Under Bed/Dual Battery Installation






When I first started building my truck for wheeling, *the* thing to get was the Optima red-top battery. So I got one and replaced the aged lead-acid battery that was leaking all over my fender. Seemed like just after that, *the* big thing was dual batteries. My single red-top filled the area under my hood and worked fine for nearly two years of hard wheeling, but after picking up a Premier Power Welder, I had to find some room to install the large control box for the welder.

So, what you might ask, does needing more room under the hood and dual batteries have in common? Well, needing more room led me to explore alternate locations for some of the equipment already living under the hood. I ruled out the engine electronics, my 12V air compressor and that basically left the battery. Batteries are big and heavy, so I thought if I could find someplace low and out of the way for the battery, I could free up valuable space, lower my center of gravity and get my welder installed.

Here were my design goals:

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So, to start the process, I used the highly scientific method of a creeper and a tape measure and basically measured the dimensions of all the likely spots under my truck where a battery might possibly be attached. I had not yet picked a battery, but I had a few types I had dimensions for and then checked the available locations to see if the battery volume would fit. Besides the location I found, below, I also noted that the area above the rear axle is quite open and would also make a good battery location, although I plan to install either a 2.5 gallon air tank or an air bag helper spring there.

Dual Battery tray parts Dual Battery tray assembled Red dual battery tray installed
A: Battery tray parts B: Battery tray assembled C: Battery tray installed

Lo and behold, I found a location for two small batteries up above the rear drive shaft, below the passenger seats. The battery tray is made of 1x1x1/8 steel angle, cut and welded into a frame as shown in photo A above. The bottom tray has two square recesses for the batteries, the top frame is the same except with the top web cut out to clear the battery terminals, as shown in photo B above. I actually welded the top and bottom trays up at my old welding class. I purchased all the steel angle at a local hardware store, them pre-cut all the pieces ona table saw equipped with an abrasive cutoff wheel. Then took the bag of parts to class and spent most of the night gas (oxy-acetylene) welding the parts together. I figured why spend 3 hours welding scraps of steel together that would be tossed into the scrap bin afterwards, when I could make something useful instead! Had a great welding teacher and he was cool with students doing their own thing.

To attach them, I used 4 threaded rods that essentially hang from the floor. I drilled 4 holes in the floor pan beneath the rear seat bottom. A nut on the top of the floor supports the weight, another nut snugged up against the bottom of the floor panel, with a length of pipe to serve as a spacer, holds the threaded rod tight. Then a third nut holds the battery try in place from below. In photo C above, you can see the threaded rods running along the side of the battery tray. And what is that silver/gray stuff wrapped around the tray? Looks like duct tape and that's what is is. I used the duct tape to strap the upper and lower trays together while I slid it up onto the threaded rods, that in turn pass through holes in tabs welded to the sides of the trays. Once the nuts are tightened down, the trays clamp the batteries in between them snugly and nothing can move. As it is, the pair of batteries are sandwiched between the gas tank and the heat shield above the muffler.

There's not a lot of room, but the tiny BP-800MJ Black Panther batteries don't take up much more room than a single conventional battery. After about 9 years of trouble-free service, I took the opportunity to replace the pair of Black Panther batteries (no longer available) with an identical size/capacity unit from Odyssey, a PC925 model. As an added benefit, they are totally metal jacketed and completely sealed, so should be up to the harsh environment down there. The batteries are bolted up through the floor, and 1/0 welding cable is used to bring the power up, under the body to a pair of 350A disconnects, pictured below:

Dual battery under-truck wiring

These disconnects serve several functions:

The grounds are tied together after the connectors and the positive leads terminate on insulated studs (these can be seen at the far left of the image above). A pair of 1/0 cables run up to a 200A continuous duty solenoid which can be controlled in a number of modes. I ordered both batteries at the same time, so hopefully they'll be out of the same lot. This is important to minimize problems with the two "fighting" when in parallel. Also, the Black Panther batteries are true deep-cycle units and can be safely discharged to 0V without damage.

The main battery is tied to the engine fuse box, while all the auxiliary circuits (including the winch) are fed from the auxiliary battery. A disconnect on the front of the vehicle supplies the winch or a jumper cable. A connector in back serves a similar function. But, you may ask, doesn't an electric winch draw a lot of current? Yes they do, easily more than the 200A capacity of the solenoid, but not all 400A going to the winch comes through the solenoid. Ideally, 200A of it would come from the auxiliary battery (before the solenoid) and the other 200A from the main battery (via the main battery). Practically, a bit more may come over the solenoid since the 160A alternator is behind it, too. But in several years of use, I've not had any problem with the solenoid or winch operation. A solenoid current rating is typically determined by the amount of current it can make or break, the actual running current can be much higher in practice, as long as I don't try switching the solenoid on or off while at full pull on the winch.

The solenoid control logic, from Painless Wiring, allows for 3 modes of operation:

  1. Mode one is used for most conditions. In this position the system has twice the normal starting amperage automatically. Batteries are isolated with the ignition off, thus preventing the "dueling battery syndrome". When the ignition is on (and the engine running) both batteries are charged (in parallel) from the alternator.
  2. Mode Two is provided to lock out the auxiliary battery. In this mode the secondary battery can be held in reserve when heavy winching or extended cranking are expected. Or, in my case, I have the winch connected to the auxiliary battery, so this way the winch will not drain my main battery. In this mode, the main battery is charged by the alternator, but not the auxiliary one.
  3. Mode Three is used if the main battery won't start the vehicle. This mode allows a self jump-start from the auxiliary. Both batteries are charged by the alternator in this mode, however, the solenoid remains energized even if the engine is off.

While it seems that this is a very primitive and simple circuit, I find its remarkably effective and easy to use, or should I say not use. In day to day use, you never touch the switch. In fact there are probably entire years that I don't touch the switch or even think about it. Its just left in the Mode 1 setting, which turns on the solenoid when the ignition is on, putting the two batteries in parallel. When the engine is shut off the solenoid automatically turns off, separating the two batteries. Only on the rare occasion I kill the main battery, instead of hauling out the jumper cables and finding a willing battery donor, I just flip the switch to Mode 3, wait a few seconds for the aux. to charge the main, turn the key and start the engine. After a few seconds, flip the switch back to Mode 1, where it'll stay for another year. Have never needed Mode 2 other than to feel how slow the starter turns with one instead of two batteries. Now I only carry jumper cables to jump someone else (if needed) or to rig up a 24V output for welding purposes.

I've slightly departed from this recommended setup, by placing all my auxiliary devices, like winch, power points, CB, lights, etc. on the 2nd battery. Since I use a 12V DC cooler, I want to be able to run it all night off the 2nd battery and not have to worry about draining the main battery. Anyway, the idea is that in normal operation, ignition power energizes the solenoid to connect the batteries together, for charging and double starting current. With the ignition off, the batteries are separate. The jump start mode allows the solenoid to be energized manually.

For safety and convenience, I made up a pair of 5' extension cables, with a 350A connector shell on each end as well as a pair of 5' jumper cables (with a 350A connector on one end and battery clamps on the other end). I'll be installing a similar, single battery system in my pickup and intend to share these cables with it. For emergency welding, I can unplug the two battery connectors, plug in extensions and jumper cables, tie one "-" to the other "+" lead and obtain 24V DC from the remaining clamps. While I plan to have a Premier Power Welder to do the main welding chores, for the price of a few connectors and some cable, I can provide a backup welding system, so why not add it to the system? The front and rear connectors allow for safe battery jumping (no battery fumes to explode) and it just so happens that the quick disconnects that portable winches use are the same as I used, so if needed, I can hook up a portable to my rear bumper.

So, now with the battery out of the stock location, I also decided to move the air box into this place and then place the PPW control box where the air box used to live. The battery control switch lives up next to the formerly empty switch location on the dash. I had previously moved the rear window control switch into this location.

After about 10 years of use, the old Black Panther batteries started getting a little tired. With the small 25AH capacity, it did not take leaving a lot of stuff running to drain them. So I picked up a pair of Odyssey batteries of the same size and swapped them out. I figure to get another 10 years out of this pair. Surprisingly, when I got the BPs out, I found they still held a charge fine. I think the main source of the problems was that one of the posts had loosened up. After getting an allen wrench inside to tighten up the retaining screw, that battery seemed fine. But since I already swapped in the new ones, I put the old batteries to use in my solar heating/cooling setup in my house. Solar panels charge the batteries and a thermostat operates a fan that either pushes sun-heated air into the house, or pulls cool night air in. So those batteries are now in their second "career" and enjoying a second life!

I am really happy with this dual battery setup. It is very reliable, have not had one problem with in since installation. And with the long lifetime of the batteries, they must "like" it too. If not, they would not last that long. I never even think about the dual batteries in day to day use. They are just there, just like one battery. The only time you think about it is if for some reason the main battery gets drawn down too far to start the engine. In that case, instead of popping the hood and dragging out the jumper cables, just flip a switch on the dash and off you go.

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Parts List:

Excuse the "crude ASCII art" but this is how it is all hooked up:

Wiring Schematic 

       |                            AUX              |
       R                  Batt2-<>---+               F  |
       E                            SOL              R  W
       A                  Batt1-<>---+---ALT         O  I
       R                             |               N  N
       |                             |+-<>-STR-FUSE  T  C
       |                             ||              |  H
       |                             ++              |
       |                                             |

       2 - 6' extensions   2 - 6' jumper cables
       >------<  >------<  >------C  >------C

">" is a 350A connector shell
"STR" is the starter
"FUSE" is the engine fuse block (internal 80A fuse)
"SOL" is the 200A solenoid
"AUX" is the auxiliary fuse block
"ALT" is the 160A Premier Power Welder Alternator
"WINCH" is the Warn 9000i front-mounted winch
Note: Ground circuit omitted for clarity, but ground cables are run in parallel w/ power to minimize ground loops.
Parts Supplies:
Wrangler Power Products
Painless Wiring Products
Premier Power Welder
Madco Welding, Mtn. View, CA.

Part List for dual battery wiring

Part Description Qty
34-902G 350A Connector Shell 15
32-902-1 1/0 terminals 30
34-902DC Dust Cover 1
34-902M Connector mounts 2
34-902L Connector locks 2
30-101CR/B Positive power studs 4
32-6410-38 Cable lugs 10
32-4110P/N 1/0 battery clamp 2
32-4210P/N 1/0 battery clamp 2
32-317R/B Battery clamp insulator 2
32-318R/B Battery clamp insulator 2
32-653B/R 1" Heat shrink tubing 4'
34-915B/R 350A battery clamps 4
38-101 1/0 terminal crimper 1
1/0 Welding cable 50'
BP-800MJ 800A Batteries 2

Rating: bananabananabanana

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Welding with Batteries:

So, can you really weld with a couple of 12V batteries?

Yes, quite well in fact. I find 3 in series (36V) work better that 2 (24V) but in a pinch use what you got. You'll need some welding rod, a rod holder (jumper cable clamp might work in a pinch) some jumper cables and some batteries. If everyone in your wheeling group has a set of jumper cables, you are set. Hook cable to each battery, then connect the red of one to the black of the next one and so on, finally ending up with one black clamp and one red one and 24 or 36V between the two. Keep the batteries as far back from the welding location as you can, especially if they are not sealed units. A fire extinguisher is a good idea and some basic welding safety equipment like heavy gloves and a face shield with minimum #10 tint lens.

So what kind of rod should I get:

Shielded Metal Arc Welding rods are designated by a multi-digit code as follows: