rear mounted starter batteries

[rockcrawler31]

Hi all

I've been thinking about rear mounting my starter batteries for several reasons. A. to clear a bit of engine bay space, B. better weight distribution and C. to have the batteries nearer a rear mount winch.

Is it as simple as running a heavy cable to an insulated pole in the engine bay and hooking up all the original loom cables/starter motor supply to that? Suggestions as to cable size over an approximate 3 meter run for a diesel with twin starter batts would be nice.

I'm guessing it would be better to have a second dedicated earth cable running along the positive rather than using a chassis earth or would that not be necessary. would earthing the batteries to the chassis and mounting a pole on the chassis in the engine bay be sufficient?

Is there any reason the charging of the batteries would be affected or less effective?

[rockcrawler31]

Also since i'd be running a massive cable along the chassis/body capable of carrying boggetyteen amps during a short, what's the best way to protect against short at the battery, given that any fuse would have to allow for cranking amps while not burning down the car if the cable rubbed through?

[chimpboy]

Basically yes to the first question. It is just a case of running a heavy cable to the engine bay. One way this is sometimes done is to run your new cable to the starter motor lug, which and then carry on to the original wiring from there.

AFAIK - forget a fuse, there is no good option for this, maybe someone else will suggest one but imho you just need to mount it in such a way that it cannot rub through.

On one car I had the battery was in the boot (factory setup), the cable ran through the trans tunnel. No problems at all. On a G60 patrol I had it ran from under the driver's seat under the car. In both cases it was to the starter motor lug. This makes sense as that is where you want the shortest run and least voltage loss. From there it ran up to the fuse box. Cable was slightly bigger than a typical battery cable IIRC, but I don't know what gauge it was. I think you would want 1 gauge or 0 gauge but maybe less would do bearing in mind that a starter motor could be pulling 100-200 amps (rough guess).

BTW I think there are some cars where it runs through the cabin instead, under the carpet.

[rockcrawler31]

Thanks chimp

It's more or less what i expected i just wanted to see if i had left something out of the process. I guess there's plenty of trucks around that have 24v going fromf multiple batteries behind the cab to the starter in front of the cab.

[chimpboy]

I missed the bit about the earth. In the vehicles I've seen with a rear-mounted battery standard the earth cable just went from the battery to the body or chassis. There was no earth cable running to the engine bay.

[Foo on patrol]

Don't forget, you will loose a certain amount of amperage through the distance involved. (take that into account)

[-Scott-]

Don't forget, you will loose a certain amount of amperage through the distance involved. (take that into account)

Sort of.

The reduction in current is due to voltage drop, which is proportional to both length and cable size. Increasing cable length will increase voltage drop, so there's less voltage at the starter, so the starter draws less current.

A larger cable can offset the longer length, and restore the voltage drop. Theoretically, increasing the size of the cable by enough can restore current to it's original level. In reality, it's unlikely to be practical to increase the cable sufficiently to restore voltage drop completely, but bigger is generally better. Which Milo probably already understands.

[CanberraMav]

This is exactly the question i wanted to ask.

How did you go with this? Any pictures etc.

What method did you end up using?

Cheers

[rockcrawler31]

I probably won't get it done for a while yet, but there's plenty of advantages to having the batts up the back if you've got the room for it.

[Clanky]

You will want to consider the installation carefully, double insulate the cable and fit grommets etc so you make sure there will be no chaffing and rub through of the insulation. A short is effectively a welding cable and could be quite hazardous - fire & smoke (potentially deadly inside if its PVC sheathed), plus other issues such as having the crap scared out of you.

[Clanky]

The reduction in current is due to voltage drop, which is proportional to both length and cable size. Increasing cable length will increase voltage drop, so there's less voltage at the starter, so the starter draws less current.

Hmmm in confused by this statement in relation to an electric motor - a fixed resistance agreed.
A stalled motor is effectively a short due to its low impedance, and the current reduces as the motor spins to its speed. IE inrush current of inductors.
If the motor is rated at 'x' power and the voltage reduced would you not see a higher current as the motor is effectively not producing its max power thus its not achieving its speed and hence higher current draw due to the lower impedance? Of course that assumes that theoretically the battery can deliver the power in the first place.
I am thinking for example:
i=p/e
i=12w / 12V = 1amp
i=12w/10V = 1.2amp

That sound right? Im a little tired so may have not worded that right


P.S Not having a go, just thinking out loud

[-Scott-]

The reduction in current is due to voltage drop, which is proportional to both length and cable size. Increasing cable length will increase voltage drop, so there's less voltage at the starter, so the starter draws less current.

Hmmm in confused by this statement in relation to an electric motor - a fixed resistance agreed.
A stalled motor is effectively a short due to its low impedance, and the current reduces as the motor spins to its speed. IE inrush current of inductors.
If the motor is rated at 'x' power and the voltage reduced would you not see a higher current as the motor is effectively not producing its max power thus its not achieving its speed and hence higher current draw due to the lower impedance? Of course that assumes that theoretically the battery can deliver the power in the first place.
I am thinking for example:
i=p/e
i=12w / 12V = 1amp
i=12w/10V = 1.2amp

That sound right? Im a little tired so may have not worded that right


P.S Not having a go, just thinking out loud

Good pickup. Yes, normally a DC motor at lower speed will draw more current, not less - if the voltage is constant.

We're not talking constant voltage - at the motor. Because of the longer cable run, more voltage is lost in the cables, and there's less voltage at the motor, so the motor draws less current AND turns slower (given the same load).

Yes, the motor has a low impedance, which is what determines the stall current. But the relationship between current & speed (at constant voltage) isn't set by the power rating of the motor.

The rotor impedance is the same. It never changes. What changes is the "back EMF". The spinning rotor is literally a generator, which is generating a voltage that fights the voltage that is applied at the motor terminals. The faster the motor spins, the generator voltage increases, so there's less voltage effectively spinning the motor - and it's this voltage across the rotor impedance that determines the motor current (from memory...)

If we have less voltage available to the motor, but any given rotor speed generates the same "back EMF", then the motor can't draw the same current - because there's less voltage remaining across the fixed rotor resistance.

If you're mathematically inclined, Power = volts x amps = torque x rpm (ignoring losses). If we approximate that starting the engine requires the same torque regardless of cranking speed, and we know we have less voltage and less current, then we must have less rpm.

I think. Feel free to discuss.

[Clanky]

Hehe, yeah thats makes sense
In essence the impedance of the extra cable (causing the voltage drop) will be added to the motor impedance thus in effect the system as a whole is slightly higher in total impedance. Therefore less amps

Yay.....I knew that

[Tiny]

My 73 has the battery mounted in the rear cranking. 6.2 chev, HD pos and neg cables

[CanberraMav]

Do you have to run the charge wire from the alternator all the way back to the battery or can it go to the starter pole or other?

Sorry im electrically retarded

[Clanky]

Do you have to run the charge wire from the alternator all the way back to the battery or can it go to the starter pole or other?

Sorry im electrically retarded

Starter pole is as good as any. Also best to hook up to the big neg bolted to the engine too

[-Scott-]

Do you have to run the charge wire from the alternator all the way back to the battery or can it go to the starter pole or other?

Sorry im electrically retarded

Starter pole is as good as any. Also best to hook up to the big neg bolted to the engine too

It depends on how everything is wired. The idea of the separate sense line is to let the regulator know what's happening at the battery, independent of any voltage drop due to charging current flow in the cables. If the cable between starter pole & battery isn't part of the charging circuit, that would be fine.

Having said that, as long as it's connected, you won't damage anything. Worst case, your battery won't charge quite as fast as it might with ideal placement of the sense line - for most people, they'd never notice the difference.

If the sense line becomes disconnected, the alternator can go to max output (trying to increase voltage to where the regulator wants it to be) and boil your battery. Install a voltmeter, and keep an eye on it.

[CanberraMav]

So what your saying is that its best to run the alt charge wire straight to the battery so the regulator knows whats going on?

[-Scott-]

So what your saying is that its best to run the alt charge wire straight to the battery so the regulator knows whats going on?

As far as I know, that is the most reliable way to ensure that the system works at its best. Depending on how the system is wired, the main wire to the starter may be a simple equivalent.

But, for most people, I don't believe it will make a huge difference where it's connected - as long as it's connected.

[Matt_85Lux]

What is better to use for the cable thin (fine) strand or thick (coarse) strand ( if that makes sense) cable?

[-Scott-]

Basically, the finer the individual strands in a wire, the more flexible it is and the better it resists vibration.

In an automotive application it probably reaches the stage where finer individual strands no longer offer any additional benefit.

Just about any "automotive" cable sold will use sufficiently fine strands - you only need to work out what the size really means for current carrying capacity. In the case of an alternator sense line, current is negligible, so use whatever you've got that you can terminate properly.

Don't use household mains wiring unless you really understand the previous stuff. Most household stuff (that goes in the walls/ceiling) isn't really "flexible" (despite being called "flex") and doesn't really like vibration.

[Matt_85Lux]

I just had both sitting here and wasn't sure which would be better to run to the starter, I might go with the finer stuff. Cheers

[De-lux]

MY setup in my 40 series..

750cca CAT battery above the rear wheel arch, super thick cabling earthing out on the chassis just below the battery, super thick cable that I managed to fit snugly in an old hose to use as insulation. As the thick + cable gets to the engine bay I have a 250amp ANL fuse before it gets to the starter.

The only pics I have are below...





You can see the big fuse and positive insulated cabling in the foregrount