[Sentoa] Electrical Upgrades to Respite

Mark Laffin mark.laffin at gmail.com
Sun Sep 12 07:53:31 EDT 2010

I did some upgrades to the DC electrical system over the summer that I
wanted to share with the group.  Prior to the upgrades, my tug had two
Interstate flooded lead acid 4D batteries in a box on the Port side. It
appears that this was the original configuration from the factory.  The 4Ds
are large capacity deep cycle marine batteries that are designed to deliver
a steady current over a long period of time.  They are not really designed
to deliver large bursts of current that typically occur with starting the
engine, running the windlass, or pulsing the bow thruster.  They also take a
lot longer to recharge than a starting battery.  My boat also is equipped
with the Cummins 6BT5.9M engine with the standard truck alternator that
comes on the engine.

We like to spend time at anchor or "boat camping" in some of the more remote
places in our cruising grounds (Lake Ontario and the St Lawrence River).
The 4Ds can meet the DC power needs of the boat well for a couple of days if
they are not drawn down by using the bow thruster or windlass prior to
shutting down the boat.  The thruster and windlass draw over a 100 amps each
and can take a lot of Amp-Hours (Ah) off the batteries quickly.  I decided
to break up the DC loads into two more battery banks: One for the
thruster/windlass, and a second dedicated battery for starting the engine.
The original 4D batteries would continue to serve the house needs of the

The shelf on the Starboard side of the engine room was empty, so that's
where I decided to put the new batteries.  It was also close to the DC
wiring for the thruster, windlass, alternator and engine starter motor.
After doing some research, drawing up my wiring diagram and talking to a
couple of marine electricians, I decided to go with three new Group 27
starting batteries.  I put two of the G27 batteries in parallel to power the
thruster and windlass, and dedicated the third battery to the engine
starter.  The batteries are in battery boxes and secured to the shelf.  I
also added a battery switch between the thruster/windlass batteries and the
starter battery so I could put all three in parallel if the engine start
battery was low for some reason.  The switch would be normally open.  The
G27 starting batteries are designed to deliver large amounts of current in
short bursts and they recharge relatively quickly compared to the deep cycle
4D batteries.

To get the benefit of three  independent battery banks while still being
able to charge them from a single alternator, you have to isolate them from
each other.  I selected an FET 200A battery isolator for the job.  Diode
isolators have a 0.7V drop across them that can fool regulators and/or not
allow the batteries to come to a complete charge.  The FET battery isolator
has a 0.05V drop at full current, so the batteries see the full charge
voltage but, are still isolated from each other.  

If you haven't had the pleasure of working with 2/0 battery cable before,
you're in for a real treat.  It's heavy and stiff and requires some planning
and thought when routing it and the orienting the crimp lugs (you can't
twist this cable easily to orient the lugs once they are on).  I purchased a
heavy-duty battery lug crimper made by Ancor and a set of heavy duty wire
cutters (Jamestown Distributors had the best price).  Another handy tool
that Ancor had was a battery cable jacket stripper that cuts the plastic
jackets precisely without nicking the wire inside.  I also purchased an
assortment of the crimp-on lugs.  The threaded studs on the various devices
have different diameters for the threaded posts (1/4, 5/16 and 3/8 inches),
so you have to make sure you put on the right lug.  Another very important
thing I got was a bottle of Kopr-Shield terminal grease.  Kopr-Shield is a
conductive grease that seals the ends of the wire to prevent corrosion and
provides a highly conductive seal between the wire and crimp lug, and the
lugs and their wiring posts.  I've removed every electrical connection on
the boat that I can get at, sanded it to remove any corrosion, applied some
Kopr-Shield, and re-made the connection. You'll also need a supply of
wire-ties, wire-tie mounts, adhesive-lined heat-shrink tubing and plastic
terminal caps to cover the exposed 12V lugs and threaded posts.  I found a
great source for most of the wire and electrical components at very
competitive prices: http://shop.genuinedealz.com/. 

I mounted the battery boxes first, and then installed the G27 batteries in
the boxes to make sure they were secure and I had the working clearances
that I needed.  Next, I used pieces of 12 AWG solid wire to rough in the
wire routing and lengths. The 12AWG would serve as patterns when measuring
and cutting the 2/0 battery cables later.  After I was satisfied with the
wire routing, I mounted the FET isolator on the forward engine room bulkhead
and then separated the thruster/windlass wiring from the rest of the boat
I removed the +12VDC and ground cables for the 4D batteries and wrapped the
lugs with electrical tape. I also turned off the battery charger.  These 4Ds
can deliver a LOT of current (can you say arc-weld?) that can be very
dangerous. Never do any serious electrical work on a boat with live
batteries attached.

 The next big job was cutting, stripping, and crimping the 2/0 cables.  I
always crimped one end first, attached it to its stud,  routed the wire and
determined the orientation of the crimp lug on the other end, then crimped
the lug on (don't forget the Kopr-Shield before crimping), and finally put
on the heat-shrink tubing to seal out moisture, provide strain-relief, and
protect the interface between the lug and wire.  Once the heave cables were
in place and secured, I ran 8 AWG cables from the battery charger to the two
new battery banks. I have a Xantrex 3-output TrueCharge 40+ charger. The
whole job took a full day (about 12 hours) to complete.  I retraced every
wire and compared it to my diagram to make sure that I hadn't miss-wired
anything. Then I labeled everything. Once I was satisfied, I connected the
cables to the batteries and everything worked as planned.

Overall, I'm very satisfied with the results. We've been able to go over two
days without topping off the house batteries.  The engine is very happy to
have its own dedicated power source, and the thruster and windlass can be
used without affecting our on-hook time.  Next spring I'm going to replace
the alternator on the engine with a 200A Balmer and external 3-stage
regulator to complete the project (spent the boat budget this year).

We also replaced all the incandescent bulbs on the boat with LEDs.  That
made a big difference.  You'd  be surprised how much current those halogen
G4 bulbs in the salon draw. The LED bulbs draw less than 10% of the current
that incandescent bulbs do, with virtually no heat, put out as much or more
light, and last more than 50,000 hours (long after I'm done with boating).

Why not just get a generator, you may ask?  We have one; not a dedicated
marine one, a Honda EU3000i mounted on the upper deck behind the pilothouse.
I couldn't see spending $10k or so for a dedicated generator because we
don't use or need it that often.  The Honda provides us all the power we
need for the brief periods we need it, is very quiet and fuel efficient.  We
used it to power the aft air conditioner on one particularly hot, humid and
windless day this summer when we were crossing Lake Ontario from Canada to
the US. It ran flawlessly for the 6 hours we were underway and still had
fuel in the tank. It was 90 degrees outside with 90% humidity but, a
comfortable 76 degrees inside the boat.

I'd be happy to provide more details and pictures of my battery project to
anyone interested.

Mark Laffin
MV Respite, NT32-135
Victor, New York

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