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Dual Batteries for Any Vehicle

Engine View
Posted December 1, 2001

The Real Way to Be Safe in the Boonies

Step By Step

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  • The simplest way to get dual batteries is to order them with your new rig. That’s how this Chevy came. The second easiest way is to use factory parts and have someone else install them. It may not be cheap, but everything fits. For those who can’t get the factory items, they can use this kit from Painless Performance and use the factory trays and cables if your vehicle has that option.

  • Most custom rigs like this Jeep have room under the hood for a spare battery. Some companies such as Tomken have battery trays that fit two Optima batteries side by side in the stock location. Optima also offers 6-volt models which are half the size of the 12-volt styles. By separating them you can save room and still have 12 volts by running them in series.

  • A relay is used to switch most systems like this, which is similar to the fender-mounted Ford starter relay. However, these starter relays are not for continuous duty, and will fail if used in the system. Special relays are available such as this one on the firewall, but we prefer the big Painless relay, which is good up to 100 amps.

  • We tried to use a Tomken dual battery tray on our Ultimate A1, but since we cut out the fenderwell there was no place to mount the support brace. Since we liked the quality and design of the Tomken tray and hold-down, we chopped it up and stuck it under the passenger seat, and slipped the Optima in place.

  • We used the two-gauge cable and ends Tomken supplied, and crimped and soldered the ends. Mr. Gasket offers a battery relocation kit with two-gauge wires, so we used its long cable to run from the second battery to the solenoid and to the main battery.

  • The only place left to mount the Painless 100-amp solenoid was on the inside of the firewall. So we removed the main battery and drilled some holes, being careful not to drill into any other wires on the back side. The bolts are inserted from the engine compartment, so the bolt won’t protrude into the case of the battery.

  • Out of site and protected from the elements, the solenoid is still easy to access to mount the wires. We used two-gauge cable on all battery hot and ground applications, and made sure the body is grounded to the frame and the engine is grounded to the body and frame. Big cables and good grounds make for an efficient electrical system.

  • Our Optima has both side and top posts, so we used one of each since we have too many wires on the battery already. With the new system, we’ll probably move a few wires to the second battery for shorter wire runs and a cleaner installation.

  • The simple three-way toggle switch is dash-mounted along with the two small indicator lights. The left position illuminates a green light indicating both batteries are hooked together when the key is on. When the key is off, the second battery is isolated. The center position isolates the second battery from charging or depletion in any key position. The right position illuminates the red light, and both batteries are hooked together regardless of key position.

  • Our finished install under the seat is compact, hidden, and safe. In tight locations like this, make sure the posts don’t come in contact with the seat springs or dangerous arcing or battery explosions could occur. We’ll make some covers for the terminals so that nothing will be wedged between them, even in a rollover.

We’ve all experienced that dreaded sound under the hood when you twist the key: Click. Click Click. A dead battery is more of an inconvenience around town (depending where you live), but out in the boonies it takes on a real significance of the worst kind. If you’re smart enough to wheel with friends instead of by yourself, maybe someone has some jumper cables, or maybe some tools to swap batteries. Better yet, if you are lucky you’ll have parked on a hill and you can bump start the beast. OK, so you are by yourself, have no tools, and drive an auto-box 4x4. You lose. You could spend that freezing night out in the middle of nowhere and hope someone noticed you are missing. Yeah, right.

The miserable scenario we just painted for you is to let you know that there is another way, which is simply another battery. We’ve carried a spare Optima with us for years, safely wrapped up in a sleeping bag under the seat. These batteries don’t leak acid, can be stored in any position, and can be slapped on top of another Optima battery without cables to emergency jump an engine.

However, the real way to be safe in the boonies is to install a dual battery system, and one that doesn’t screw around with your stock setup. Standard battery isolators only have the ability to charge and discharge one way, while we want to be able to choose which battery is on line, and which isn’t. The old fashioned way of battery selection is to use a big, monster rotary switch found in many emergency vehicles and motorhomes. That big round sucker has positions for Battery 1, Battery 2, Both, and Off. This is a durable unit, but big and cumbersome for our rigs, and it requires major electrical wiring to hook it up.

There is another way we found, and the simplicity makes this setup a no-brainer. Painless Wiring offers a dual battery controller that hooks up with a switch, solenoid, indicator lights, and a few wires for a quick hookup. The universal nature of the kit allows virtually any vehicle to sport dual batteries, even when there’s not any room under the hood. We installed the whole shebang in about an hour in our Ultimate A1, including mounting the Optima battery from Performance Products and running cables. Check out how simple it is, and never be caught with dead cells again.


Mr. Gasket
Performance Products
Buena Vista, CO 81211