wrote in :

I'm trying to learn about inverters, and how to put a
system together. I want to use it pretty much just
for my refrigerator (about 800W), and my TV/VCR
etc. It seems like it would be cheaper to get two
less powerful inverters, than one powerful one.
So far getting two of the 1200W for $120 each at:

Ahhh...one of my favorite subjects....

Powerful inverters are now dirt cheap to produce with great profit margins,
especially in marine stores where they command amazing prices.....but,
alas, read on.


http://store.worldstart.com/customer...=16468&cat=307

seems like the best deal for me. That would leave
me plenty of head room--especially on the TV
circuit--for including other things. Does anyone
know of a better deal?

Oh, no problem at all getting the OUTPUT POWER to run just about any 115VAC
product you can buy. POWERFUL INVERTERS are real cheap with the digital
switching technology of today.....but, alas, read on.


How do I figure what size batteries to use? Should
I keep the batteries separate for each inverter, or
is it better to connect two or more batteries and go
from there? If so, how to connect the batteries
together, and then how to go from there to the
inverters?

Oh, oh. The slick salesman in the yacht club shirt, tan yachtie shorts and
expensive deck shoes didn't talk to you about BATTERIES actually powering
it, did they? NOT.

Let's use your load specs and have a look.....

Power isn't produced by inverters...it's converted at a small cost of
efficiency...over 90% in today's technology. Let's look at 800 watts, you
example, at the top. Discount the fridge doesn't run 24/7 while we look.
How long it runs depends on how often you open the door and how hot it is
in the boat.

Volts X Amps = Watts, so Amps = Watts/Volts...whether it's at 115VAC or
12VDC. The important part is the 12V side. 800 watts divided by 12V =
over 66A, about what the boat's starter draws after the engine starts
cranking over. There's the problem.....How long can we crank a starter
before the battery goes dead? Days?...no. Hours?....no. Minutes?...we
getting warmer. Seconds?...ah, there it is. It's about ampere-hours, and
how batteries are rated.....

First, put starting batteries out of your mind. Starting batteries will
produce a LOT of current for a few SECONDS, then they are dead....as any
boater already knows...(c;

Let's look at some common deep cycle batteries made for this kind of
service. A common deep cycle battery, the size of a large car battery, is
rated at 130 AH (ampere-hours), usually at a 20 hour rate (we take 20 hours
to discharge it). In reality, if you drain the battery over 50% of that,
the battery will soon be useless, so you have 65AH of "power" available.
Geez, that battery will only produce 66 amps for ONE HOUR?!! Well, no, it
will only produce 66A for a lot LESS than one hour because we didn't
discharge it over a 20 hour period, giving the chemical reaction time to
eat away at the lead plates in an orderly fashion. No, as you can see,
this isn't going to work, no matter how we're wishing it.

We need 66A of 12V power for a couple of days, not an hour! 66A x 48 hours
= 3,168 ampere-hours! WOW! The biggest 6V golf cart battery, the big tall
one they want $179 for, is only 700AH and we need TWO of them in series to
get 12V! 3,168 AH is gonna take BANKS of these monsters to get 800 watts
for TWO days! That guy in the Sperry Docksiders never said anything about
this problem, eh?

Back in the days of the diesel submarine, the battery banks were made of 2
volt, lead-acid cells that were 6,280 AH capacity, which we could draw
3,168 AH out of without destroying them, easy! Each CELL was about 4'
wide, 7' high and 3' deep. We'll need 6 of them to get 12V. Where we
gonna put 'em? Can we FLOAT 'EM?!! Each cell weighed about a ton or
so....(sigh)

Oh, oh. Now we're gonna have to RECHARGE THEM!.....

Whatever battery bank you use, it must be RECHARGED, FREQUENTLY if it is to
survive for any length of time. Batteries are NOT instant-charging
capacitors. The chemical reaction takes LONGER to recharge one AH than it
did to discharge them. You're also limited by the size of your
ALTERNATOR's output capacity to produce amps....lots of amps. So, if
you're a power boater running the engine all day, this is no problem. The
engine's running, anyways. Ragbaggers take a dim view of running the
engine to recharge big battery banks. My captain is one of them. He
thinks a battery should recharge in 20 minutes....(c;

To put 3000 AH back INTO the battery banks, we're going to have to put in
MORE than 3000 AH because lead acid batteries are AWFUL inefficient places
to store electrons....slowly.


I'm lead to believe that any inverters I can afford
would produce a square wave instead of a sine
wave. How to know what influence that would
have on my TV's performance? The TV works
great using the Generac 4000W generator I'm
using now. Can I expect it to be different using
the inverter?


No longer true. You can buy a 4KW synthesized sinewave inverter for under
$1000, easy. I paid $109 for a 500W Tripplite. Sinewave inverter power is
CHEAP and EASY to build with our computer technology.

All we need now is a 5000 AH battery the size of a starting battery that
can produce 500 AMPS for 10 hours without being destroyed in the process.

It's why there are NO BATTERY POWERED CARS on your street charging in 2
hours from the plug in the garage. It's just not practical at this time
because of the battery power problem.....

The power stored in a 10 gallon diesel or gas tank to run the Generac would
be the size of your garage stored in lead-acid batteries and weigh 20
tons.....Genset is still the answer....

Larry

NOTHING is funnier than a boater with a new 4KW inverter carrying his
electric heater down the dock with a big smile on his face....(c;