Dual Batteries

Got Spark?

This has been one of my slowest "projects" in the making.  Mostly due to the fact that I had the complete project in mind up front (I knew what I wanted) but I didn't have all the resources (buck$).  There are lots of articles out there in print (and on the web) talking about how to install a dual battery system.  My final decision relied on the most-common advice: the Battery Manager System by Wrangler NW Power Products.  However, not to be entirely identical to all the other articles, I made a few changes to my system.

Of course, the batteries always shown in those articles were the Optima Batteries.  This is a popular battery for off-road vehicles because of its ability to withstand the rigors of the off-road environment - up to 15 times more shock resistance than a conventional battery.  You can read all about them, including their deep cycle marine batteries, by following this link to Optima Batteries.  While these are not inexpensive batteries, all I can say is that these batteries are well worth the money.  (Note:  It may later prove, over time, that the added initial expense will come out to be less expensive (in cents per day) then a conventional battery when you compare the added lifespan of these batteries.) 

The first step in my process was the dual battery tray.  My stock tray had basically a pool of white battery acid and rust in it.  Why the battery, with all its weight and the bouncing around, didn't fall through the bottom, I don't know?  A rectangular piece of tin had even been placed beneath the battery to help support the integrity of the stock tray, but it didn't really do much good.  I shopped around and found a nice tray from Tomken Machine for $79.95.  Wrangler Power Products also makes one that sells for about $5 to $10 more.  Both mount to the body in the same way, using the existing battery tray mounting holes on the firewall AND the top two holes for the stock jack mount.  This means, with the dual battery tray, you will have to do away with the under-hood jack mount (no great loss since they are just about worthless if you have larger tires).

Dual battery tray - one battery

For what it's worth, comparing the two trays (Wrangler NW Power Products verses Tomken), the Wrangler's tray has a solid bottom and Tomken's tray has only a "ledge" that surround the batteries.  One could argue which one will be stronger, but I was concerned about having water or residue pooling in the bottom of the tray and promoting rust - like what happened to the stock tray even with its drain holes!  Very little water will pool in the Tomken tray due to the lack of a solid platform.  Both trays are powder coated, and both trays are of high quality.  Both trays use a similar 1/4" steel rod and long treaded "J" hooks that hold the batteries down across the top.  However, the Wrangler's post uses three "J" hooks (middle and two ends), and the Tomken's post only uses two (one for each end).

View of remounted plug ports, (below the tray now on the fender well)

Once you've made your own decisions and have picked your tray, installation is fairly straight-forward.  One note, the stock tray holds (on the outside) the bracket with the diagnostic plugs.  I had to remove that bracket from the old tray and relocate it to the fender wall, where the jack was and below the tray.  The Wrangler tray may provide a mounting location for this unit, but I am not sure.  I may reconsider my new location, and re-mount the diagnostic plugs to the front of my tray - we'll see.  (Side note:  With the Mopar MPI or the Howell TBI fuel injection conversions, these stock diagnostic plugs are removed altogether.)

The next consideration will be how you will use your two batteries.  You will at least need a battery isolator.  The isolator's job is to monitor both batteries, and charge whichever one is lowest.  Looking around in the four wheel drive shops, I found Warn makes an isolator for this purpose.  It sells for about $60 and there are several models depending on what your needs are - just which one I needed was a little confusing.  So I looked around some more, and found the same isolator at the local NAPA auto parts store for about $32, almost half of the Warn version and it did the same thing!  This one didn't have the "Warn" label on the package, but appeared to be identical as both were made in the same country, and both were anodized in the same blue color.  Go figure?  However, what amperage I needed was still a bit confusing.  (For a simple view of how to wire a dual battery system with just an isolator, )

For my needs, I wanted the second battery to be available for the winch on those long days, and possibly my other electrical accessories like the air compressor or radio, too.  I also wanted the ability to have a back-up system in place - extra power for "jumping" another vehicle, or maybe even myself!  I wanted more than just an isolator alone, I wanted a "battery manager system" and Wrangler NW Power Products makes a really neat one.  Specifically, the Wrangler's battery manager allows you to select various modes for using the two batteries, and you can make this selection from within the cab.  Perhaps what was even better was the fact that Wrangler had a complete kit for my Jeep, so I didn't have to guess what size isolator I needed.

The Wrangler "Battery Manager System" includes a three-way toggle switch (with LED), the wiring harness, and the various attachments to patch it into your electrical system.  Of course, you also need to get their isolator, and it was helpful to get their two-battery wiring kit, too.  The two-battery wiring kit is simply a set of heavy gauge, color coded wires to tie the two batteries together, and some "military" style terminal connectors.  Once I had all the "parts" necessary to wire all the stuff together, I only needed the second battery...  Three months later, I was finally able to make the final leap!  I now had all the components necessary to complete my ultimate dual battery system.

How to spend lots of money!!!

Since I know very little about electronics, the whole project seemed a little intimidating to me.  All I can say was that after it was finished, it was very easy.  This was primarily due to the fact that the folks at Wrangler made it very simple.  I only wish they'd have spent a little more time writing their instructions out in more detail, but it was still fairly simple once you got into it.

What's involved...

The first step is to put both batteries into the tray and secure them in place with the 1/4" steel rod and the "J" hooks.  I guess I should point out that I also purchased Wrangler's 1/4" steel rod to replace the rod that came with the Tomken tray.  I did this because Wrangler's rod includes the hardware to mount the isolator on top.  After I installed the rod, I attached the isolator to the rod using the two nuts and washers provided.

Next, arrange the stock battery cables and new two-battery wiring cables making sure they'll easily reach the posts of the batteries.  There is going to be a lot of wiring going on in this small area, so take the time to plan out the route of all the cables carefully (about 45 seconds was all I needed).  Once this is done, you can secure all the cables to the batteries.  I'd suggest securing the cables loosely as no doubt, before you're done, you'll be making some changes.  Usually when working on your vehicle's electrical system, it is advised to disconnect the Negative cable from the battery so you don't short out something else along the way.  It's probably good advice to give here - but I didn't do that.  Also, the very last step is to wire in the Battery Manager's wiring harness into the system, and you'll need to tie two of those wires to each of the military clamps on the Auxiliary battery - so again, don't tighten every thing down just yet.

  Optima Set-up from side.

Step three was actually the hardest.  This step involves choosing the location of the toggle switch.  Well, that part was easy, what was hardest was drilling the holes through the bottom lip of the dashboard!  Somehow, trying to fold my body into some small origami figure and fit into that small space is always frustrating for me.   Once I determined where I wanted to mount my toggle switch, I discovered the two holes they provided on the backing plate were too far in the back of the plate.  So I had to make some modifications to this part, and I drilled two more holes allowing the front edge of the backing plate to rest flush with the dashboard (and the other pressure gauge I had installed for my on-board air tank).

Batter Manager Relay Switch

The toggle switch is located between the cigarette lighter and white gauge beneath the dash.

Once the toggle switch was mounted, I routed the cable attached to the switch over toward the fuse panel.  The wiring provided was more than enough to reach this distance!

The fourth step is routing the wiring harness from the batteries to the opposite side of the engine bay.  I zip tied the one end to the harness (with all the loose wires) above the batteries on the grill support rod just to hold it in place while I routed the harness across the top of the rear firewall.  I used several large zip ties (not provided in the kit) to hold the harness in place.

Step five is to drill a 1/2" hole through the fire wall.  Again, there was plenty of wire to reach, so I chose a location that best suited my needs - right along side the fuse panel.  Feed the plug-end of the wiring harness (really, the only end you are dealing with) through the hole and then seal the hole with the rubber grommet provided.  From the inside, attach the harness plug with the plug on the wire coming from the toggle switch.  Once this was done, the overall length of the two wires (harness and switch) is finally established.   I used more zip ties to carefully wrap-up the excess wire and secured it up under the dash.

Gray wire (harness) going through fire wall (rubber grommet also provided).

Step six is to find a power source from the fuse panel.  This was really easy, even for a non-electrically inclined person like myself.  The only requirement is to find a fuse that only works with the ignition in the "on" position.  So don't use a brake light or other lighting fuse.  I chose the fan motor fuse.  The Battery Manager System kit includes all these necessary little parts, including this nifty little metal band that fits over a standard fuse.  Simply pull out the fuse, slip the band over one of the two blades on the fuse, and clip the band to the back of the plastic part.  Then insert the fuse back into the panel.  The end of this band (attached around the fuse) has a male bladed end.  There one last wire provided in the Battery Manager System kit (red, with a green stripe).  This wire has a small fuse patched into its middle.  Attach one end of this wire to the back of your fuse, and the other end to the pigtail wire near the end plug of the wiring harness (that you've just ran through the 1/2" hole and connected with the toggle switch wire).  You can't miss it when you see the wire.

Finally, the last step is to now attach the other end of the wiring harness to the isolator.  This is the end you zip-tied above the batteries.  There are three wires coming off this end of the harness and each one is labeled, and color coded.  So there is absolutely no doubt as to where each wire goes.  With all the little wires attached, now you can tighten down all the battery cables and fasteners.  You're done!

Opt Dual Wiring.jpg (46459 bytes)

How it all works...

The toggle switch has three positions, or modes:  Mode 1 is the "normal" operating position.  The LED light will be green, and the system will use only the "main" battery for starting and operating the vehicle.  However, the isolator will be charging BOTH of the batteries in this mode - keeping the auxiliary battery ready.  Mode 2 (middle position) does not display any light.  This position totally shuts off the auxiliary battery - it's not being charged and it will not operate any system that is connected to that battery.  I'm not really sure when this would be necessary unless you simply need your alternator to ONLY concentrate on charging the main battery.  Mode 3 is the "emergency" mode and displays the red light.  In this mode, the amperage from both batteries is grouped together (parallel), but not the voltage.  Use this mode for large draws on your electrical system, such as jump-starting others, or long periods of winching.  You can even use this mode to jump start yourself!

One red-top Optima battery provides 800 cold cranking amps (CCA), a considerable increase over my former NAPA Legend 75's 530 CCA rating.  Add the second Optima, switch over to Mode 3, and now I have 1600 CCA's.  Or, in terms of reserve capacity, one Optima battery has 112 minutes of reserve capacity.  Join two together and you have 224 minutes of reserve capacity.  Good for a piece of mind in the snowy winter months.