wants to move the battery to the back, good idea?

[El Jefe]

I'm thinking I want to move my battery. Any suggestions on this? what gauge wire would I need? should I upgrade my altenator? I was planning on getting one of them CS144's.

Thanks in advance.

[FSJ Guy]

I have thought about this, too. The cabling is what will be expensive. You'll have to run two cables (at least) to the remote battery. $$$

[Eric S.]

I know why you would want to move the battery to the back of a sports car to improve weight distribution. But why do you want to do this on something this big? I'm interested.

[El Jefe]

I'm thinking bout it because I got rid of my inner fenders. So the battery in the back would look cleaner.

I think I can get the cable from work. Not sure yet. Would have to ask my boss.

[Joe Guilbeau]

Bad idea, too much IR loss in the cables, now you have to run another cable for the Alternator, Bad BAD BAAADDDDD idea

[247pia]

Originally Posted by Joe Guilbeau

Bad idea, too much IR loss in the cables, now you have to run another cable for the Alternator, Bad BAD BAAADDDDD idea

what is IR? I know I = intensity and R = resistance

enlighten me wise one.

Oh wait am I dumb? Internal resistance?

[Walt]

Originally Posted by Joe Guilbeau

Bad idea, too much IR loss in the cables, now you have to run another cable for the Alternator, Bad BAD BAAADDDDD idea

Wow, that's a pretty strong opinion...
There are plenty of cars that run their batteries in the trunk from the factory. There's one sitting in my driveway right now. Somehow it manages to crank over every time I turn the key...
Heavy battery cable is expensive (0 or 00), but that's all that's required to get past any potential voltage drop issues.

[Walt]

V(oltage)=I(Current)xR(esistance). The smaller the cable, the larger the R(esistance), the greater the voltage drop between the battery and the starter. R is a function of the resistivity of the cable x the length of the circuit.

[247pia]

Originally Posted by Walt

V(oltage)=I(Current)xR(esistance)

I know that but he said too much IR. Don't get it.

[Walt]

Implying that the voltage drop (IR) will be excessive

[Tripwire]

If ya want to run without fenders - ok- just tons of crapola inside the engine compartment all the time....

I have relocated my battery on a few vehicles....you are gonna want a LARGE - the biggest you can afford- ( as big as your thumb) welding cable to reach to the back. Get as many "stranded" numbers as possible as more individual strands can carry more current when all added up.

You only "need" one cable - for the positive. The battery in the back can ground to the frame....be sure to use the same size cable and killer big terminals for this.....IMHO i would go to a marine shop and have them professionally terminated and heat shrunk to avoid future issues.

The battery box should be a marine /automotive grade one, and since its inside i would buy one with a vent to vent out farty battery gasses

the box base and the battery base WILL need to be bolted to the floor as you dont want to end up killed by a hit in the head or disfigured by battery acid ----or even both---in a accident or rollover

[Joe Guilbeau]

Originally Posted by 247pia

what is IR? I know I = intensity and R = resistance

enlighten me wise one.

Oh wait am I dumb? Internal resistance?

E = Voltage
I = Currrent
R = Resistance

I x R = E

[247pia]

Come on Joe

E=electromotive force measured in volts
I=intensity which is current measured in ampere's
R=resistance measured in ohm's

[Joe Guilbeau]

Originally Posted by Walt

Wow, that's a pretty strong opinion...
There are plenty of cars that run their batteries in the trunk from the factory. There's one sitting in my driveway right now. Somehow it manages to crank over every time I turn the key...
Heavy battery cable is expensive (0 or 00), but that's all that's required to get past any potential voltage drop issues.

0 awg wire is more than 1/3 - inch in diameter for one thing.

Second is that for the alterntor to charge the battery propery, the charge wire from the alternator should be in close proximity to the positive battery post.

How are you protecting the cable runs? With a fusible link or a circuit breaker or ANL Fuse?

Those large battery wires will need to interface with some circuit protection in some manner.

Alternators are not designed to supply your vehicles load, they are designed to maintain the batteries charge, and there is a difference between the two.

If the sense wire to the alternator's regulator is not placed at the right point with a battery mounted at the rear of the vehicle, then the alternator will take on the job of supplying the vehicles load.

When this happens, the alternator's output is at max voltage, and thus will lessen the life of many, if not all batteries.

If the vehicle comes from the factory with a battery in the rear, then the engineers who designed the electrical systems of that vehicle did their part and designed it right, our Jeeps were not designed that way, and thus we must overcome this.

I am interested in learning how you overcame some of these challenges.

[Joe Guilbeau]

Originally Posted by Tripwire

If ya want to run without fenders - ok- just tons of crapola inside the engine compartment all the time....

I have relocated my battery on a few vehicles....you are gonna want a LARGE - the biggest you can afford- ( as big as your thumb) welding cable to reach to the back. Get as many "stranded" numbers as possible as more individual strands can carry more current when all added up.

You only "need" one cable - for the positive. The battery in the back can ground to the frame....be sure to use the same size cable and killer big terminals for this.....IMHO i would go to a marine shop and have them professionally terminated and heat shrunk to avoid future issues.

The battery box should be a marine /automotive grade one, and since its inside i would buy one with a vent to vent out farty battery gasses

the box base and the battery base WILL need to be bolted to the floor as you dont want to end up killed by a hit in the head or disfigured by battery acid ----or even both---in a accident or rollover

And how did you run the alternator wire to charge the battery and to sense the voltage at the battery?

An additional isse is the Starter and if you supplied a battery to just start the vehicle, as a long run from the back of the vehicle to the starter is also an issue, not so much with small vehicles as the starter current draw is not so much as with our vehicles.

[Joe Guilbeau]

Originally Posted by 247pia

I know that but he said too much IR. Don't get it.

All wire have resistance, and the current that runs through a wire will develop a voltage, this is the IR voltage drop that is lost in the cable and is turned directly into heat.

The reason for the suggestion to use as many strands of copper wire as you can find on a suitable large cable 0-awg or larger is that current is actually carried on the outer surface of the single conductor of wire, and the more strands that are available the larger the current that will be delivered to the load (battery in this case).

The main reason for many strands is that it is flexible, and the appropriate bend radius may be realized.

You will still have to find a way to deliver the alternator charge voltage to the battery, otherwise with a single wire to the positive battery post, you must overcome the IR loss of the battery cable and you are having the alternator send current to the battery with that same battery sending current to the loads in the vehicle.

This, if effect forces the alternator to supply the loads in the vehicle, and it is not designed to do this. The alternator is designed to maintain the charge on a fully charged battery.

Thus it will be charging almost all of the time, and outgassing of a lead acid battery will occur, as well as premature failure of the battery due to the charge being applied to it as well as the alternator being constantly on.

Under light loads this is not so much of an issue, when the loads approach 100 amps or more it becomes something that must be addressed.

My reasoning is that if this is just to avoid seeing the battery, it is a BAD, BAD, idea...

Get a spray can and paint said battery

[Joe Guilbeau]

Originally Posted by Joe Guilbeau

All wire have resistance, and the current that runs through a wire will develop a voltage, this is the IR voltage drop that is lost in the cable and is turned directly into heat.

The reason for the suggestion to use as many strands of copper wire as you can find on a suitable large cable 0-awg or larger is that current is actually carried on the outer surface of the single conductor of wire, and the more strands that are available the larger the current that will be delivered to the load (battery in this case).

The main reason for many strands is that it is flexible, and the appropriate bend radius may be realized.

You will still have to find a way to deliver the alternator charge voltage to the battery, otherwise with a single wire to the positive battery post, you must overcome the IR loss of the battery cable and you are having the alternator send current to the battery with that same battery sending current to the loads in the vehicle.

This, if effect forces the alternator to supply the loads in the vehicle, and it is not designed to do this. The alternator is designed to maintain the charge on a fully charged battery.

Thus it will be charging almost all of the time, and outgassing of a lead acid battery will occur, as well as premature failure of the battery due to the charge being applied to it as well as the alternator being constantly on.

Under light loads this is not so much of an issue, when the loads approach 100 amps or more it becomes something that must be addressed.

My reasoning is that if this is just to avoid seeing the battery, it is a BAD, BAD, idea...

Get a spray can and paint said battery

A final note is that when a Lead Acid battery blows it's top due to outgassing and a spark, it is not a pretty sight.

What is particularly funny is those vehicle without key for the trunks or if you loose your key to the trunk, and the battery goes dead... how funny is that!

Or if the lead acid battery in the trunk has a shorted cell, then that alternator will work overtime supplying the battery load from a battery that will NEVER charge right, and the other vehicle loads placed that the battery would normally supply and the alternator must now supply.

I guess I could go on and on...

[Tripwire]

Joe hit it on the head with the description of the strands - I coulda have elaborated more

the way it was "splained" to me in univeristy is that the electrons actually flow over the surface of the individual conductors and not thru the middle of the conductor. Its kind of like I5 thru downtown LA...you can have a single lane road thats a hundred feet thick to carry the cars - or you can have 5 lanes thats 20 feet thick....

also as joe and I said - outgassing is a serious issue and it must be adressed if you are driving a street rig....off road and no windows - no issue

[Joe Guilbeau]

Guys,

I am not aying that it cannot be done, car audio folks do this for a living and have some pretty spectacular designs to boot.

Here is a response to someone asking me why I say that the alternator is not desinged to supply the vehicles load, after all, they reasoned; that the alternators amperage rating is has to be taken into account when purchasing.

While it is correct to theorize that the alternator must have enough amperage to supply the battery and all other electrical devices that require voltage and current in a vehicle as loads, it is the batteries prime directive to supply loads in the vehicle.

The primary directive of the alternator is to top off the battery, this is why a vehicle may run with an alternator disconnected or no longer working.

The alternator’s primary job is to keep the battery charged to a potential of 14.4Vdc, which is the optimal charge of a 6-celled 12Vdc car battery. Various regulators do this by regulating the current through rotor coil windings, by either a pulse width modulation of a square wave at 400Hz to supply voltage to the rotor field windings or simply by turning that voltage on/off to start the charge or stop the charge process.

While it is true that the alternator cannot self-start and needs a supply of voltage to push current into the Rotor coil windings, it is the batteries job to supply this load to get the alternator up and going. Once the alternator is spinning and supplied with current and voltage it can sustain itself.

Notice that the alternator cannot function properly with either a shorted battery or a battery that is no longer in the circuit. The other loads in the vehicle have not changed, but the primary supply for all of the loads in the vehicle certainly has, and in this particular case the alternator cannot start or supply lights or horns or radio.

The alternator is a load for the battery until it is spinning and generating voltage on its own. Once it is producing it only sees the battery as a load, and not the other device in the vehicle that require a voltage with sufficient current to operate.

This is the drawn out explanation of why I made that statement.

Consider the ammeter in most early vehicles. The ammeter display was negative amps when the starter was engaged, then to positive amps as the battery took a charge to replace the depleted charge that the starter took out. The output of the alternator went to the ammeter and a splice that carried it to the fuse block and other devices. The shunt bar on the ammeter carried the voltage and current flowing back to the battery thru a fusible link or fuse and then to the battery.

This amp gauge was a battery charge indicator and not an indicator of how much current the alternator was delivering at its output. One may notice that it would indicate negative amperage when the vehicle started and immediately swing to the positive amperage to show that the alternator was replenishing the batteries depleted energy reserves that the starter pulled.

The alternator has no idea what the loads of the vehicle require; its sole purpose is to keep the potential of the battery to within 1Vdc of 14.4Vdc. It regulates the voltage of the battery and has no idea of the current loads of the electrical system that the battery supplies.

Therefore, it is the battery job to supply loads in the vehicle, while the alternator job is to supply voltage to the only load that it senses, which in “our Jeeps” is the battery.

Recall that in some early alternators, the alternator would not begin to charge until the shaft rpm of the alternator reaches some specified threshold, and therefore it was incapable of supplying any load in the vehicle. Thus, it is not the job of the alternator to supply loads in the vehicle, although it certainly does do this by maintaining an appropriate charge to the battery.

Think of it in this manner, could an alternator that is rated for 65 amps supply the necessary current to start our Jeeps? I think the answer is no to that question. The battery is there to supply as much current as the load requires, the alternator cannot possibly be tasked with this load, unless it is rated for that 300 amp charge and whatever else the vehicle loads place upon it. To be sure, it will severely lessen the life of an alternator if it is require to supply starter current, in lieu of the battery.

When the engine is not running the alternator cannot supply loads, the battery can and performs this function, as it is the primary means of supplying loads.

The alternator is not a battery charger; it is a battery maintenance device. This is a bit of a fine distinction and somewhat elusive to grasp. Think of the voltage regulator of the alternator as a voltmeter used by the alternator to measure the battery voltage.

From a battery manufactures viewpoint, their batteries are only there to supply the starter load and the get enough current and voltage to the alternator so that when the rpm threshold of it’s specific design is met, then it “takes over” and supplies the loads.

While this is true, I have indicated that the alternator has no idea of the loads in the vehicle and cannot possibly be configured to sense them, it only senses voltage of the battery and then only if the voltage on the battery is within 1Vdc (plus or minus) of 14.4Vdc.

For instance, if you placed a separate battery of 15Vdc to the sense wire of a vehicles battery and disconnected the original sense wire then the battery will run itself down and you will experience a dead battery, as the alternator thinks that the battery is at 15Vdc and it will not supply the rotor with the required voltage and current in order to produce a charge.

What we are focusing on is the fact that the battery is used to supplement the alternator while the alternator’s output is insufficient to supply the required loads that are imposed upon the electrical system. From the battery standpoint this is, in fact the case. A large capacity battery can better withstand the load requirement than a small capacity battery.

Thus in the above paragraph, the battery will attempt to supply any load that exceeds the current charging systems capacity. Thus, it also acts as a filter or “shock absorber” for the charging system. The battery size should be selected in order to supply loads of the vehicle when the charging system is no longer working. This is referred to as the Reserve Capacity of the battery, and thus the battery is the primary load-supplying device in a vehicle.

The amp hour rating of a battery is a measurement of how long a battery can supply a current for 20 hours at 80-degrees F, before the cell voltages drops below 1.75Vdc (10.5Vdc for a 12 Vdc car battery). So five amps for 20 hours is 100 amp-hour rated.

This brings us to optimum charging rates for vehicle batteries, which happens to be 1/10th of the amp-hour rating. In our above example, 1/10th of a 100 amp-hour battery is 10 amps.

Now, consider that an alternator’s output can vary from 34 amps to 300 or so amps. How is this good for the battery, if we wish to maintain an optimal slow charge? It is all a game of trade-offs and compromising to achieve best overall results at the cheapest price possible.

In our vehicles, a slow trickle charge will not work out very well when we attempt to start the vehicle 10 times in short 1-minute drive times. We would deplete the batteries reserve capacity and the alternator would not be able to charge the vehicle in time for the eighth or ninth start (using a low reserve battery and limited amp-hour rating and low Cold Cranking Amps CCA rating of a battery to illustrate this point).

Therefore, the bottom line is that it is true that the alternator does not supply loads of the vehicle once a specified rpm threshold is overcome, it supplies the battery with a maintenance float charge. Keep in mind, though; the alternator can only do this when higher rpm rates are met. This it cannot be the primary supply for loads, this is the battery job, and the battery can be placed in a charging mode when these rpm thresholds are met.

Also, consider the case when an alternator’s voltage regulator fails and overcharges a battery, the voltage can go as high as 16.5Vdc, which might damage some sensitive electrical loads in the vehicle when it spikes. A battery could never do this, although it could short or deplete its charge.

I have owned several Full Size Jeeps and my trade is as an Electronic Technician, trained in the Navy and 2 year technical degree. Navy training was in 1974, started my first job in 1978 in electronics.

[will e]

Ok, I understand most of what Joe said as he does a good job of explaining it.

I run two batteries, one at each fender. I took the positive cable from the second battery and fed it into power steering return hose to help protect it in case I get into an accident and it somehow gets hit. This isn't fool proof by any means but it does add a level of protection against the cable getting cut and basically becoming a spark machine as it tries to weld itself to the nearest ground (near gas lines, carbs, fuel filters and who knows what else could be a source of fire).

So, yeah, one draw back of having it run the entire length of the truck is if it gets cut and this will most likely happen during some violent confrentation of some other object and that may result in fuel or whatever being leaked.

Maybe I am a little paranoid.

If you do this, put the cable in some rubber hose. Make sure it is mounted secure but able to 'move' a little and with some slack that can be taken up. A quick disconnect or kill switch would be even better.