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On 18 Aug 2003 19:31:53 -0700,
(Richard Malcolm) wrote:

hey guys, thanks for your messages. I will reread them and then
proceed, but first let me update. great weekend on the boat with no
problems. went to rockport,mass.
I have a couple of thoughts. first the inverter, it is a xantrex 1750.
I did install it myself, with 5ft 4o cables with a 250Ma fuse

1750 watts plus the loss of conversion because it's not 100% efficient
will draw around 160-170 amps at full load. Yeah, I know, crazy isn't
it? That's the physics of it, anyways. So, you should WIRE it for
200 amps so you don't catch the boat on fire melting inadequate wiring
and POWER it with batteries that won't explode at a constant 200A
load.

Now that we both see that isn't reality in all but the large yachts,
the alternative is to limit the load on the existing batteries to a
much safer level than 1750 watts. Let's look at the microwave, the
biggest load. 900W at 12V...by the way, the more load you put on the
inverter, the lower the battery voltage drops, which makes the
inverter pull even MORE amps, lowering the battery voltage even
further in a downward self-destructive spiral caused by the voltage
regulation circuits in the modern inverters. 900W at 12V = 75A but
we'd better say 80A because the inverter isn't perfect. So, to be
safe, let's wire the inverter for a maximum 100A load on the battery
bank....for the 3 minutes you want it. We STILL have to wire it for
100A to the inverter. ANY wiring MUST be fused to protect the wiring
and the battery. This big inverter will EASILY blow the 100A fuse if
you go over 100A load on the battery.....when the wife plugs in the
1500W hair dryer, for instance.

On a short run to the inverter, the shorter the better, you can use #2
starter cable if you can keep the run down to a few feet. I highly
recommend the inverter be mounted as CLOSE to the house batteries as
possible because the big loss in cabling is on the 12V side, not the
AC side where the current is much lower. Buy an appropriate 100A
marine fuse to put next to the battery in an appropriate holder. West
marine has them that use the new spade fuses and seal from the water
intrusion. They mount with 2 screws on a bulkhead near the battery.

I see LOTS of dangerous, haywired, very powerful inverters drawing way
too much current through way too small cables that can burn the boat
to the waterline from simply drawing their rated currents. If anyone
ever tells you a battery exploded on a boat.....go have a look at what
the results are. Even if the wires survive, boiling the house
batteries IS very dangerous.

(something like that, installed not in the engine compartment to keep
spars away from engine. I use the inverter mostly for 2 things. 1 a
900 watt microwave oven for 3 minutes or so at a time, and 2 a CPAP
machine, which I used to help me breathe when I sleep, so it is on for
7 hours or so. The CPAP machine worked OK last year with my 300 watt
inverter which plugged into the cig lighter, but that connection was a
joke. I have not worn the battery down to the point that it could not
start the engine in the morning. However, I ussally start the engine
with the other battery.
2- I have been told not to switch the battery switch while the engine
is running. this is something that I have been doing, My question is
if I ran down one battery and started the engine on the other, how to
I charged the weak battery without switching the switch while the
engine is running?

There are many ways. The easiest way is a "battery isolator", which
is two huge diodes on a big heat sink to draw off the heat from them.
Diodes only conduct in one direction...from cathode to anode. So, how
they work is to connect both anodes to the + terminal of the
alternator and each separate cathode to each battery bank. This
isolator completely bypasses the battery switch. It's connected like
this:
-----anode--cathode------Battery 1+
|
alternator-----|
|
------anode--cathode-------Battery 2 +

The terminals on the device are marked alternator, battery1, battery2.
Current cannot flow from battery 1 to battery 2, or the other way
'round, because one of the diodes will be reversed bias and block it.
It's fully automatic and you cannot forget to set it because nothing
is to set.... Simply unwire the original battery output wire from the
alternator and wire the new one in. (Please disconnect BOTH ENDS of
the original wire as it will still be hot, otherwise, and use the same
heavy cabling to wire in the isolator the original wiring used.)

The other possibility is the ON-OFF switch bypassing the current
Battery 1 - Battery 2 - Both - OFF switch you now have installed you
shouldn't move when the engine is on. West Marine has these, too.
They are rated at 800A, bulkhead mount and look like the original
heavy battery switch with the big knob pointer on them.

This switch is wired with the same heavy cable used for the starter
from the BATTERY 1 to the BATTERY 2 posts on the original battery
switch, effectively hooking them together when this switch is ON. You
must remember to turn the switch OFF when the engine is not running.

What happens to the B1-B2-Both-Off switch is that between positions
the contact comes off both batteries, possibly leaving the raw
alternator hooked to the electrical system and putting 17-20V pulses
on them....in short, it makes a surge. The crossover switch between
them doesn't do this because no loads are hooked to it. It merely
connects one battery bank to the other....in the scenario we're
talking about, the good starting battery with the alternator on it, to
the dead house batteries, NEVER allowing the alternator to come off a
battery so this surge cannot happen. The alternator will then charge
the dead house batteries through the new switch ASAP as you drive.

3- I have borrowed the battery tester and have a multi meter (not
digital, but I should invest in one)

tommorow, I plan to test the batteries by doing the following. Does
this sound right. First, turn battery switch to off, connect the
tester to battery one, push button on tester, hold for 10 seconds and
read. repeat for battery 2. Then turn battery switch to battery 1,
hook up tester to battery one, start engine and read the tester. turn
engine off, switch to battery 2, and repeat. this will help me decide
if battery or charger is working right.
this is where I will start. (keep in mind that I have made an
appointment with my regular boat mechanic for wed, but would like to
learn as much as possible before I have the pros take over. I
understand that removing the altenator is not to tough of a job. any
opinions? Thanks again for all your help.

Sounds good to me. Good plan. If we're going to buy a meter, let's
buy a PANEL DC voltmeter for the battery switch! (PS - you don't HAVE
to pay $100 for a panel voltmeter if you don't tell your yachtie
friends and buy them from the NAPA auto parts store.) Real nice 2",
0-15VDC square panel meters, cat # 541-MSQ-DVV-015 are available from
Mouser Electronics (800-346-6873) for $20. At this price, you can
just buy 2 and hook one to each battery, permanently, and mount them
someplace nice so you can look at them as you pass. A panel voltmeter
draws microamps of power and would take 100 years to run down a boat
battery, not an issue at all. This will let you see what the
discharging battery voltage looks like under load, continuously
(around 11V dead - 13.8V charged), while the AC charger is on (about
14.2V fully charged) and while the alternator on the boat is charging
them (hmm...look....Battery 1 is 14.6V but Battery 2 is only 12.7V! I
musta forgot to switch on the crossover switch, again!)

Larry W4CSC

Maybe we could get the power grid fixed if every politician
regulating the power companies wasn't on their payrolls.

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