hairdriers, etc.. when I'm anchored.
Northern tools sells a sealed Marine invertor for 399 its 3000/1500W which
works for my purposes.. doubt it would run the AC..
but I guess I'd have to get a larger on for that..

Has anyone installed this redundant power source defore? I'm curious how
long you can run on it without depleting one battery..
Actually thinking about installing a third battery for this purpose.

Thaks,

John

You can use an inverter to power small AC loads for fairly long
periods, or large loads (like a microwave or electric kettle) for
short periods. If you have a 1200 watt 120 V load such as a kettle,
it will draw 10 amps at 120 volts, but the inverter will draw a little
over 100 amps from the batteries to produce that power.

A Group 27 battery (about 25% larger than a traditional car battery)
would be rated about 110 amp-hours, if I recall correctly, but that
rating assumes the battery is discharged over 20 hours, which would be
a draw of only 5 amps. Drawing 100 amps, you will likely only get
around 50% capacity, so you could only run your kettle for about 30
minutes before the battery is completely discharged.

To determine the battery capacity you need, you will have to look at
the likely AC loads you will want to use, and the length of time you
will use each one, to compute the total amp-hours required per day.

You will also have to make some provision to recharge the batteries -
this will require a high capacity battery charger, and probably an
upgrade of the alternator and regulator on the engine, if you expect
to be away from shore power for more than a day or two.

You should also re-arrange the batteries so that one battery is
dedicated to engine starting, and can't be discharged by the inverter
or other "house" loads.