I just got an '86 F150 with an "in-line" 6-cylinder. I want to take this truck out and travel, and may want to run LOTS of accessories when stopped. (Lights for setting up camp, TV, radio, etc...)

I have always had the idea of setting up a "dual battery" system. Whether I heard or read about it somewhere or made the idea up in my head, I don't know. But it seems to me that if "Cold Cranking Amps" is good, with "more being better", the best way to increase overall amperage is to hook up 2 batteries in parallel.

I've had some electronics training, and the idea makes perfect sense to me, but I've looked around on the internet and ran text searches and can't find any "hit" where someone has done this.

As I "envision" it, it would simply be a matter of setting the 2nd battery next to the first, and joining them together, hot to hot (positive) and ground to ground (negative). Is there any reason why this shouldn't be done ? Is it this simple, or is there more to it than this ?

It occurs to me that there might be other reasons for doing this. Battery redundancy for one. If one goes bad, you can remove it from the circuit and rely on the 2nd until the 1st gets replaced. Also, it seems the alternator would work less, and thereby increase gas mileage. (The idea here is that it's easier for an alternator to charge two batteries up "a little bit" over a long time, than to have to recharge one batterey "a lot" over a short time. Seems reasonable to me, am I right or wrong ?

Plus, you are less likely to be stuck in the middle of nowhere with a dead battery, as it will take twice as long to completely go dead.

Anyways these are my thoughts. I plan to do this unless someone has reason to believe it is a bad idea. ANy thoughts appreciated.

 

You can hook two batteries together, but it has some disadvantages.

If one battery goes dead for some internal reason, the other battery will be pulled down with it. They are hooked together, so their performance is tied together. If you have one good battery you just replaced, and still have one older battery, the new battery will only perform as well as the old battery. This includes charging too.

You are correct that if you have a drain, it will take longer to run the batteries down when hooked together, but the possiblity exists that you will drain them both enough to where neither one has enough oomph to start the truck.

The best way to do it, especially for the purposes you have described is with a battery isolator. All this thing amounts to is a large diode that is put in series with the charging wire from the alternator. What happens is you don't hook the batteries together, except through the isolator charging circuit. And then you seperate your loads according to what you want to power with the truck off.

What this will do is let the main starting battery sit there and it's only job is to start the truck, and power the rest of the truck as normal. Your extra battery gets charged through the isolator anytime the truck is running, but any loads connected to the aux battery will not drain the main starting battery. What you can also do is buy a deep cycle marine type battery for the aux battery. This way you can drain it way down without damaging it. A regular car battery will be damaged if it is drained way down repeatedly.

And no, you do not get a free lunch by hooking batteries together(less strain on the alternator). You are using a certain amount of energy to start the truck, no matter if you have two batteries or one. So this energy always has to be made up, no matter if it's very quickly with a lot of load on the alternator, or over a longer period of time with less load on the alternator.

 

Dave? Franklin ?

After I posted this, I ran some searches on "Dual Battery System" and found a bunch of aftermarket systems designed to do what you describe. Since the thing IS effectively a "big diode", why are they charging @ $320.00 for the kit ?

Could I build one myself ? Are there places to get schematics ? I like the idea, but not enough to drop $320.00; I could better spend that money somewhere else.

What are the possible problems with simply running the two batteries in parallel ? Seems that they would (individually) work less. And, as you say that automotive batteries are harmed by repeatedly being run-down, seems that the two should last longer collectively than they would consecutively.

Also, I do get your point regarding the "free-lunch" as far as the principal of conservation of energy is concerned, but what about "wear and tear" ? Surely it's got to be better for the alternator to output say (I have no idea what a reasonable number might be) a constant "3 Amps" over a long period of time compared to having to labor over an output of "6 Amps" on an occasional, yet regular basis.

But overall, given your obvious expertise, what is the viability of this proposal as far as it's cost to benefit ratio ?

A few hours labor, a couple cables and an extra battery. Does this seem a reasonable investment ?

 

Here is a GREAT URL...

http://fourwheeler.com/techarticles/129_0401_ford/

Hope this helps...

 

That link is a great "store bought" item.
Plan "B".....
Get a battery tray from the wreckers out of the correct year group truck.
Buy a new starter solenoid. Hook the solenoid into the positive cable joining the 2 batteries. Install a lighted switch in the cab & rob power from any point that only operates with the key in the run position to trigger the solenoid with.
It's a cheap way of doing the job, but it still works.
AL.

 

That is why I like FTE!!! GREAT imagination Dealford! I like that Plan "B".

 

JCWhitney has them from $21.00 to $120.00. And you hook it up and forget it. Go to their site and type in "isolator" in the search box.

You can do the solenoid trick, but you will have to think about turning the switch on and off. Forget to turn the switch on, and the aux battery will not be charged(you need to charge the aux battery on a regular basis, even if you are not using it). Forget to turn it off, and the main battery will be discharged along with the aux battery.

And you also run the risk of overheating something if the aux battery is really dead, and you flip the solenoid on. The main battery is going to want to equalize with the run down aux battery right away, and it has enough current output to possibly make things interesting(smoking wires).

There are many ways to do it. I think the isolator is a cheap, slick solution.

 

This is exactly what I was looking for as well!!! Great thread!!

 

I thought a "Solenoid" was an electro-mechanical device that throws a gear out when power is applied. I don't understand what it is doing here.

 

Technically you are correct, but mechanics sometimes use the word solenoid to mean "starter relay" which is really the correct term.

 

So how do I apply this suggestion to my truck? Where would I find the "Starter Relay" ?

What function does it do ? I assume it is an electrically activated switch. How does this fit into the overall plan of installing a second battery. Am I switching from one battery to another, or switching the second battery in and out of the circuit ?

I understood your description of the "large diode" allowing a charge to go into the battery as it was available, but preventing a discharge from going in the other direction. I assumed the idea was to drive the accessories from the second battery, keep the first dedicated only to running the factory electrical equipment and the diode's purpose was to allow both batteries to re-charge when the alternator was producing excess power.

Did I get this right ?

If this IS right, I don't see how to translate the operation of a Relay into this idea. The two concepts seem very different to me, and I don't understand the intent of the second. Could someone please clarify ?

 

Always use a battery isolator in the charging circuit. Use a continuous duty solenoid/relay available from RV dealers, not a starter solenoid. A starter solenoid will burn up. Some Parts stores may have continuous duty relays also.

Batteries become mismatched in service and one battery will discharge into the other one which will eventually destroy the batteries (it's a Chemistry thing). They can not be connected in parallel.

There are many threads here on this subject.

 

Yep, you got it right. And the take the diode out of the circuit, and replace it with a very large relay and you have the other solenoid/relay concept. Turn the relay on, both batteries are connected. Turn the relay off, the batteries are seperate.

 

I was looking at the JC Whitney Isolators they seem to be the cat's a$$, and a sight better than $300 that is shown from the article, but no explanation from JC Whitney on how to wire the batteries, or even if they come with wiring instructions. Plus the question of the switch into the cab to change from one to both batteries etc

 

Here's a diagram they had if you clicked "info buy".




You would not need a switch unless you wanted one with one of the relays JCWhitney sells for that purpose. You can make it as simple as the diagram above, or as complicated as you want, depending on what the desired purpose is.