Moving battery: cable sizing, grounding ?'s

Just for convenience, I would use welding cable. The kind used for Arc Welding.

As for grounding, it depends on how your vehicle chassis is assembled. If you can get wires long enough to reach from the engine to the trunk area, read the resistance of those leads. Subtract that test wire reading in ohms from what you get when you measure from the engine block to a non-painted area in the trunk compartment. If you have less than 1 ohm, after subtracting the DVM test leads resistance, you're good to go with grounding to the chassis in the trunk area. On an unpainted area. IOW, scrape/file down to bare metal, tap, put lugs on the B minus, and bolt on the cable.

STILL! ( this is important ) run the welding cable from the battery B minus up to the ENGINE BLOCK and bolt it on there, where it is probably already bolted on.

You want two grounds.

One back to the engine block

One to the chassis of the vehicle, if you can get under 1 ohm chassis resistance from the front of the vehicle to the back. This is possible if you have a Unibody welded chassis like I do. On an older vehicle, you can put ground straps ( braided cable shielding ) between the front end and the passenger compartment, and another strap or straps between the passenger compartment and the rear of the vehicle.

My ideas.

Lg

does the car going to have to start in cold weather? The starter is the #1 user of current from the battery and may draw a few hundred amps.

Do a google search for "AWG resistance" to get an idea of what the resistance per foot is for various wire thicknesses (gauge). Find out the resistance of the current wiring; then calculate the wire gauge needed to get the same resistance for a longer run.

On Sat, 04 Feb 2006 14:42:20 -0600, Lawrence Glickman wrote: ...

Unless you use a two dollar meter, the leads' resistance is going to be less than a half ohm. Most 3 1/2 digit DVMs only offer a 200.0 ohm range as the lowest, giving 0.1 ohm resolution.

Unless you have a three hundred dollar meter with a 4 wire probe (no current through the sense wires), you're going to have a tough time measuring the resistance of a battery cable. Most of the resistance is going to be in corrosion at the connectors. IIRC, a battery cable is typically 4 or 6 gauge wire. 6 gauge wire is .47ohms/1000', 4 gauge is 0.24ohms/1000'.

The existing + cable to starter is probably #6 and is 2' long. Since my new cable will be 7.5 times longer, to keep the voltage drop the same I will need to use cable that that 7.5 times less resistance per 1000 feet. That would be either 2/0 or 3/0 cable - massive stuff, heavy. So, I think I have to be prepared to accept some voltage drop, the only question is how much is acceptable.

If I was to not run a full sized ground from the battery forward, I would still run a bigger wire from the body over to the engine block.

Jim

AZ Nomad wrote:

You could always cheat and pass a known current through the conductor, then use Ohm's Law to calculate resistance... but as long as your voltage drop wasn't too horrible it wouldn't matter what the resistance is, now would it?.. Jim

Dunno about your battery, but my new auto battery weighs virtually nothing. And moving a battery to the back is percentage wise not much of a change in weight distribution. Or ... seems like a lot of trouble for nothing. Does remind me though, that my old VW bug had the battery under the rear seat ... but that was for practical reasons ... close to the alternator (in the rear) but not enough room in the engine compartment.

It depends on the vehicle. Is it bolted together or just spot welded like my Unibody. An older vehicle with a FRAME is likely to have more continuity problems than a Unibody like mine where everything is joined together as one piece.

Still, think of I=E/R, or better yet, P=(E)(I)

I move 260 AMPERES of current through my vehicle when I hit the starter motor.

And use 66 amperes Plus, when I'm winter driving with the lights and heaters on.

You don't need a lot of resistance to see that some places on the chassis can become *hot spots* in the literal sense if they produce resistance with that kind of current flowing through them.

That is why I recommended welding cable for the main battery terminal connections. I wouldn't try this *experiment* with anything less. But it ain't my car, so...

Lg

I have used

Vince has a pretty nice site, and you may find a lot of information there.

By the way, his cables were spectacular, and have the price of OEM:

It's not nothing.

20 pounds from the front to the back is a 40 pound "net" weight change. On a 3000 pound car, that's 1% of the weight.

Although, on a FWD wagon I don't see the point - for drag racing a FWD car, you WANT weight in the front. And, unless it's a Subaru wagon it's probably not being autoxed.

But it's the OP's car, he can do what he want with it. He can mount the battery on the roof if he wants. :)

If you're measuring voltage with a high impedance meter (even cheap meters are typically 10M), the voltage drop will be negligible.

Get a 0.01 ohm shunt, and wire up the battery cable to a car battery and a nice load like a headlight. Measure the voltage drop at the 0.01 ohm shunt and you'll know the current through the entire circuit. Measure the voltage drop across the battery cable and you'll be able to easily calculate it's resistance.

Yeah, it was because those pathetic excuses for cars had no heater to speak of, so if it was really cold, you just got someone heavy to sit in the back seat, pushing the seat springs into contact with the battery terminals. Presto: Instant heat, lots of it, and regardless of engine temperature.

Merde! I hate these autocorrecting routines. They were HALF the price of an OEM.

CAUTION.

It will work with big cables BUT be very careful of the installation/ INSULATION of the hot wire. It will be big and run a long way. If it ever wears thru the insulation the car will be gone before you smell the smoke. Because the starter draws large currents there is no practical fuse and a short of the hot wire to chassis will be deadly. When the battery is near the starter the unfused wire is very short and mostly in the air between the battery and the starter solenoid.

Your best bet will be running multiple wires. 1 to the starter, 1 to the fuse panel under the hood. depends on the year of your vehicle. Also you will need a good strong wire to the alternator since that is very very important for charging your battery without loosing volts and efficiency. Of course if you ave room, run the 1-2 jumbo wires back to the former battery location, thats simplest.

The best is to move the whole fuse panel when you move the battery, but I don't know the car to say how well/easy this will be.

Without fusing you increase the fire hazard of your vehicle. Will you run these wires under the carpet in the car? I don't think there is any inobtrusive place to run these wires.

Didnt tell us the car type or if its truck, unibody, etc. I would say look at the rear lights and see if they are grounded to frame or to wire runnign back to front of car. I really need to know the grounding strategy of the car to give a solid easy answer. without that i have to recommend treating the ground just like the B+ sans the fuses. Do not ground the battery at the rear. It could work or you could end up with ground loop issues in different systems. Safest way is to stick to existing architecture and simply compensate for voltage drop by using multiple wires.

Battery should not be in passenger compartment. Maybe you want to give more reasons why your doing this and we can give more options.

In addition to the details of involving venting of hydrogen gas to the outside have you provided for some sort of drain?

Most battery boxes designed for trunk mounting provide a vent hose intended to be routed through the trunk floor. There is no drain as the idea is that the plastic or stainless box will contain any inadvertant acid spills.

nate

Good point. Which makes me think of another point. He is putting this in the rear of a wagon. I wonder where he is putting it.....hopefully not on the storgage compartment floor.

I'm not sure I understand what the problem is that will be solved by this exercise.

Well, if I really really needed to remove ... and that's remove, not relocate ... 20 lbs from the front of my jeep, there are smarter "weighs" do it than relocate the frikken battery.

Jim is answering some of the questions posted:

I want to do it to shorten the braking distance.

It is a brand new compact wagon of Japanese unibody design and US manufacture. The battery will be inside the rim of the spare tire, under the rear floor.

Everything, including the rear lights, is grounded thru the body - no separate ground wires except for a heavy one from the engine block over to the battery minus terminal.

The proposed positive cable will be under the car, with heavy duty PVC insulation and covered by the black nylon sheathing typically used for car wiring. Small sections subject to movement or slight abrasion will be inside lengths of rubber hose. I have done this before, and have one installation that is now 20 years old with no abrasion problems. However, that old vehicle does not have a computer so probably is not sensitive to the voltage spikes that occur when you shut off the starter motor. Thus, I am interested in dnoyeB's suggestion to run separate hot wires to the alternator, computer, and starter. Any idea what gage each should be? (Would like to keep the weight down.)

Some posters have identified other cars that have the battery in the rear. Does anyone know if they run separate wires for alt, comp, starter, or take other precautions?

Jim

bowgus wrote: