Winter 2008 $10
Libya Archaeology
Dual Battery Comparison
Hole-in-the-Rock, Utah
Overland Journal Winter 2008
Gentlemen’s Dual
By Chris Marzonie
Photography as noted
An introduction to dual battery systems, and a review of seven
products to keep the fridge cold, and you from being stranded
Overland Journal Winter 2008
It’s that sinking feeling—
you know the one. You’ve rolled up the camp and packed everything
into the vehicle; you jump into the driver’s seat, turn the key and . . .
Overland travelers are equipping their vehicles with more and more
aftermarket products that rely on electrical power—everything from
two-meter radios to GPS units, fridges to laptops, winches to driving
lights. The power required by these devices can be substantial, and
adds to that needed by the factory equipment: computerized engine
management, OEM stereo systems, OEM lighting, perhaps power
locks and windows—and one of the most important: the starter. You
are ultimately putting a lot of trust in electrical components that operate in unison to keep you moving when you want to move, and comfortable when you’re parked. Primarily, you’re relying on the battery,
the starter, and the alternator (or generator in older vehicles), although
most newer vehicles also have a computer control module that can
wreak havoc if it malfunctions. In this article, we’re focusing on one
of these components: the battery.
The original factory battery was designed for operating the original
factory equipment. Aside from the vital task of turning the starter
motor, the design intent includes circuitry assumed to operate with the
engine turned off, such as hazard flashers, convenience lights, instrument lights, audible indicators, the horn, and maybe a bit of stereo
operation. Beyond that, the battery relies on the alternator—via a running engine—to provide power for continuous, high-draw devices. It
also relies on the alternator to recharge any power lost from the battery
when starting the car or running equipment with the engine off. When
you ask a battery to do much more than this, you begin to stretch the
limits of the original design intent, reducing reliability and shortening
its life.
A dual battery system solves these problems. By correctly installing
a second battery, you are doubling the odds of having power when you
need it, and reducing the chance of total failure by half. Another option is to replace the sole battery in the vehicle with an upgraded version that has more robust construction and higher capacity, along with
a tolerance for repeated discharges. This may be appropriate based
on your intended use and budget, but generally speaking the added
reliability and redundancy of a dual-battery system is hard to beat. In
the most basic sense, you are always carrying a spare battery with you,
which can mean the difference between a brief interruption of a solo
trip and being stranded.
Electrical loads
The first consideration is how much power you’ll need. You should complete this exercise
before you buy that microwave oven, the portable blender, or the extra strand of party lights. Load
calculations can be grouped into three categories: Power while moving, power while stationary,
and power for winching.
Power while moving includes things such as auxiliary lighting, stereos, navigation equipment,
fridges, etc., that are in use while the engine is running. Accessories such as high-wattage auxiliary
lighting and high-power stereos will draw a substantial amount of the power available from the
charging system, and that effects how long it will take to recharge the batteries while moving.
You can use this information to roughly determine how long you may need to drive the vehicle
to recharge the second battery, and likewise to determine if your factory alternator is up to the
task of supplying sufficient power for all of your equipment, while still charging the batteries in
a timely fashion.
Next, you’ll want to determine how much power you will use while stationary, with the engine
off. (See the sidebar for a sample calculation.) Typically this will include any electrical items you’ll
be using while camped. Some of the most common are fridges, lighting, communications radios,
stereo systems, and rechargers for small batteries in laptops, phones, and cameras. Err on the
conservative side with your numbers, so you’ll have a buffer. This will compensate for things such
as unusually hot days (longer fridge run times and less-efficient power transfer) unusually heavy
gadgetry sessions, aged batteries, or those late night soirees when you and your buddies stay up to
watch the moon set. Remember to account for additional loads when using AC power inverters.
Some of us won’t be able to outfit batteries big enough to keep up with all of our vices, especially
while boondocking for more than a day or two, so at that point it may be wise to look into solar
and/or wind charging systems.
Finally, let’s examine the relationship between winching and batteries. Winches demand an
enormous amount of current. Using a second battery to power the winch helps preserve the start-
Overland Journal Winter 2008
ing battery (or reserve battery depending on your system) and extend winch operating time. If
you consistently find yourself doing long and/or frequent winch pulls (such as in mud and snow
country), you would do well to invest in a high-output alternator along with the largest deep-cycle
battery (or batteries) you can afford and have room for, to satisfy the power-hungry winch. Find
out what your winch’s maximum draw is, then use the load calculation, substituting the winch for
the 12V accessory. (Additionally, you’ll need to consider Peukert’s Law, which means the faster
you discharge the battery, the less capacity it has. An online Peukert calculator is available at Factor in additional loss due to heat, inefficiency, and less-than-ideal
battery conditions, and you’ll get an idea of how long you can run your winch from your auxiliary
battery. In most cases, occasional winch use to recover yourself or another vehicle, or move a fallen tree from the road, will be handled just fine with a single battery in good condition. If frequent
or long pulls are expected, then you may want more reserve power. This can be achieved by either
adding a third battery, or combining the primary and secondary battery in a dual system. You may
also want to consider supplying the winch from the main battery so it can take direct advantage of
the alternator’s output regardless of the isolator’s switch position. If you plan to combine batteries, you’ll need a combiner switch that is capable of handling the large, continuous current loads
the winch demands. Slee Off Road documents a fine example of such a system on their website,
and offers installations for this configuration. It employs a heavy-duty manual battery switch with
a bypass feature that locks the dual-battery isolator out of the circuit, thereby protecting it from
the large currents. This allows the operator to disable the isolator solenoid from the circuit, and
then use either battery or both batteries for winching operations, or to self-jump-start the vehicle.
Blue Sea Systems also has a product of this type, called the “Add a Battery.” (Note that the system
shown on Slee’s website may require an Automatic Charging Relay (ACR) for newer vehicles.) See
the resouce listing at the end of this article.
Choosing the batteries
The second step in selecting system components is to choose the battery or batteries. Many
people keep the original starting battery, especially if it’s in good condition. But if you tend to
discharge it on a regular basis (especially if you set up the secondary battery strictly as a reserve),
you should consider installing an AGM deep-cycle battery with high cranking amps, which can
handle both heavy starting loads and repeated discharge.
When adding a secondary battery, a simple rule of thumb is to install the largest capacity battery you can afford, both in terms of price and available space. Quality is another consideration;
an impressive amp-hour rating will do no good if a cheap battery fails early on. Don’t neglect
the mount—a generic battery tray held to an inner fender with self-tapping screws just won’t do.
The mount must be engineered to hold the battery securely in place in the event of a collision or
The subject of batteries and varying types of deep-cycle designs is enough for an article of its
own, so I’ll keep this to a brief summary of what’s available to the automotive and RV market.
With the exception of hybrid cars, virtually all automotive batteries are lead-acid. Lead-acid is
just one of many different battery chemistry methods, and has traditionally been used for automobiles because of the properties associated with it. Capable of safely producing large amounts
of current, lead-acid batteries are relatively predictable in their operation, reasonably durable, and
economical. The basic principal of a lead-acid battery is to combine lead plates or grids along with
electrolyte, otherwise known as battery acid (sulfuric acid and water), to create chemical energy
that can be converted to electrical energy and vice versa. There are three types of lead-acid battery: flooded-cell (wet cell), gel, and AGM (absorbed glass mat).
Flooded cell batteries are the oldest design and use a liquid (wet) battery acid. They usually
tolerate overcharging and undercharging better than AGM or gel batteries, although they have
some limitations for charge current, as the acid can boil and off-gas if too much current is applied.
They require periodic maintenance, including cleaning, since they vent hydrogen gas and acid
Load Calculation
Electrical loads are measured by amp-hours
(AH). One amp-hour equals a load of one amp
for a period of one hour. Batteries are commonly specified based on a 20-hour rate, whereby
they are discharged down to 10.5 volts over a
20-hour period, and the resulting amp-hours
supplied are measured. However, that’s unrealistic for our calculations, because typical
batteries should not be discharged lower than
50 percent of their capacity, or they will suffer
premature wear. In a 12-volt system, 12.7 volts
is considered a 100 percent state of charge,
while 12.06 volts is considered 50 percent. (To
calculate amps, divide watts by volts; e.g., a
60-watt, 12-volt light draws 5 amps. Multiply
that figure by time used to get amp-hours)
Here is a sample calculation of a typical
vehicle’s electrical requirements.
Fridge: 40 AH (5 amps, cycling on 50 percent
of the time, x 16 hours—turned off for 8 hours
Laptop (running): 6 AH (6 amps x 1 hour)
Laptop (charging): 6 AH (3 amps x 2 hours)
2-Meter Radio: 1.4 AH (0.7 amps x 2 hours)
Lights: 3.75 AH (three 5W lights (.417 amps)
x 3 hours)
Total AH required in a 24-hour period =
For this example, a typical 105 amp-hour
deep-cycle battery would almost be sufficient
for a 24-hour period, provided the 50 percent
state-of-charge regimen was followed. Note:
Idling your engine, even at high RPM, is a
poor method of recharging your battery while
camped. Assuming a 50 percent discharge,
it can take several hours of idling to bring the
battery back up to nearly fully charged. The
amount of pollution created and fuel wasted
is not worth it. Either bring an extra battery, or
use a solar/wind charging system (to be covered in a future article) or a small, efficient, portable generator.
Overland Journal Winter 2008
mist, which leads to corrosive build-up on exterior parts. Most have accessible cell ports with caps
to allow distilled water to be added to replenish the electrolyte mixture. On a cost per cycle basis,
they are the least expensive by a large margin, and can last a long time if properly maintained.
Disadvantages are the more intensive maintenance, susceptibility to vibration damage, and intolerance for non-use (the lifespan is shortened if left to sit without being used or charged).
Gel batteries suspend the acid in a firm silica gel, eliminating the need to replenish the water
in the cells. The advantage to the gel electrolyte is that the battery can be mounted in any position, and there is no acid to splash or spill. Gel cells can be left idle for long periods of time, and
hold a charge much better than a flooded cell. The gel cannot circulate among the cells as in a wet
cell battery, so the plates must be kept somewhat thin to accept a charge in a reasonable amount
of time. The biggest disadvantages of gel batteries are their finicky charging requirements and
expensive price tags. They must be charged at a slow rate, per the manufacturer’s specifications, in
order to last. Overcharging can create damaging pockets in the gel that reduce performance and
can cause premature failure.
AGM (absorbed glass mat) batteries are often improperly referred to as gel batteries, and
are sometimes labeled as “dry cell.” Although similar in concept to a gel battery, instead of gel,
an electrolyte paste is absorbed into fine fiberglass mats to about 95 percent saturation (often
referred to as “starved electrolyte”), which is sandwiched between the plates. The famous “spiral
cell” design, with cylindrical cells packaged in cases resembling a six-pack, is one example of
AGM construction, but the standard box-shaped cases are common as well. The box case allows
more cell volume in roughly the same overall dimensions as a spiral-cell counterpart, and therefore typically boasts higher reserve power. AGM batteries are considered by many to be the best
choice for 4WD and overland applications. The nature of their construction makes them highly
durable and vibration-resistant, with low internal resistance. They can sit for long periods of time
with little self-discharge. Charging capacity is limited by heat, not necessarily current, so you can
dump a large amount of amperage into them for a much faster recharge than gel batteries. Their
biggest disadvantage is price, often twice as much as flooded-cell batteries.
Types of dual-battery systems
How do you like your eggs? Poached, hard-boiled, or scrambled? Ok, over-medium it is. There
are different styles of battery isolation systems to choose from, and debates abound about which
one is best.
The four common types of isolators include manual switches, diodes, solenoids, and MOSFETs (metal-oxide-semiconductor field-effect transistor). The term “isolator” refers to the device’s ability to isolate the second battery from the main battery when a charging current is not
present. This allows you to run your accessories from one battery while preserving the other for
starting duty. In addition to isolation, some devices have the capability to combine the two batteries to increase the available power output. This can be useful for winching or self jump-starting.
• The manual switch is the simplest isolator. It employs heavy-duty contacts, selected by a rotary
knob, which allow the operator to combine, isolate, or individually charge the batteries. While
simple, rugged, and inexpensive, this method requires user involvement. So if you are a person
like me, who once left an expensive camera lens sitting on an ARB bumper, from which it fell
to be run over by a 35-inch mud-terrain tire, you might want to opt for an automatic system,
lest you leave both batteries connected and wonder why your fridge is doing so well. It’s been
running for how many days without flattening the battery? Man, we could stay out here forever!
(And you might get your wish.)
• A diode isolator is essentially a switch made of semiconductor material that only allows current
to flow in one direction. It’s like a one-way valve for electricity. A diode isolator is commonly
configured as a trio of metals studs, set into a chunk of extruded aluminum cast with cooling fins, also known as a “heat sink.” The cooling fins are necessary because as current passes
through the diodes, they heat up. This heat means there is some loss of voltage. The chief
downside to a diode isolator is that higher current results in more heat and a higher voltage loss
Overland Journal Winter 2008
(up to a full volt in some models). The lost voltage on the outlet side of the isolator may not
be seen by the vehicle’s voltage regulator, resulting in a less than fully charged battery. Diode
isolators are very common and generally affordable, as well as easy to connect to the vehicle’s
charging system. Units rated for larger amperage can be quite bulky, and therefore require adequate space for installation.
• A solenoid isolator (also called a relay) consists of heavy-duty mechanical contacts similar to
those used on starters and winches. When the charging voltage drops below a preset level,
the device switches to isolate the batteries. When charging resumes and the voltage rises to a
preset level, the device connects both batteries to the charging current. This type of isolator
is often economical, simply constructed, and easy to install. Solenoids are capable of handling
high currents, and many can momentarily combine battery current to allow self jump-starting
of the vehicle. The disadvantage is that the contacts can theoretically become pitted and oxidized over time, eventually leading to increased resistance, voltage loss, and possibly failure.
They don’t discriminate when it comes to direction of current, so once connected, a severely
discharged battery may be subject to a huge inrush of current from a fully charged (starting)
battery. Solenoid isolators often come with companion “smart” controllers that allow remote
(in-cab) operation, overrides, and even custom voltage settings.
• Recently, the MOSFET isolator has become popular. A MOSFET is a solid-state device able
to switch power quietly, without moving parts, with minimal voltage loss and virtually no heat.
MOSFETs are also compact for a given capacity, so require less space for installation. MOSFETs were originally used in smaller electronic devices and with lower currents than seen in
vehicle applications, but developments in technology have made them viable for high-current
applications such as dual-battery systems. Depending on the construction, MOSFETs can
potentially be sensitive to voltage spikes and power surges unless they are specifically rated for
continuous duty under such high loads.
A word on inverters:
If you install an inverter to run typical 120V
appliances and accessories, choose one
that will waste as little power as possible.
This includes selecting a size appropriate for your intended use. You don’t need
a 2,000-watt inverter to run a 100-watt
laptop. Inverters waste power in the conversion process, in addition to requiring
power just to operate. Large inverters can
be very inefficient at inverting small loads.
Many list a peak efficiency of 90 percent,
which means 10 percent of the power used
is wasted, and that’s a best-case scenario. Run a small load through a 2,000
watt inverter and the efficiency could drop
precipitously, wasting much of the power
consumed from your battery. That means a
loss of precious amp-hours, and less time
that can remain stationary without supplemental charging.
Configuration options
Having reviewed the different types of systems, let’s discuss ways to wire them. For basic dualbattery duty, there are two prevalent schools of thought: One is to use the primary (starting) battery to run all of the accessory loads, leaving the auxiliary battery unmolested and fully charged as
a reserve supply. The second approach is the opposite—run all the accessories from the auxiliary
battery, and reserve the primary battery for starting the engine and powering stock equipment.
Many argue that leaving the secondary battery isolated and unused unless needed for jumpstarting the vehicle is a waste of a perfectly good battery. Batteries need to be cycled periodically
(that is, discharged and recharged) to keep them alive and healthy. However, with the advent of
AGM batteries and their ultra-long shelf lives, combined with occasional jump-starts and periodic manual cycling by the owner, there’s not much reason to doubt a long and reliable use period
for such a system.
On the other hand there are those, like myself, who prefer to keep a standard-size, highcranking battery for starting the vehicle and running the OEM circuits, and install a monster
deep-cycle model with a high amp-hour rating to run all the accessories. This keeps both batteries
cycled with regular use, but still isolates the primary battery from the secondary battery while the
engine is off.
And for those with manual switches, well, you can slice it anyway you like. Just don’t forget to
turn the knob.
Whichever way you choose, be sure to use wire that’s up to the job. Somewhat unintuitively,
12VDC systems require heavier-gauge wire than 120VAC systems like those in your house. Use a
wire chart to determine the correct gauge for the length of run you need.
Overland Journal Winter 2008
Product Profiles
Hellroaring Technologies Model: BIC-95150B
By Brian DeArmon
Remote toggle switch (center) for manually combining the batteries (Photo: Brian DeArmon)
The Hellroaring BIC-95150B installed on the firewall of Brian’s Jeep TJ (Photo: Brian DeArmon)
By Jonathan Hanson
The complete National Luna kit (Photo: Jonathan
Overland Journal Winter 2008
$175 (isolator) $47 (switch)
The Hellroaring isolator is a small, fully potted (encased in resin and sealed from dirt and
moisture), solid-state device (combined diode/FET) with an integrated heat sink, and can be
configured in several ways, allowing you to set up the electrical system as you desire. The most
common 4WD application is the basic auxiliary battery isolation, but it can also be configured for
starting battery isolation. The BIC-95150B comes with an exhaustive 10 pages of instructions,
which makes things seem more complicated than they really are. Refer to the included wiring
diagrams while reading the instructions and it becomes clear that installation is a breeze.
In its basic configuration, power is supplied to the isolator from the alternator (usually tapped
in at the positive terminal of the OEM battery). Power is then supplied from the isolator to the
auxiliary battery’s positive terminal. The auxiliary battery is grounded to either the chassis or
the OEM battery, and finally the isolator itself is grounded via a fused connection (provided by
Hellroaring). It’s that simple. To spice things up a bit, two options exist. First is a remote switch
that allows both batteries to be combined in parallel, allowing for heavier loads to be placed on
the system without excessive voltage drop. The second is a remote LED indicator for the remote
switch. Both of these options are single wire inputs. The Hellroaring isolators are able to handle
up to 90 amps, so unusually heavy or long electrical loads, such as starting your engine from the
auxiliary battery when the main battery is flat, should not be attempted without an external relay
with the proper load rating.
When I installed the isolator on my 2003 Jeep TJ, I opted for the basic auxiliary battery isolation. This allowed me to build a secondary electrical system that essentially removed all connections between the OEM system and any of my aftermarket electrical equipment. The only
connections between the two systems are at the batteries. The hot side of the two batteries is
connected with 8-gauge wire, via the isolator, and the ground posts are connected with a 1/0 battery cable. All of the aftermarket accessory circuits are fed off of the auxiliary battery. This leaves
the OEM electrical system completely unmolested, and should I forget to turn off the 2-meter
radio or fridge before setting off on a multi-day backpacking trip, the OEM battery will still be
fully charged when I return.
Being a solid-state device, there is very little to go wrong. My BIC-95150B has performed
flawlessly over the past year, even when mounted under the hood where temperatures are high.
I have managed to run the deep-cycle auxiliary battery down to 5 volts, essentially rendering all
of my aftermarket electrical/electronic gear useless. One turn of the key, the engine started off
of the main battery, and all excess output from the alternator is directed to the auxiliary battery,
bringing everything back to life, all without a 30-mile hike to the nearest town to summon help., 406-553-3801
National Luna Intelligent Split Charging Kit $375
After experiencing premature failure in two inexpensive diode battery isolators on our Toyota truck (I’m a slow learner), I was determined to find a more reliable system for my 1973
FJ40. Research and a conversation with Paul May at Equipt Expedition Outfitters led me to the
National Luna Intelligent Split Charging Kit.
The most telling feature of the National Luna kit is the instructions, which comprise exactly
one page. This is an astonishingly simple system to install—the major portion of my time was
spent running wires and creating a safe container for the auxiliary battery; installation and hookup of the control unit and display was the work of perhaps an hour. And the system changes
nothing in the vehicle’s existing wiring—if necessary you can revert to your stock battery setup
by removing two wires.
The next obvious feature of the system is the comprehensive and top-quality kit in which it
comes, which includes everything from 20 feet each of fine-strand, 16mm red and black battery
cable, to two sets of battery terminals, plus crimp fittings, fuse holders, and zip ties. The only
things I added were split loom protection and shrink tubing. Normally I’m leery of the term
“kit,” but this one is truly all there.
Product Profiles
The NL system employs an 85-amp (continuous duty) solenoid as its routing switch. While
perhaps not as ultimately efficient as a MOSFET-type switch (solenoid contacts can oxidize and
slightly reduce current flow over time), it’s reliable and needs no cooling fins (as diode switches
require), since inherent power wastage is scant. The brain of the NL system is encased in a small
package above the solenoid. It is timed so that only the main battery receives charge for the first
five minutes after starting, to allow it time to recover and reduce immediate load on the alternator. Then the solenoid switches charging current to both batteries. An in-cab display uses dual
LED columns, colored red to yellow to green, to indicate the state of charge in each battery. It’s
easy to see at a glance while camped if your system is in need of charging.
So far I’ve been perfectly satisfied with the National Luna system. I like having easy visual
access to the status of the batteries, and it’s equally important to me that I could quickly disconnect the unit in the unlikely event of trouble in the field. The kit is simple enough, and the
instructions clear enough, that even an amateur mechanic could easily accomplish a professional
installation., 866-703-1026
The National Luna solenoid on the passenger-side
firewall of the JF40 (Photo: Jonathan Hanson)
Painless Performance Products 250-Amp Dual
Battery Current Control System #40102 $131
In an attempt to be prepared for those times when Murphy’s Law becomes reality, and to provide power for future accessories such as a refrigerator, I chose a simple and inexpensive manual
system manufactured by Painless Performance Products for my 2003 Tundra. I researched several
articles about dual-battery systems, but Painless’s kit was available locally at a popular retailer.
The kit comprises a 250-amp solenoid, an SPDT center-off waterproof switch, indicator
lights, a length of 4-conductor wire similar to trailer wiring cable, and needed connectors and other
hardware. The instructions suggest that one should run everything off the primary battery, keeping the auxiliary charged and ready for emergency backup. That philosophy made sense to me, so
I followed it in all of my accessory wiring.
Both the instructions and installation were very straightforward for anyone with basic electrical circuit knowledge. The operator chooses manually from three possible switch positions. The
center position disengages the auxiliary battery from the charging circuit entirely, essentially replicating the factory configuration. The “green” position engages the solenoid to charge or use power from the auxiliary battery while the ignition key is in the accessory or run positions. The “red”
position engages the solenoid at all times, combining the batteries as if they were one. Operating
the system normally, the batteries are isolated with the switch in the center. To charge the auxiliary
battery, select the green position. To enable a cable-free jump-start for a weak primary battery, or
provide extra power to high-demand accessories such as a winch, select the red position.
I decided to relocate both batteries and the solenoid to the space behind the rear axle, to save
space in the engine compartment for an air compressor. This required a long run of positive 2/0
gauge cable. Because I felt there was a risk of water intrusion into the solenoid at the relocated
position, I used RTV silicone to seal all the seams in the solenoid.
One of the shortcomings of the Painless kit was the implication that it had everything one
needs, when in fact it lacks any heavy-gauge cable or connectors for the secondary battery. If the
application calls for long lengths of cable (mine did), it can be a very expensive add-on (mine
was). Likely as a cost-reducing decision, the Painless system also excludes components to measure
the voltage available from the individual batteries without extra wiring or third-party voltmeters.
There is also no voltage-sensing relay to automatically charge both batteries and isolate the secondary battery if the primary vehicle charging system voltage drops below a minimum threshold.
After a year of use, I am pleased with the product, but I wish I had more data on battery status—and I must admit it would be nice to have the auxiliary battery charged automatically instead
of having to remember to do so manually., 817-244-6212
By Gary Greer
Painless Performance #40102 installed under
Gary’s Toyota Tundra (Photo: Chris Marzonie)
Remote switch with indicator lights for switching
among isolation, charging, and combining of batteries (Photo: Chris Marzonie)
Overland Journal Winter 2008
Product Profiles
By Dan Streight
Power Gate SR-200 installed on the firewall of
Dan’s Toyota Land Cruiser (Photo: Dan Streight)
(Photo: Perfect Switch LLC)
By Pierre Michaud
Perfect Switch Power-Gate - Model SR-200
Single Rectifier Switch $268
Like most overland rigs, the engine bay on my 1999 Toyota Land Cruiser is packed with just
about everything except an abundance of space. Yet I still needed to install a dual battery system
in my engine bay. Even though Toyota engineers left the perfect location for the second battery,
figuring out where the typical solenoid-style isolator would fit was giving me, well, fits.
Fortunately I had been reading about a new entry in the battery isolator world. This new
isolator design was a departure from all other systems on the market, and had no moving parts.
Nada, zero, none: Just pure electronic circuitry.
I’ll be the first to admit: I know very little about electronics. I’m okay with simple circuits in
AC or DC, but you’ll quickly lose me when you start talking about field-effect transistors and
the like.
It turns out a MOSFET is a good thing. Perfect Switch, a company that invented and manufacturers the Power-Gate battery isolator, has created a rectifier/isolator that uses MOSFET
technology. Aside from the benefit of not using any moving parts, the Power-Gate MOSFET
isolator is also very small in size. This technology keeps waste current to a minimum, thereby
eliminating the need for a typical heat sink that adds bulk to conventional diode isolators.
The installation was easy and straightforward, as Power-Gate provides excellent full-color
instructional diagrams. Just find a spot for a small metal and epoxy package about the size of a
deck of cards. On my Land Cruiser that was on the upper portion of the driver’s side firewall. I
used the Power-Gate as a template and center-punched where the holes would be drilled. Four
self-tapping screws are all that’s required to secure the unit along with the supplied plastic spacers to allow air circulation between the Power-Gate and the mounting surface.
My Power-Gate has, for the last several years, done its job quietly, reliably, and without taking up much engine bay space. What more can you ask of your isolator?
Editor’s note: Although the single-rectifier isolator will keep the starting battery isolated, it will allow the
main battery to draw from the auxiliary (the auxiliary battery is not isolated). Perfect Switch says they will soon
release a dual-rectifier model that will isolate both batteries., 858-720-1339
Sure Power Battery Separator/Interconnect Model
1314-200 (and 1315-200) $125
The Sure Power system appealed to me due to its affordability, simplicity, and availability
through a large network of suppliers. The solenoid-switched, 200-amp unit automatically combines the auxiliary and main batteries once the charging system reaches 13.2 volts, and will disconnect the batteries should the voltage drop below 12.8. A time-delay feature is built in to prevent unnecessary switching due to voltage fluctuations. The user can manually connect the main
and auxiliary battery should the need arise. I’ve found this feature useful during hard winching
sessions when you need additional battery capacity.
I originally purchased the model 1314-200 and mounted it on the firewall of my 2003 Toyota
Tacoma. I had some concerns regarding the effects of high temperature, but the specifications
indicated the unit was suitable for operating conditions ranging from minus 40º C (minus 40º
F) to 85º C (185º F). This location was close to the main and auxiliary batteries, minimizing the
length of wire required for the connections. Per the manufacturer’s recommendations, I used
4-gauge wire with copper lugs and shrink tubing. This mounting location provided a hidden benefit; I could clearly hear when the solenoid would connect or separate the batteries. I mounted
the manual intervention switch on the console surrounding the shifter boot for easy access. I
chose not to use the manual intervention/ auxiliary start indicating light, because the switch I
used had a red indicating light built in. The overall installation was simple, thanks to the excellent
instructions. However, no wiring supplies are included.
After a week of operation the unit failed. I could no longer hear it operating, and determined
that it was no longer connecting the two batteries automatically. It would do so using the manual
Overland Journal Winter 2008
Product Profiles
override function, but would automatically disconnect once the vehicle was turned off, defeating
the whole concept of an automatic system. After a call to the retailer and some troubleshooting,
we determined that the unit was defective. They were quick to supply a replacement unit, shipped
at no cost. It was a model 1315-200 (which monitors either battery level to initiate charging,
rather than only the main battery as with the 1314), in this case, due to availability. The replacement unit has been in service since then with no issues to report, although I am aware of three
other units that have failed in a similar fashion.
One other minor gripe I have with this setup is the lack of a battery monitor, easily solved by
installing a voltmeter. Overall I’ve been very satisfied with the performance of the Model 1315200. It has kept the main and auxiliary batteries charged, and even provided a self-jump start
on one occasion. Truth be told, if I had to do it all over again, I’d still go the Sure Power route., 800-845-6269
Xantrex - Pathmaker 100-amp Model 84-2051-02 $215
Xantrex is well-known in the marine, RV, and solar industries for mobile power products. I’ve
been using their Pathmaker 100-amp isolator/combiner system for about six years with excellent
results, and no failures to date.
The system comprises a solenoid-type switch, a control module with status indicators, a selector switch with momentary position, and dial controls for customized user settings. Different amperage ratings and battery bank configurations are available (up to three batteries and 500 amps).
There is an optional remote switch with indicator available.
Installation is straightforward using the comprehensive owner’s manual. You’ll need to procure the necessary cables and connectors to suit your installation. I found a suitable mounting location for the solenoid on the firewall of my 1998 Tacoma. One cable from the solenoid connects
to the positive terminal of the starting battery, one to the positive terminal of the auxiliary battery,
and one to ground. That’s it, pretty simple. I added some complexity by remotely mounting the
control module in the cab, which required removing the module from the solenoid mounting
plate, extending the wiring from the solenoid through the firewall to the module, and fabricating
a simple box in which to mount the device.
The control module senses both cut-in and cut-out voltages for connecting and disconnecting
the batteries, and incorporates voltage overload protection. All of these voltages are user selectable using the dial controls. Normal operation has the toggle switch in the center position. A
green light indicates that the batteries are connected (at or above connect-voltage). A yellow light
indicates that they are isolated (at or below disconnect voltage). A red light indicates a high voltage
disconnect. When I charge the auxiliary battery at home with a charger, I use the toggle switch
to manually disconnect the batteries (my charger will not operate when two batteries are present). I can use the momentary switch function to combine the batteries for emergency starting
(self-jump starting), an option I have gratefully used on a couple of occasions. The combination
automatically resets to auto position after five minutes, or with another push of the switch.
Overall I have been very satisfied with the convenience and reliable operation of the Xantrex
system. However, there are some characteristics supposedly associated with the design that I’m
curious to learn more about. Apparently when the batteries are connected via a solenoid, there
is nothing to prevent the two from naturally equalizing with one another. In other words, if I
severely deplete my auxiliary battery and the solenoid connects it to the charging system once
the connect voltage is sensed, the available current from the nearly fully charged primary battery
will rush into the secondary battery until they have equal voltages. This intense transfer of power
is potentially hard on both the solenoid contacts and the batteries—but I have no quantifiable
results to prove the extent of such effects. Regardless, the system has worked as advertised for a
long time, and continues to provide me with a safe starting battery or self-jump capability when
I’m on solo adventures out in the hinterlands, and that is something I really appreciate. xantrex.
Sure Power Model 1314-200 installed on the
firewall of Pierre’s Toyota Tacoma (Photo: Pierre
Switch (close-up view) (Photo: Pierre Michaud)
By Chris Marzonie
Xantrex Pathmaker solenoid with battery cables
attached (Photo: Chris Marzonie)
Xantrex Pathmaker control panel (Photo: Chris
Overland Journal Winter 2008
Product Profiles
By Matthew Carter (special thanks to
Robert Gorrell)
Xantrex Pathmaker control panel (Photo: Chris
Xantrex Pathmaker solenoid with battery cables
attached (Photo: Chris Marzonie)
Wrangler NW Power – Dual Battery Kit
100-115/100-120 $347 (kit), $187 (Isolator)
One of the first projects I undertook on my 2003 Toyota Tacoma was to install dual batteries.
I wanted one battery reserved for starting purposes, and another that would be dedicated to aftermarket accessories and back-up starting. The research began.
I couldn’t find an all-inclusive kit available for the early Tacomas, so I decided to fabricate my
own. After making room under the hood and building my tray and tie-down, I loaded it with a
red-top Optima for starting and a yellow-top Optima for deep-cycle duty. Now it was time for an
The companies I learned about during my research included National Luna, Sure Power, Hellroaring, and Wrangler NW (a.k.a. AmFor Electronics). After speaking to each of these companies
and talking to owners of each system, I decided on the Wrangler NW product. My choice was based
on system simplicity, owner opinion, price, word-of-mouth reliability, and customer service.
I called AmFor Electronics and spoke to Stan, who spent at least an hour on the phone with me
answering questions. After telling him that I had mounted my batteries side by side, he suggested
that I send him a picture. After reviewing the photo, he recommended their Jeep TJ kit minus the
tray. I added the optional in-cab battery manager kit, a pre-wired, three-position switch that provides
both dual-on and dual-off positions and an emergency-connect position. Total price was $300.
The parts showed up in a matter of days. The quality of the materials was top-notch. All wires
were labeled and pre-terminated with the proper connections and shrink-tube. Since my installation was custom, it took a little imagination on my part, but overall was easy. For performance and
a clean look, I added marine-style connectors to the factory cables. These were the only extra parts
I purchased.
Since the installation, the solenoid isolator and in-cab manager switch have worked flawlessly.
The system has held up to 10,000 miles of heat, cold, dust, and vibration without a hiccup. I currently have an auxiliary fuse block connected to the yellow-top, and I’m running a couple of low-amp
accessories and my winch with it. The Red-Top is my primary starting battery and powers all factory
accessories. I love this system and would recommend it to anybody., 800-962-2616
This was a different kind of review, since we couldn’t measure the
comparative efficiencies of these units, installed in vehicles thousands
of miles apart, with different batteries, alternators, and lengths of wiring
run, all of which could produce spurious differences in readings even
if we assembled everyone in one place. So we couldn’t determine an
editor’s choice by strictly empirical means. Instead, we documented the
known strengths and weaknesses of each type of isolator, then investigated the products in terms of quality, ease of installation (and ease of
de-installation should a problem arise in the field), comprehensiveness
if a kit is offered, price, and customer service. The three principals on
the staff—myself, Scott Brady, and Jonathan Hanson—have each had
first-hand experience with different systems, so we pooled our individual
conclusions, which turned out to be unanimous.
We suggest avoiding diode-type isolators, which, while lacking moving parts, waste current and can fail from excessive heat buildup. Combination diode/FET systems offer better efficiency, but if you are considering one, check the manufacturer’s rated voltage loss, especially when
the battery is discharged.
Our preference is either a solenoid or a MOSFET. The MOSFET’s
advantage is solid-state construction, but product choices are limited,
and until dual-rectifier units are available, a discharged primary battery
Overland Journal Winter 2008
can draw down a secondary battery in a MOSFET system. Many solenoid-based systems have been in continuous use for years, so they have
the advantage of a longer proven history. Despite the mechanical contacts, they are durable and long-lasting performers.
Our Editors’ (plural) Choice went to the National Luna. As a kit, it
is exhaustively complete, child’s play to install, and will accommodate
almost any application and second battery location. If you’d rather put
together your own system, the NL Intelligent Solenoid and monitor are
available separately. Our only wish is that a higher amperage capability
were available.
We gave the Value Award to the Xantrex Pathmaker, another solenoid-modulated unit. The Pathmaker offers a number of features not
found on even the National Luna, such as adjustable voltage cut-in and
higher amperage capabilities. The only questionable aspect of this choice
is that the Pathmaker has been discontinued by Xantrex. The company
claims to have a two-year supply, and warranty coverage is still there. The
best source is now not Xantrex, but a Google search for one of many
online vendors.
Of course, a viable alternative to any of these is a durable, reliable
manual system—just don’t leave your camera lens sitting on the bumper
while you pop into the cab to switch batteries.
Dual Battery Comparison
National Luna
Intelligent Split
Charging Kit
Painless Performance Products
- 250 Amp Dual
Battery Current
Control System
Perfect Switch
Power-Gate Model SR-200
Single Rectifier
Sure Power Battery Separator/Interconnect
Model 1315-200
Wrangler NW
Power - Dual
Battery Kit
346.72 kit
187.25 isolator
Xantrex - Pathmaker 100-amp
Model 84-205102
$175 (Isolator)
$47 (Remote)
$375 kit
$140 isolator
0.6 lbs/0.27 Kg
9.48 lbs/4.3kg
(kit weight)
1.6 lbs./0.7 kg
1.16 lbs./0.53 kg
1.9 lbs./0.86 kg
Approx. 1.5
lbs./0.68 kg
(kit weight)
1.3 lbs./0.59kg
4.8" x 3" x 2"
12.19 x 7.62 x
5.08 cm
4.5" x 5" x 2.25"
11.43 x 12.7 x
5.72 cm
4" x 3.25"
10.16 x 8.26 cm
3.5" x 7" x
8.89 x 17.78 x
2.22 cm
3.28" x 4.06"
x 4.02"
8.33 x 10.31 x
10.21 cm
3.69" x 3.3" x
9.37 x 8.38 x
7.11 cm
6.3" x 7.4" x 3.6"
16 x 18.8 x 9.14
-40C to 65C
-40F to 149F
-40C to 12C
-40 to 257 F
-40C to 88C
-40F - 190F
-20C to 85C
-4 to 185F
-40C to 60C
-40 to 140 F
-40C to 65C
-40 to 149 F
-28.9C to 48.9C
-20 to 120F
Status lights
Manual switch/
No (avail. w/BIC95300B)
Wiring and
Voltage indicator included
Audible alarm
(can be turned
DC current
95A continuous,
150A/20 secs,
85A continuous,
400A peak
240A continuous, 300A peak
Up to 500A
800A/15ms peak
200A continuous, 600A peak
200A continuous, 600A peak
100A continuous, 400A peak
Voltage range
Not available
Not available
Country of
South Africa
Slee Off-road,, 888-494-7533
Blue Sea,, 360-738-8230
Discover Batteries,, available from,
Overland Journal Winter 2008
Review Team
Brian DeArmon
Pierre Michaud
Dan Streight
Pierre Michaud is a system specialist and project
manager at a CANDU6
nuclear power station in
New Brunswick, Canada
(at least, until recently—
we hear a move to Australia is in the immediate
offing). So he’s used to
dealing with electrical currents of slightly
higher amperage levels
than those discussed in
this review. With a job like
that, we imagine relaxation time is vital for his
blood pressure. Until very
recently, Pierre relaxed
with the help of his 1999
Toyota Tacoma, which
took him fishing, hunting,
mountain biking, and hiking. But apparently he’s
now on the hunt for a new
Toyota somewhere in Oz.
From an early age, a desire to explore the outdoor world was instilled
in Dan and his siblings
by their parents. Whether
fishing, hunting, mountain biking, backpacking, hiking, back-country
ski touring, camping, or,
now, overlanding in his
100-Series Land Cruiser,
Dan has outdoors in his
DNA. The past dozen
or so years Dan has been
exploring a playground
right where he lives and
works: Nevada. It’s said
that 85 percent of Nevada
is public land, with some
1,000,000 miles of roads
to explore. That ought to
keep Dan busy for at least
a couple more years.
Overland Journal Winter 2008
Riding on four wheels
or two, the soles of his
boots, or the saddle of a
horse, Brian DeArmon
has spent most of his life
exploring the wonders of
nature. This obsession
has taken him from the
Rocky Mountains, to the
Gold Coast of Australia,
the beaches of the Seychelles Islands, the frozen
landscapes of Alaska, and
a few places in between.
Settled now in the Sonoran Desert, Brian is enjoying a lull in the fast pace of
life before the next adventure begins.
Gary Greer
Gary Greer lives in Houston, Texas, with his wife
and two teenage children.
His entrepreneurial career
in the energy industry affords him time to spend
in the Sam Houston Area
Council, and as an active
Boy Scout leader. Scouting has fueled his passion
for service, leadership development, conservation,
and the outdoors. While
he enjoys four-wheeling
in his 2003 Tundra, he
especially enjoys the rare
opportunity to overland
with his kids. Because
Texas has little public access land, this involves
extensive highway travel
to overland trailheads,
severely limiting trip frequency.
Matthew Carter
Matt Carter is a paramedic with Pridemark Paramedic Services in Denver,
Colorado. He has been
working on vehicles since
the age of 16, taught by
his buddies Rob Gorrell,
Kyle Voigt, and Chris Hill.
His latest project vehicle
is a 2003 Toyota Tacoma,
currently being modified
for overland travel in the
West. When not working
on his truck, Matt enjoys
using it to go snowboarding, mountain biking, and