My Dual battery setup.

I did a bunch of research on dual battery setups before doing mine.  There are a lot of different options and setups available.

I wanted mine to be simple, reliable, and uncomplicated.  I rejected the designs with fancy isolators and those using multiple solenoids

because I felt the potential for failure in these parts, leaving the entire electrical system inoperable, was too high.  Plus they're harder to

setup, understand, and maintain.  Also, even though many claim success, the solenoids you see used are not designed for this, even if they are

"constant duty - more on this below.

So I ended up with a very simple system consisting of 2 batteries, a large marine switch with positions 1/2/both/off, all wired with 1/0 welding cable.

A lot of conventional wisdom recommends 2 identical batteries, but I wanted to try to reap the strengths of both kinds of batteries, so I installed a marine deep cycle battery (battery 2) and a regular heavy duty automotive starting battery (battery 1).

Here's my system:

This is the switch, available from  Note that they have switches available rated up to 1000 amps !!

Mine is rated for 230 amps continuous, 350 amp intermittent (5 minutes).  More on these ratings in a bit.

This is the back of the switch, with the terminals labelled.  The field terminals are for use with an externally regulated alternator, such that 

accidentally switching the switch to "off" with the engine (alternator) running, will not harm the alternator.  However, as my alternator is internally regulated, I do not, and cannot use this feature, and must not switch the switch to off with engine big deal.


Here's an e-mail exchange between myself and Guest Co. on the topic:


My question:

From: []
Sent: Saturday, July 07, 2001 12:36 AM
To: <>
Subject: Use of AFD battery switch with internally regulated alter...

I recently purchased a battery switch model 2100 with AFD.

I am confused and worried by the information on your web site.

On this page:

you state:

"When using switches with outboards or alternators with internal voltage
regulators, be sure to choose a switch without alternator field disconnect.

But on this page:
 you state
"The function of #2100, #210, #2300A, #3100, #3200 selector switches will
not be affected if the field circuit disconnect feature is not used".

So, can I use the 2100 with AFD with 2 batteries and an internally regulated
alternator, no isolator?

If so, will damage occur if accidentally switched to "off" with engine

Thanks for your help,

Bill Ansell

Their answer:

You must hook up the AFD to both the alternator and the regulator for it to operate properly. Since you have an internal regulator, you cannot use the AFD feature and you will harm the alternator by going to or through the Off position of the switch with the engine running.
Sue D.

Here's where I installed the switch....not for everyone I know, but I like it.  My friends joke that I could launch nuclear missiles with that switch !


Another installed switch shot

Last one :-0

Here's a look under the hood.  All the cables marked with an * come together at the junction block seen in the pic just above the relay.  See the schematic for details.

Here's the other side under the hood, showing the second battery in the stock air box location.  To mount it, I bought a generic battery tray, and welded different length bolts to the bottom of it to create a level stable platform for the battery in this location.  The pic is also labelled wrong, this is actually the Automotive starting battery, bat 1, which is much smaller than the big marine deep cycle battery.

Here's another pic of the left side.  Once again, the pic is mislabelled, THIS is the Marine Deep cycle battery, Bat 2.


Another shot of Bat 1 in the air box's old spot.  It just fits nicely.

Here's the very simple schematic for my system.

Now, here are some exchanges between myself and MADXJ's own Dave Kamp, that should shed some light on the subject:

My questions are in white, and Dave's responses in blue.


The marine battery switch is rated for
continuous-duty service under explosion-proof conditions.  Continuous-duty
means you can run it constantly at 230A without burning contacts.  It is
not recommended for you to THROW the switch under 230A load, but it will
survive it, without striking an exposed arc (that's what explosion-proof
means).  It will easily allow you to crank the engine, but don't throw the
switch while cranking.  the 350A intermittent rating is when you're
initiating a cranking situations... like turning over a big V8 Diesel...
you'll see momentary cranking currents in the 350A range, but not for very
long.  The switch will handle 350A for several minutes, but if you demand
more draw than that, your batteries will be seeing a virtually infinite
load, so don't worry about killing the switch.

The two auxiliary terminals are for disconnecting the FIELD of an
alternator with external regulator.  In marine applications, the
alternator, regulator, and distributor are required to be explosion-proof
(gas fumes in bilge go boom!), and boats with multiple batteries usually
also have an isolator block, to allow charging two batteries at the same
time, without bridging the two together.  (I.e., if one is dead, and the
other fully charged, 'bridging' would cause one battery to rapidly
discharge into the other, causing a very dangerous situation).  The
isolator is nothing more than two big diodes biased TOWARDS each battery...
that makes 'em reverse-biased to each other.  There's about a 1v drop across
the diodes, so the marine regulator is usually either set a volt higher
than normal automotive, or there's an external voltage sensing wire at the
battery terminal, to regulate against.

With an external regulator, but not using that auxiliary contact to cut the
alternator's field, you could accidentally disconnect the battery switch,
and since the alternator is still generating, the alternator field would
become powered directly by stator output, causing the regulator to lose
control of regulation.  Without regulation, alternator output will jump to
over 70v... doing damage to quite a few things.

Two ways to solve this problem:
1)  (if your vehicle uses an external regulator, find your engine wiring
diagram, find the FIELD wire, cut, and hook it to those two terminals
2)  only flip the switch to the OFF position AFTER the engine has
stopped running...

If your alternator has an internal regulator, just hook up the batteries,
and leave the two auxiliaries unconnected... internally-regulated

alternators usually don't have any problems with battery switches. 
The deep-cycle has thick plates, and is intended for deep discharge over a long
period of time, not deep discharge at high current.  If you're running your
headlights 'till it's dead, the deep-cycle will survive more
charge/discharges, but DC's can't handle high-current without damage... and
sometimes they'll explode.

>  Bummer is, I thought it would be groovy to be able to switch to "both"
and be able to winch with the power of both batteries??

oh yeah- I crank with both batteries- no problem... make sure you've got
similar batteries, though- so you don't draw one down faster than the
other-  the imbalance is hard on 'em. and boats with multiple batteries usually
also have an isolator block, to allow charging two batteries at the same
time, without bridging the two together. (I.e., if one is dead, and the
other fully charged, 'bridging' would cause one battery to rapidly
discharge into the other, causing a very dangerous situation).

>  By bridging you mean connecting in parallel?


>  So this could be a problem for me too? If I say drain battery 2 (the
deep cycle) running accessories engine off in camp, If I switch to "both" I
would be in trouble?  How then would I get battery 2 recharged? The switch
is laid out with the detents in the following positions: 12 o'clock is
"both", 3 o'clock is "2", 6 o'clock is "off", 9 o'clock is "1".  IE there is
no way to go from 1 to 2 without passing "both" or "off".  With the engine
off I could switch from 2 to 1 through off, start the Jeep, but how will 2
charge unless I switch to "both"????

Depending on how far down you dragged it... if you killed it dead, I'd
recommend you install an isolator, so you don't get a high rush of current
betwixt the two.  If you do it accidentally, so be it, but lookout, as
applying a high-current charge can be really dangerous (boom).

It'd be best if you install an isolator and find a way to set up your voltage regulator for
external sensing, to accommodate the voltage drop of the isolator's diode block

>Does anyone know the amp ratings for the continuous 'ford-style' solenoids,
and would this be a practical alternative- bat 1 for normal functions, bat 2
for extras like winch, lights etc.  and using the solenoid to place
batteries in parallel?

Most starter solenoids have between 1/2 square inch and 3/4 sq in. of
surface area... they'll switch under load (obviously, it's their job) but
as a result, the contacts are assumed to operate under intermittent load.
The switching action will cause arcing, which burns away the contact
surface, leaving corrosion and pitting.  Starter solenoids manage a fair
lifespan that way... but they're considered a consumable part.

You'll see those solenoids, and some others, rated as 'continuous duty'...
in this case, they're not referring to the contacts-  they're referring to
the COIL.  See, when you crank an engine with a 12v battery, the load of
the starter, with the battery cable's resistance, leaves quite a bit less
system voltage available for other things (like ignition, fuel pump, and
starter sol coil).  Most of the starting-critical stuff is designed to work
at lower voltages... usually around 9v.  Your EFI controllers will function
just fine at 7ish volts, while the fuel pump, ignition coil, and injectors
will reliably operate between 7 and 9v.  This makes for reliable starting
under worst conditions. 

The catch is... when the engine is running, system voltage comes up to
about 14.2... plenty high to cook those other devices... that's why you
have a ballast resistor on the fuel pump, and why you used to have a
ballast resistor in the ignition system.  Don't need a resistor in the
ignition anymore, and not in the injectors either... they're pulse-width
modulated (or in other terms, dwell-controlled) so the control unit keeps
'em from cooking.  As far as the starter solenoid is concerned, you
wouldn't be using it for very long, and when you were, the system voltage
would be low... so the coil won't burn out on it's own.

So if you take a normal starter solenoid and hook it to a 12v battery,
it'll last oh... somewhere between 35 seconds and a few minutes... then
it'll cash it in.

As you noted, there are some 'continuous duty' rated solenoids... whether or
not they're actually wound for 12-14v hard to really find out... the best
supply source for solenoids with 'real' ratings is golf cart shops (THEY
are very specifically rated).

You can use the A / B battery scenario, with a solenoid to shunt them
together (in parallel), but I wouldn't set a system up that way for the
same reason Bill mentioned-  You're likely to loose solenoid trigger power
when you need it most... furthermore, they draw a fair amount of power
where a battery switch doesn't.

There are quite a few ham operators out there who run high-power linear
amps mobile, and use a secondary battery set to power the amp... and run a
power feed from the 'main' system (with a diode block isolator) to help
charge the batteries...


>  How can I connect my winch through the switch (i.e. take + from the "feed" terminal when  most every 8 or 9000 lb winch will draw between 370-450
amps at full pull?
>  Is it because, as you explained, the ratings on the marine switch
(230/345) are for a no arc condition while switching at that load, and
therefore the switch can actually handle more?  I'm getting confused again :-)

Well, how LONG will you be grinding the winch at full-pull?  Are your
battery cables getting hot?  The trick to designing electrical contacts for
not-switched-under-load use... is to design the contacts with much more
surface area that the conductor's carrying power.  So if you've got 3/4 sq in
of contact surface, but you're running #2 cable, you'll be fine-  cables
will melt before switch does!

For the most part, winches draw insignificant amounts of power until
you load 'em up... Can't remember when, but there's been pull-offs in some
of the 4x mags, and they show no-load speed and current.  As you noted,
winches with higher pull ratings will draw less current at a given load
because of that reduction... but if you get a high-pull winch with FAST
retrieval rate, you'll have some pretty high current draw.  You'll also
find that shunt-wound motors draw more current than series-wound under
stall condition, while blah blah blah, but what it all boils down to... is
that a fast, high-load pull requires more power, while a slow, high-load
pull draws less...  it's that Power=Work*Time thing  &  Work = Force *

The best way for the electrically inept to deal with current-draw issues is
to use the thermal indicator of ohm's law:  if a conductor has too much
resistance for the load being drawn, the conductor will get hot.  This is
not only a good way to determine if you've got big-enough cables, it's also
a shoe-in method to finding BAD cables or connections-  they look good on
the outside, but if they're rotten on the inside, they'll be cookin'.

>  If it turns out I shouldn't run the winch through the switch, should I
hardwire it to the auto battery, the marine, or both then?  Can I just run
a lead directly from both + terminals to the winch?

I'd use the automotive battery for the winch...

>  Just had another thought, could I run through the switch but say have a
350 amp breaker in both + leads before the switch to protect it?  Where on
earth does one get 350 amp 12v breaker?

--From an electrical utility company... Nah- I wouldn't bother.  If you
push it too hard, it'll let 'ya know.  Don't worry about the cables-
they'll give you warning.  Worry about the battery-  it won't.

You'd be better off winching with a large cranking battery, rather than a deep-cycle, though-  the
deep-cycle has thick plates, and is intended for deep discharge over a long
period of time, not deep discharge at high current.

>  I have both a high power automotive cranking battery (Energizer 900 amps
/ 750 cca) and an energizer deep cycle marine 700 marine ca / 550 cca - 140
reserve (dunno what the 140 refers to).  I figured by having both types I
could exploit the strengths of both depending on the situation, but it's
becoming clear I have no real understanding of what I'm up to! :-)

Use the high-CCA battery- its internal resistance is lower.  Deep
cycles don't do high discharge rates... they'll push a 10A load for days,
but don't like pushing a 100A load for more than a minute...

>  oooo - that sounds bad.  I only suggested winching from the DC because
if you were to run it flat, it would be recoverable whereas reg. auto
batteries will not recover even from a single total discharge, I know
because I've gone through 3 in the last year - mostly by leaving the
battery connected and not driving the Jeep for 3 months while I hack off
axles and suspension and the like.

Yeah, you can do any battery good harm... I've found that in many cases,
you can bring a DOA battery back just by connecting battery charger and a
half-dozen resistors (to limit charge current) and let it charge for a week
or two... Use a 20A charger, but limit the charge current to 5A or so...
pull out a resistor every three-four days to keep it at the 5A level... you
can get 'em back, as long as you haven't shorted a cell or broken a plate....

>  Will I be ok using the two different batteries I described above?

I wouldn't... I haven't had much success cranking the boat that way-  I
usually get one new battery every season (so I've always got one new one,
and one that's just a year old)... I always moved the older battery to one
of the other vehicles for highway use.  Cranking with dissimilars tended to
make the batteries die young.

>  I guess if I kill either dead, I could leave them out of the circuit till
I can get home and charge it separately with the battery charger?


>>  I stayed away from an isolator just to eliminate further complexity
(stuff I don't understand) and possible failure point.  I have read several
horror stories of isolator based systems going belly up and leaving the
vehicle without an electrical system at all.  Plus a good isolator is quite

Nah, not really-  it's just a big damned diode...  I MADE mine... looked up
the right part in an ECG catalogue, bored/tapped a 1/4-20 hole in a piece
of alum heat sink, and hooked up the wires... you can buy the same thing
out of a boating catalog for about ten times the price of the parts...  of
course, it'll have a pretty anodized finish, and a decal with some brand

It'd be  best if you install an isolator

>  Yes, best, but too complicated for me :-|  M'bee I should try some fancy
book learnin' :-)

Look for "The 12v Doctor's Bible"... available through West Marine,
BOAT-US, and some of the other marine suppliers-  you might even find it at
some car shops-  has a very good section on alternators, starters, basic
ignition systems, and modifications.  Not really big, and not hard to read
(it's got that Forrest Mims III feel to it).


Finally, here are some great Marin/ battery / dual battery links:

 check out the tech and wiring diagram sections. :

lots of good links:


Other setups:

Tech info:

 A great battery FAQ :



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