The two batteries are for higher cranking amps, which are needed for the high resistance load that a diesel engine requires during starting. (it takes alot more power to turn it over) The compression in a diesel engine is a hair over 17:1 compared to a gas engine of around 9:1.
The voltage system in 99.9% of regular production vehicles is a 12 volt system. The batteries in your truck are wired in paralell, whereby the positive terminal of each battery are hooked to one another as well as the negative terminals. This electrical configuration is called paralell.
If you hooked them in series the voltage would be additive and would double the current (amps). For example four batteries in series would produce a rated voltage of 48 volts and produce a quadruple current flow. Dont ever try this, you will fry every sensitive electrical component in the system.

Bret

 

Thanks Bret. That makes sense

 

Don't forget the glow plugs also use a lot of juice before the engine is ever turned over.

 

So to reset the ECU, do we need to remove the positive cable from both batteries or just one?

 

Bret In a parallel connection you double the amps, but the voltage stays the same. In a series connection you double the voltage and the amps stay the same.

DO NOT ever hook up a series system on your truck and be careful of hook ups if you jump start yours or others vehicles or you will burn out well over a $1,000 wort of electronics including your sound system and maybe some wiring harnesses.

We have a very large amp draw on these trucks, especially when cold. the glow plugs stay on after the wait to start light goes off and the starter on most diesel trucks is a reduction gear type + the compression ratio as mentioned before make for a tremendous load on the batteries. In the summer a PSD will usually start with one weak battery, but when the first cold snap comes through and if it seems to turn over slowly, you best get the batteries checked out or look forward to a no start situation.

My first diesel was a 80 Toyota 4 banger and it had two batteries.

Take care /ubbthreads/images/graemlins/crazy.gif - Ed

 

Disconnect the lead on the battery that is BEING fed from the second and/or the battery that has the leads going to the starter and power distribution unit.
I currently dont own a PSD, but I would have the see the actual setup to tell you specifically.

Bret

 

You are doubeling the capacity (cranking amps). Cranking amps is a term used in the battery industry for battery capacity. It's based on the total surface area of the plates. Don't confuse that with actual current flow outside the powerscource (battery)
There is a formula to remember that electricity is based upon. It's called Ohm's Law: E=IR whereas your electromotive force (E) in volts equals the current (I), in amps times the resistance (R) in Ohms.
So, if you have a 12 volt battery in a circuit with 6 ohms of resistance, the current will be 2 amps.

E=IR
12v = 2 amps times 6 ohms (two times six equals twelve)

If you had two batteries in series the voltage would be 24 and the current would double assuming the load (resistance) is the same.

E=IR
24v = 4 amps times 6 ohms

For our application on the PSD, the batteries are not in series, they are in paralell where the voltage is still 12 volts.

When you put batteries in parallell the volatage stays the same, but the available power is increased because there is double the lead -acid plates to provide power.

An application where batteries are in series would be a common 4 battery cell tubular flashlight like a maglight since the positive end of one leads into the negative of another. If you have four 1.5 volt batteries, the total battery supply is 6 volts for the load, which is the combined resistance of the light bulb and the return ground of the metal case.

 

Isn't that basically what the below says?

"Bret In a parallel connection you double the amps, but the voltage stays the same."

When I was in the Navy years ago I spent my first six months on a APA (USS LENEWEE) in the battery locker. We had peter and mike landing craft and they had Gray Marine diesels. Four six volt batteries in series thusly a 24 volt system. Yep went through EM A school and I know what you are presenting.

/ubbthreads/images/graemlins/crazy.gif - Ed

 

[ QUOTE ]
So to reset the ECU, do we need to remove the positive cable from both batteries or just one?

[/ QUOTE ]

My dealer disconnects both negative terminals. Ford also had him turn on the headlight switch, and leave it like that for 30 minutes before re-connecting. I have done this procedure myself. It works.

 

What's the correct procedure for boosting?

 

Well... what he tried (and failed) to explain was why two batteries in parallel make more power. And it's not (entirely) do to plate area.

You see, E=IR has a few other applications. The circuit has resistance and the batteries have resistance. You don't normally talk about the resistance of a battery unless you have an incredibly high draw. But batteries do have resistance.

When you start your truck, you are going to draw hundreds of amps. Each of the eight glow plugs take power (20A or 40A each, I seem to recall ... they have to go from 0F (cold morning) to 1000F (heating the cylinder) in a few seconds) and then while they are still running, the starter has to crank. I know that gas engine starters are 40-ish-Amps, so I suspect that this truck has an 80 or 100 amp starter.

So while you're trying to draw this power, you run into the limitation of drawing that power at 12V ... and drawing that power from the battery. As you draw more power from the battery, the voltage it offers lowers due to it's internal resistance.

So you can view the two battery situation in several ways. One way to view it is that two batteries in parallel offer less resistance to the circuit. This is the same as larger plate areas in the battery offer less resistance. (Two resistances "n" in parallel offer n/2 resistance.)

In fact you can understand it in whatever way you like. That's how E=IR fits into the situation. People probably tried starting diesels with one battery for awhile. After that got tiresome, they put in two. In fact, on effect is likely that the two batteries allow the system to work further down the age scale of the battery ... such that you might even get more lifetime out of two batteries together than you would out of two single batteries.

 

I agree with you. Heck in time the plates sulfate and this adds resistance. Some of the lead sulfate falls into the bottom of the case and the batteries keep getting weaker you get a dead cell. The charging system forces the led sulfate back into an ionic solution and the process keeps cycling until they fail. You can have an internal connection come loose and that causes it to fail.

All of that is why when I need to replace a batter from age I replace them both wit the highest rated ones that will fit.

Heck we do not have but a bout 3 or 4 companies that manufacture batteries in this country now.

Johnson Controls, make the most , Champion, Optra, and I forget the rest.

/ubbthreads/images/graemlins/crazy.gif - Ed

 

Pull both positive cables
DENNY

 

There's a reason for disconnecting the negative terminals first.... if the wrench slips and contacts the fender or such, if we are n the negative, nothing will happen. If we attack the positive terminals first and the wrench slips.......

We would first disconnect both negative terminals... it doesn't matter which one is first. Then, once the coachwork and mechanicals of the truck are no longer a ground path, we can disconnect the positives with little fear of a light show or worse.

 

[ QUOTE ]
Don't forget the glow plugs also use a lot of juice before the engine is ever turned over.

[/ QUOTE ]

And they use that lot of juice just when the batteries are reduced in cranking capacity, when it's very cold. This is one of the major reasons we have 2 batteries. So when it's very cold the batteries are at their worst, the glow plugs are calling for a bunch of current AND the oil is very thick(er) creating a lot of additional drag(more amps)

 

"Then, once the coachwork and mechanicals of the truck are no longer a ground path, we can disconnect the positives with little fear of a light show or worse."


A high tech version of the "Northern Lights" ?? /ubbthreads/images/graemlins/eek.gif /ubbthreads/images/graemlins/eek.gif /ubbthreads/images/graemlins/eek.gif
Ian /ubbthreads/images/graemlins/biggrin.gif

 

All the electrical equations are good, but here's the simpler way I look at it: Ford would love to put a 1600-2000AH battery in our trucks, but they don't make them! So they put two 800-1000AH batteries in instead. It's a lot cheaper to put in two fairly standard batteries than to go get a battery company to make some custom monster battery.

Having gained some experience now with a single-battery diesel ('82 Mercedes) I am looking at some way to add a second battery. Which, like all good cold-weather-performance-related projects, has to wait until warm weather, at which point I'll forget about it until next winter! (The glow plugs in the '97 were like that, until the next to last one died and I had to do the replacement in 10 degree weather, argh.)

Duncan

 

V=IxR, I=V/R, R=V/I, P=IxV, I=P/V, V=P/I, ELI the ICE man........

You have two batteries because the electrical load at start up requires it. You should be happy, you have one more battery then those girly gas powered trucks! /ubbthreads/images/graemlins/biggrin.gif

 

[ QUOTE ]
V=IxR, I=V/R, R=V/I, P=IxV, I=P/V, V=P/I, ELI the ICE man........

[/ QUOTE ]

We apologise for posting anyting too technical for you to comprehend.

 

Actually, it can be a little more "graphic"..... I admit to trying the odd shortcut, now and then.... A misguided wrench can "clamp" itself to two very bad objects and start to grow very warm to the touch in a very short time. It's bad enough when you think "I just know I'm going to regret this..."..... I can only imagine what it will seem like to either the neophyte or (ever so prevalent) "Sooper DIYer".

 

Not to be combative, but I do feel we need to make a point.... While I do drive a "girly" truck.. I do it with great aplomb and I do it with a lot of wisdom..... a LOT of wisdom. I live in a climate that is nothing less than cold for several months of the year. I commute. I run empty or very lightly loaded 99% of the time. I make many short trips. If I owned the truck of my dreams, I would have no end to concerns with it. My driving style and conditions precludes a diesel.... I cannot drive it the way it needs to be driven...... <I could repeat that in capitals>....

Might be nice to drive a diesel... but if we can't use it the way it needs......

 

The charging system is also disable for the two minutes after the truck is started while the glow plugs are active. Only the batteries are running the injectors, headlights, glow plugs, trans etc....

Repeated cold starts would kill a single in a hurry.

Also, one battery is adequate but it essentially would be "deep cycled" every time you started the truck negating any kind of battery life.

 

Just thought I'd keep the ball rolling here... The only thing missing from all the theory here is why we use I for current (you had 2 out of 3 Brett!)... "I" is for the Intensity of the EMF. "ELI the ICE man" brought back a flood of acronyms best forgotten! here's my .02... Bad Boys Race Our Young Girls Behind Victory Garden Walls- Get Started Now. There's a "dirtier" version too, but not right for this forum!

AE "A" school
BS Aero Eng
A&P and FCC
...and now I'm just a Stick Monkey /ubbthreads/images/graemlins/biggrin.gif

 

[ QUOTE ]
Just thought I'd keep the ball rolling here... The only thing missing from all the theory here is why we use I for current (you had 2 out of 3 Brett!)... "I" is for the Intensity of the EMF. "ELI the ICE man" brought back a flood of acronyms best forgotten! here's my .02... Bad Boys Race Our Young Girls Behind Victory Garden Walls- Get Started Now. There's a "dirtier" version too, but not right for this forum!

AE "A" school
BS Aero Eng
A&P and FCC
...and now I'm just a Stick Monkey /ubbthreads/images/graemlins/biggrin.gif

[/ QUOTE ]

Did you mean "Bad Boys Rape Our Young Girls, But Violet Gives Willingly" ?

For anyone wondering what that is, it's a saying to remember the color code chart for identifying color bands on resistors to determine the resistance value when a meter is not available.

 

/ubbthreads/images/graemlins/blush.gif LOL, he said it, not me!!