Dual Batteries & more
Last Update:  December 7, 2004
My electrical system has undergone a number of changes over the last few years.   The most significant was
when I had as many as three batteries and two separate alternators working at the same time.   You can read
more about that system below.  I later removed it, when I installed air conditioning, as one of the alternators
was located in place of the A/C compressor.

Today, the system includes only two batteries and a single alternator, but includes a few more features.  The
batteries and electrical systems are controlled and isolated electronically.

The system uses an Optima Red Top battery in the stock location.  It normally powers all of the truck's stock
electrical systems in addition to being the primary power for the starter motor and winch.

A second Yellow Top deep cycle battery is located in the bed.   It is connected to the Red Top via a 200
continuous duty amp rated relay system that is controlled either automatically when the ignition is switched on
or via a manual override switch.   The Yellow Top battery is the primary source of power for all other auxiliary
systems, such as the driving lights, interior lights,  Ham radio, refrigerator and all power outlets.  It is wired to
power a rear winch, although I currently don't carry one.  The long main power lead between the two batteries
is protected by a 200 amp fuse at both ends.   The Yellow Top battery can also be completely disconnected
from all electrical systems via a second 200 continuous duty amp relay.

The chief purpose of the system is to provide a back up starter battery in the event that the primary battery
fails or drains unexpectedly.   In addition, the deep cycle battery is able to be isolated and used to power
equipment overnight while camping.   Both batteries can be charged at the same time and connected together
to provide ample current for winching operations.
The front Red Top battery and
basic view of the front wiring,
including relay that connects the
two batteries.  It's not the cleanest
set up from having to modify it a
number of times, but it works
and it's reliable.
The basic wiring diagram of my
current system.  It may seem
complicated, but that's because
it's the result of several
modifications from the original
triple battery- dual alternator
The rear Yellow Top deep cycle
battery and relay that completely
isolates the battery from all
power sources via the flip of a
switch.  Also note the 200 amp
Electrical System Completely Turned Off
When turned off and isolated, the volt meters read the current voltage of both batteries, which in this case is fully charged at
about 12 indicated volts (actual voltage is about 12.6, but there is a slight drop in the wiring leading to the gauges)   The
main relay switch, which controls the connection between the two batteries is in the off position.  Both batteries are totally

The rear battery relay switch is also in the off position.  This completely isolates the rear battery from all electrical
connections.  The purpose of doing this is to prevent drainage from the multiple systems, such as radios, GPS, and battery
chargers, which use this battery as the primary source, when they are not in use, but still draw minimal current.
Independent, the current draw of each system is nominal, but when combined, they can drain the battery in only a few days.  
 Isolating the battery via this relay is easier than trying to unplug each electrical system when not in use.  It is also an added
measure of security in the event the hard to reach rear battery were to short out and the fuses didn't trip, the battery could be
disconnected with the flip of a switch in the cab.   
Front relay turned off, rear relay turned off, engine running
When both relays are turned off, but the engine is running, the alternator still charges the primary Red Top battery and now
indicates 14+ volts  in the picture.   The two batteries are disconnected from each other, which means the rear Yellow Top is
not charging and still indicates 12 volts.   In addition, the rear relay is still turned off, which means that no electrical system
is connected to the rear Yellow Top battery.   In this case, all auxiliary systems are disconnected and things like the GPS,
Ham radios and off road lights will not function.  However, all factory systems in the truck will function normally.

I would only  use the system in this mode in the event that there was a major short in some of the auxiliary system wiring
and I did not have time to repair and I needed to completely shut down all auxiliary electrical systems, but still be able to
drive the truck.

If I were to turn on the front relay, but leave the rear relay and rear battery disconnected, all systems in the truck, including
auxiliary systems would run off of the front Red Top battery.   I would only use this mode in the event that the rear battery
itself shorted out and failed or if the alternator failed and I wanted to stored a fully charged second battery for future use.
Front relay turned on, rear relay turned on, engine running (normal mode)
This is the most common setting.   The front relay is set to automatic-ignition.   This means the relay turns on the instant
the ignition is turned on and connects the two batteries with the driver having to do nothing.  In addition, both batteries are
automatically disconnected when the ignition is turned off, which allows for the rear Yellow Top battery to be isolated as a
back up battery in event the primary battery were to drain.  With the engine running, both batteries are equally charged at
14+ volts.

The entire electrical system is now interconnected and all systems, including factory and electrical are powered off of both
batteries as if it was one larger battery.
Front relay turned to override, rear relay turned on, engine off
This is the back up mode for the front relay.  Instead of turning on via the ignition, the front relay has a manual override
which allows the two batteries to be connected while the engine is running, in the event the ignition connection fails or while
the engine is shut off.   The most common use would be when the engine is off, but the combined current of both batteries
is needed.   For example, if the primary battery, which starts the truck, were to die, or if the engine died, but the winch was
still going to be used.    When connected, both batteries will reach the same voltage, in this case 12 volts, which also
indicates the engine is turned off.  If one battery were dead, when connected, the voltage would average out.  While it would
be significantly less then 12 volts, it should be enough to start the truck.
Front relay turned off, rear relay turned on, engine off
This final mode is used when camping and battery power is needed, but I don't want to affect the primary starter battery.  
The front Relay is turned off, which isolates the primary Red Top battery from all auxiliary electrical systems that I might
use when camping, such interior lights, power outlets, GPS, radios, etc.  If the front relay switch is set to auto, the relay
automatically turns off when the ignition is turned off.   

The rear Yellow Top battery relay is turned on, to connect the Yellow Top to the auxiliary systems.   In the event that I were
to run the yellow top battery down over night it would have absolutely no affect on the primary starter battery.  Once the
truck is fired up, the alternator would begin charging the dead battery.
This system is still being tested and is a little complicated, but has built in bypasses, should the electrical
relays fail.  Right now the current weak link is the alternator.  I've traded a combined output of 210 amps from
two alternators, to the output of only 60 amps from a single stock unit.   I plan to upgrade the single alternator
to a more powerful one in the very near future.

While I do miss the versatility, power output and uniqueness of the dual alternator system I had before, I
couldn't be happier to finally have air conditioning for those very hot summer trips into the desert.  This new
system should work perfectly fine.   The only other addition, besides the upgraded alternator that I might
make is the addition of a solar panel, which would allow me to run the refrigerator all day long, even when the
truck is not running, without worry of draining either battery.

The End

Go to my Homepage or check out my old system as outlined below.
My old triple battery, dual alternator electrical system.
This is a write up about my old system.  I'm leaving this up on the site for informational purposes.
My original electrical system design consisted of two seperate alternators
and three Optima batteries.  The electrics were split into two seperate
systems.  A stock main system and an auxiliary system.  Both systems run
completely independent of each other, but can be connected with a flip of a
switch in an emergency.   The main system, otherwise known as the stock
system, powers only the factory portion of the truck, i.e., the starter motor,
headlights, heater, ignition, etc.    All accessories, including the off road
lights and both the front and rear winches, run off of the auxiliary system.   
The only major change today is that the auxilery system which used to
have two batteries, now only has one.   The two seperate systems allows
for the winches to be run without draining the ignition system and starter
battery thus giving added security.  It also allows for an abundence of
reserve electrical capacity that can be used while camping and for
emergencies.    The main system consists of the stock 60 amp alternator
and one Red Top Optima battery.   The auxiliary system consists of one
160 amp GM style Delco-Remy alternator (located in place of an air
conditioning compressor) and one Yellow Top Optima deep cycle battery
located in the bed.  I used to have  incorporated into the system is a marine
type battery switch which allows the three batteries to be hooked together
in various combinations depending on the situation or emergency.  Today
the two systems are seperated via a 250 amp solenoid.  They would only be
connected in the event that the starter battery needed a jump start or if I
was doing an extended winching operation and needed all the batteries and
alternators.   200 amp slow burn fuses are used to protect the long
potentially dangerious battery wiring.
Wiring Diagram.  Click on image or
HERE for a larger view.
The diagram on the right is of the old set up.  I'll a
diagram of the new system up in the future.
Over all view of the engine
compartment, showing the main battery
on the left side.  This is how it looks
today after removing one of the aux.
batteries.  See below for the old set up.
Although difficult to see, this is a view
of the main stock 60 amp alternator.  
This unit has been rebuilt but is
otherwise left  stock for reliability.
The auxiliary alternator:
This is a GM style internally regulated
Delco-Remy alternator.  Stock output is about
100 amps.  This unit has been beefed up to make
a maximum 160 amps.   The alternator is
mounted where the stock air conditioning
compressor would have gone and uses the A/C
compressor crankshaft pulley.  The mount is a
Honda Civic alternator mount, bolted to a flat
piece of stainless steel which is bolted to the A/C
compressor bolt hole locations.  The upper
mount is custom made...see next picture.
This is a view of the upper bracket of
the auxiliary alternator.  The upper
bracket is custom made from a Honda
Civic alternator bracket and bolted to
the engine head.  Belt tightening range
is a bit limited but it works.

Special Thanks to my Dad, for
building both the upper and lower
brackets for this alternator
Click on logo for link
to Optima's website
Deep Cycle Battery
Starter Battery
The Optimas are great batteries.  I'll let Optima do the selling though.  Visit their
website via the links above if your interested.  One thing of special note though.  The
older Optimas, such as the ones I have, are mysteriously rated higher than the current
Optimas.  For example, the old yellow top Deep Cycle batteries are rated at 750 cca and
65 A/h, whereas the new Optimas are rated at 550 or 650 cca (depending on which
literature your read) and 55 A/h.

I inquired to Optima about the change.  They told me they simply rate them more
conservatively due to new ownership (Johnson Controls).   One distributor told me, he
thought that the old Optimas used virgin lead, while newer Optimas use recycled lead.  
It's still a mystery, but regardless, they are wonderful batteries and I would highly
recommend them.
The new system is much simplier than the old system (see below to see the details of the
old system) yet retains many of the advantages of the old system.  By using two
alternators, I can completely seperate the stock electrical system and isolate it from all
of the auxilery systems, thus dramaticly improving reliability.   Should one of the
systems fail, the other can act as a back up.   And the auxilery system, particulary the
winch, would not interfere with the stock system.  

A great example of how this can be a problem is a little experience I had in the old days
with just one alternator and one battery.  It was a cold snowy evening and I was running
my windshield wipers, heater and headlights.   I also had to winch out of a snow bank far
up a remoted snowed in road.   My stock electrical system was so overwhelmed that it
almost died.  Had it died, the engine would have died with it and I would have been
stranded.  With this set up, I can winch all day long without it ever affecting my engine,
headlights, heater, etc.  Should the winch battery and alternator become overwhelmed
and/or fail, I would still have a working engine.   In the most diar emergency, both
systems can be connected to provide two batteries and a total of 210 amps of charging
power to run the winch.
Over all view of the engine
compartment, showing the main battery
(right side) and one of the auxiliary
batteries (left side).
The old set up.  Today the stock air
filter resides here.

Main battery.
Charged by the stock
alternator, this battery powers the
engine, starter motor, and main truck
fuse box.  The main fuse box powers all
of the truck's stock accessories, such as
ignition, headlights, heater, etc.
Auxiliary battery # 1 -- and main switch.
(located in engine compartment)
This is one of two auxiliary batteries.
This battery powers the front winch and off
road lights among other accessories.  Is
augmented by a 2nd auxiliary battery
(behind cab in bed)  when the two are
connected.  It is charged by a 160 amp
auxilery alternator.
Auxiliary battery # 2
(located behind cab in cargo bed)
This is one of two auxiliary batteries.  This
battery powers the rear winch and air
compressor among other accessories.  It is
augmented by the 1st auxiliary battery when
connected.  It is charged by the 160 auxilery
alternator, but can be disconnected via the
main switch from 1st aux. battery and aux.
alternator.  It also supplies power to a cooler
located in the canopy and to running camping
This marine type
battery switch
connects all three
batteries or a variety
of combinations of
the three.
The main purpose of this battery switch is to allow for the easy connection of the three
batteries or a combination of the three for a specific purpose.   In an emergency for
example, when the main battery is dead and the truck is unable to start, one or both of
the auxiliary batteries can be switched to the main battery, thus allowing the truck to

In addition, under normal circumstances the front and rear auxiliary batteries are
connected via this switch and run in series.  They are then both charged by the 160 amp
alternator via auxilery battery # 1.  However, if desired, auxiliary battery # 2 (behind
cab in the cargo bed) can be disconnected from auxiliary battery # 1 and the alternator.  
 This would allow auxiliary battery # 2 to be used to be used overnight for camp lighting
or running an electric cooler while at camp, but still leave two fully charged batteries
available for use in the morning.
It also can provide power for an emgergency ham radio repeater, while the truck is
The main battery switching position and corrosponding results:
All three batteries are disconnected from each other.
Main battery and Aux battery # 1 are charged by their
respective alternators.   Aux. battery # 2 is stand alone and
not being charged by any alternator.
Main battery and Aux. Battery # 1 are connected as are
both alternators.  This position is only to be used when
emergency jump starting the main battery.  Upon
starting engine, switch is be turned to a different
position, to prevent voltage conflict between the two
All three batteries and both alternators are connected.
This position could also be used to jump start the main
battery by connecting both aux. batteries to main
battery to start the truck.   Also if one of the two
alternators were to malfuction, switching to this
position would allow all of the truck accessories and all
three batteries to run off of the remaining alternator.
This is the default setting.  The main battery system
and the auxiliary battery system are disconnected from
each other.  Aux. batteries # 1 and # 2 are connected
to each other in series and are charge via the 160 amp
auxiliary alternator.  When winching either from the
front or rear, both auxiliary battieries are used.
There are three main disadvantages to this system.   Weight, complication and
increased danger of fire or electrical short.

The increase in weight comes from the three very heavy batteries, each of which weigh
approximately 40 lbs.   The three batteries are the rough equivilent of one passenger in
This system originally had only two batteries, utilizing one main battery and one
auxiliary battery.  The reason for going with two auxiliary batteries is further explained
in the "advantages" section below.

This system is complicated, but not as much as it could be.  It is more complicated than
a stock system, which is why this can be a disadvantage, but it is far less complicated
than many pre-designed dual batteries systems.  I explain further in the "advantages"

There's no denying the increase in fire danger.  Due the extremely high amperage that
of the battery cables see, there is a limited fire danger.  Most of the high amperage
battery wiring are protected with 200 amp fuses.   This limits the amount of current
allowed to flow through the protected wiring.   The system has not been fully tested, so
I'm not sure if the 200 amp fuses are too small.  Since the front winch pulls 460 amps,
that wiring cannot be fused.    If a dead battery was connected to the auxilery batteries,
I'm not sure if the fuses would blow due the very high current passing from the good
battery to the bad battery.   Because the rear winch uses the smaller 1.7 h.p. motor, I'm
confident that it's fused winch wiring should work.  I'll report more as testing is

In running this system for well over a year and a half, I've no problems at all.   The slow burn
fuses help negate any fire danger and the system is actually quite simple, compared to some
electronicly control systems.    The only problem I had, so far, was the alternator bracket
vibrated itself loose on a long trip to Montana and I almost lost the auxilery alternator.  That
has since been fixed.
This system provides numerious advantages, not the least of which is security in
knowing that there is a back-up or triple back up to the main battery should it go dead.   
This system is perfect for back country travel in very remote areas.

This system provides a huge source of electrical power for front and rear winching, since
winching requires enormous electrical current.   The further the winch is from the
battery the less effecient the winch and the electrical system will be.  In it's original
configuration, the rear winch was hooked to the single auxiliary battery in the engine
compartment and had to travel 18 feet before reaching the rear winch.   This length of
wire caused the rear 5000 lb winch to lose much of it's capacity and stall out long before
it should have.   With the rear winch now hooked up to a source of electrical power about
6 feet away, it's much more effecient.   The front winch has always had it's main source
of power close by.  The auxilery battery # 1 it uses has less than 2 feet of wiring
between it and the front winch.   However, the single 750 cca battery (now rated 550 cca
by Optima) is barely adequate to power the 4.8 h.p. winch motor, even with the 160 amp
alternator.   Now, when winching with either the front or rear winch, both auxiliary
batteries can be connected giving an effective cold cranking capacity of 1500 cca. (1100
cca with the new Optima rating system)  This is a far superior winching system that
should allow for long duration trouble free winching.  The limiting factor now, is
overheating the winch motor.

With two auxiliary batteries, one or both can be used for electrical power at remote
camp sites at night.   With a fully charged starter battery disconnected from the
auxiliary batteries, I can camp worry free knowing that you'll be able to start the truck
in the morning.  For extra security, only one of the auxiliary batteries can be used and
drained overnight at camp while the starter battery and one emergency auxiliary battery
remain charged and ready to go in the morning.

Since both auxiliary batteries are of the deep cycle type, they can be fully drained and
recharged  much more often than the starter battery.   They are perfect as winch
batteries or as an overnight power source for a camp light, radio, electric cooler or any
other 12 volt electrical devise.

This system is much less complicated than it could be.  Many dual battery systems use
solenoids or isolators.  This system simply uses one mechanical switch.  The auxilery
alternator is a very simple unit.   Since the systems are completely seperate, they can
work independently or augment each other if needed.  The only likely parts to fail are
the alternators themselves.  |And with two available units, there is always a back up

This system has proven very successfull, but as mentioned in the beginning, is kind of
overkill.  For most people, 2 alternators and 3 batteries are not necessary.  The advantages
of this type of system are where winching is done on a regular basis and trips are taken into
extremely remote areas.   I rarely use the winches.  Usually only during the snow season.  
But I do drive on very long trips into remote areas, so the system has proven it's worth
here.  Having two alternators can allow the use of one as a back up.   When my auxilery
alternator almost fell off, on a very long trip,  I had to cut the belt and temporarily remove
it.  With my stock alternator still working, I was able to charge all three batteries with that
alternator and continue to operate all of my electrical systems.   The same could have been
said, had the stock alternator failed.

However, for most people, I think that a simple single alternator, dual battery system
would work perfectly fine.   I recommend using a high output alternator in place of the
fairly weak stock 60 amp unit and using 2, very high quality batteries, such as the Optimas.
  An Isolator, in between the batteries will ensure that you always have one good battery to
start the truck, should the main battery fail.   A switch could be hooked between the
batteries, should it become necessary to use both for winching.
If you have any questions, don't hesitate to email me.
Wiring Diagram
Batteries and Alternators
Batteries Specifications
All of the information below pertains to the old system.
click here for Specs.
click here for Specs.
This rear battery set up was slightly changed,
since a canopy was added.  The battery is now in
the same general location, inside a battery box.