"Wet-n-Wild Bill" wrote in
m:

Planning to install a 3000watt inverter (not inverter/charger) on a
boat with twin outboards. The House batteries are maxed out with the
electronics. I thinking of installing a dedicated battery bank for
the inverter or utilize one of the starting batteries on the outboard.
What do you think

Bill




Let's do a little arithmetic, first....

3000 watts divided by 12V = 250 amps, if the inverter were 100%
efficient, which it's not. How long do you think those house batteries
can provide 250 amps? 5 minutes? 1 minute? Hmm...it's a point.....

Now, how much is the combined DC output current of the two outboard gas
hogs hangin' off the back? 30A? 20? Some of them put out 7 or 12A to
recharge their starting batteries, but are NOT power sources at all to
save costs and to make that case as small as the customers demand it be.
Obviously, we got no 250 amps to power the inverter when the motor is
running...not at all.

Ok, so the house batteries are out....the outboard powering it just won't
happen...let's say we're going to put in a separate bank to run it and
recharge it when we get back to the dock. Those big, honkin' golf cart
batteries, the big tall ones, 6V, two in series to get 12V, they weigh a
ton, are 330AH at a 20A rate. At a 250A rate, they're only 150AH, maybe
even less. The chemical reaction to make electrons, unfortunately, takes
a lot of time. It's slow. So, if we're going to pull hard on them, and
250A is VERY hard, we'll say, just for argument and flaming on usenet,
150AH. You can use about that much, if they don't melt down at this
awful current level. So, 150AH divided by 250A = 6/10th of an hour at
full power...36 minutes...if the inverter were 100% efficient, which it's
not.

As you can see, this isn't going to happen unless we find room for some
6000AH submarine batteries. Each 2V cell weighs nearly a ton. We need 6
of them in series for 12V. Can we float it? They're 6' tall. Can we
get around 6 of them to get to the head if we mount them in the main
cabin?

What brand of generator was that you were thinking about??.....(c;
Diesel is best, but outboards use gas so we're probably stuck with gas.

I used to have a tagline that said:
"Nothing is funnier than watching a boater with a new 4KW inverter
carrying his electric heater down the dock with a big smile on his face."

Smoke came pouring out of this guy's boat at the dock, one day. Everyone
came rushing with their fire extinguishers to his aid, as it was a
Saturday morning. After the smoke cleared, the curious got looking at
what happened. He had a new inverter, 4KW I think it was. Big money!
He'd installed it, the best he could, and had plugged in a toaster (800
watts) and his coffee pot (1200 watts) to it. It was only half its
capacity, right? Should run great! Unfortunately, for him, it was
pulling 170+ amps off his big house batteries......through #8 red wires
he got from the marine store....which melted and caught the cheap plastic
insulation on fire....

Noone did any arithmetic before he installed it, obviously....(c;

Sure glad he wasn't anchored out in some cove by himself. He would have
jumped overboard as he did on the dock, and probably would have drown in
panic.

What brand of generator was that you were thinking about??.....(c;