For some reason installing two batteries where there was only one original seems to be a difficult problem. Many people don't like dealing with electricity and prefer sticking with projects that require a ratchet and a ball peen hammer.
Running dual batteries is a great project for your truck and not that hard to setup. That being said, I don't want any of your loved ones suing me, so be careful. If you have any doubts, don't. If you think for a second you're going to hold me liable for blowing up your truck (or family blaming me for your death), stop right now—It's your fault for trusting what you found on the internet. Yes, this is a disclaimer.
Anyway, I myself did some research on dual battery installation before I set out to conquer the beast... there are a lot of "good" ideas floating around about how to do it, from isolators and constant-duty solenoids to switches, even flux-capacitors. Forget about them, most are from people after your money or from others who have already paid those people. Wire them in parallel and be done with it for a minimum of cost and all the benefit. If you disagree, spend your money on some of these wonderful pinball machine parts I have and leave this advice to people who just want a truck to start in the winter.
Above: New batteries at top and lower left. Cable routing can be seen on the right side, running along the radiator support. Terminal extension bolts for the original battery location, top.
Things you must have:
Minimum length of appropriate cable
Copper "automotive style" crimp lugs
Electrical (vinyl) tape
Battery terminal extensions
First I went out and bought 12' of 2 AWG THHN cable, which happens to be gasoline and oil resistant. Measure your route before you buy wire! I chose 2 gauge as it was the smallest cable that would provide a voltage drop of no more than 2% at maximum charging current (in my case 160 amps). This is to prevent damage to the alternator and to keep the batteries from "fighting" and wearing themselves down. Calculated out, I ended up with a 2.5% drop over 6' of cable for 2 AWG at 160 amps. You may want to choose a larger cable diameter. I'm glad I didn't.
Second, you need to identify where to put the second battery! In many a Silverado the choice is simple. I located mine on the passenger side directly behind the coolant recovery. This is where the second battery from the factory is located. Some trucks are equipped with MAP sensors in this location.
I wanted to get this project under way, so I fabricated a support of 1/8" plate and bolted it down. The battery is supported off the bracket with neoprene rubber and rubber tape. To hold the battery down I fashoined a strap out of 1" webbing. You may choose to get a battery tray from a parts store / dealer. I will eventually but my old battery was dieing! I passed on the plastic trays from the local parts store.
Next, I routed the cables. This is why I'm happy I chose 2 AWG. Any larger and it would have been a difficult task. Bend them to fit kind of like you would when running brake line. At first I was going to be fancy but then reason took hold. Go easy and simple. I routed the cables from the new battery location, back to the fender, along the coolant recovery, past the air filter, turn 90 degrees, along the radiator support, down under the radiator hoses and finally to the stock battery location. Have fun! Make sure that your wires don't rub on any hoses / lines or you'll regret it later.
After you have the cable routed, crimp your lugs on. Make sure that they line up with the terminals properly or you'll have to twist the wire. Also make sure you get the correct terminal size which would be 3/8" for GM side terminal style batteries. These are available cheapest at you local hardware store in the "hardware" section in the little bins. Stay out of the electrical section or else you'll over pay or won't be able to find the right size.
Now slip on your loom (optional but highly recommended). This is the black corrugated tubing you see snaking all around your engine compartment. You can get it cheap at the hardware store. Its purpose it to protect the cable from rubbing, don't pass it up. Not to mention the installation looks much more professional this way… which is enough of a reason to warrant the extra $5.
Before you can install the system you have to secure the cables. I used nylon clamps also available at the local hardware store. These I fastened behind the bolts holding the fan duct. You may find a better location.
Now for the fun part, connect the batteries! Remember, red goes to red, black to black, not the other way around! This is called parallel. Put your extensions on the stock battery. I transferred the stock terminal bolts to the new battery. If you can't figure this out you've probably electrocuted yourself by now. The sequence I used (which I believe is correct, please someone correct me if I'm wrong):
Positive from vehicle to battery #1
Positive from battery #2 to battery #1
Negative from vehicle to battery #1
Negative from battery #2 to battery #1
You MUST connect your 12 volt batteries in parallel on a 12 volt system. Doing otherwise is called "series" and you end up with 24 volts which results in no more truck—it disappears in a flash of fire and a puff of smoke. Parallel results in the same voltage, twice the amps, and twice the reserve capacity. It's like going out and buying a battery with 1200 CCA that lasts twice as long (reserve capacity) than your stock 600 CCA battery. Old man winter is now shut out far away from your starter!
I used red vinyl tape to clearly mark the positive ends. I also used rubber splice tape and vinyl tape to wrap the copper lugs up to the terminal spade for personal safety. Rubber splice tape was also used to fashion positive terminal covers. If you can find manufactured covers for side terminal batteries, let me know, I'd like to buy some.
(12') 2 AWG Stranded THHN Cable = $35
(4) Copper Lugs = $9
(2) Battery Terminal Extensions = $9
Vinyl Tape (Red and Black) = $5
Rubber Splice Tape = $4
(2) 10' Loom = $5
Nylon Hold-Downs = $3
(2) ACDelco 78-6YR Batteries 675 CCA = $230
Total = $300
Done properly my setup would have only cost $300. It does not include tools like a wire cutter or crimper. Get good ones if you don't have them… go to Sears and buy Craftsman or better and make sure they're made in the USA. I probably spent $50 more in stuff I didn't need like crappy lead lugs from Wally-World. Get good stuff! My extensions from AutoZone will need to be replaced shortly; the threads were damaged just by trying to tighten them. Coincidently I have vowed NEVER again to shop at Wal*Mart or buy anything MADE IN CHINA no matter how small (except for disc brake rotors which is another story... arg).
Other things you must know:
SAFETY! You must have a grasp on basic electrical concepts. If you haven't learned series vs. parallel wiring, go learn it! Also, if you don't feel confident performing work on your own vehicle bring it somewhere to have someone else install this mod. Most of what we do to our trucks is fairly safe to implement, resulting at the worst some bloody knuckles. Messing this up can KILL you. Whoever said a little fear isn't healthy? You wouldn't go sticking a fork in power outlets would you? That's essentially what you're doing every time you disconnect a battery.
Batteries have a code stamped somewhere on them indicating the month and year they were manufactured. This code differs by manufacturer and you may want to learn how to read it. It is highly recommended that you get batteries made during the same month. Mine have consecutive serial numbers. I don't know if it still holds true or not, but it is also recommended to purchase batteries that are no more than 3 months old.
I chose not to ground the second battery to the chassis and run a cable between both negative terminals. I chose this for ease of installation and no *good* reason. Alternatively you could run to ground on the second battery with no cable between terminals or do both. This is probably the proper way to go, but I didn't want to unhook two grounds when I service the vehicle… so there. An electrician may now tell me if I'm wrong or right. Never-the-less, it is what I did and I can't recommend anything else.
Finally, you don't NEED a constant-duty solenoid or any other fancy $300 crap. You can choose to get it if you want, but my setup works and will work fine as long as I heed standard auto protocol like "Don't leave your dome light on."
Now listen to it crank! My-My, faster than your girlfriend!
Good luck and muddy tires!
Edited by fourwheelinfool, 15 October 2007 - 10:02 PM.