Moving battery: cable sizing, grounding ?'s

I have a typical nose heavy compact wagon and want to move the battery
from the engine compartment, to the rear of the vehicle. I have two
tech details to work out.

1. What size of batt cable to use. I will be using tinned marine grade
wire, from Ancor Marine. The run will be no more than 15' long. Engine
size is 2.5 litre, alternator is 60 amp.

2. Whether to run the ground from the battery post all the way back to
the front of the car, or just ground to the body somewhere near the
battery, or ground to body near battery and also run a smaller gage
ground all the way forward.


In case anyone is concerned about fumes from the battery, I will be
using a battery that has provisions for adding a vent hose.


Jim

On Sat, 04 Feb 2006 20:18:57 GMT, Jim >
wrote:

>I have a typical nose heavy compact wagon and want to move the battery
>from the engine compartment, to the rear of the vehicle. I have two
>tech details to work out.
>
>1. What size of batt cable to use. I will be using tinned marine grade
>wire, from Ancor Marine. The run will be no more than 15' long. Engine
>size is 2.5 litre, alternator is 60 amp.
>
>2. Whether to run the ground from the battery post all the way back to
>the front of the car, or just ground to the body somewhere near the
>battery, or ground to body near battery and also run a smaller gage
>ground all the way forward.
>
>
>In case anyone is concerned about fumes from the battery, I will be
>using a battery that has provisions for adding a vent hose.
>
>
>Jim

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

On Sat, 04 Feb 2006 20:18:57 GMT, Jim > wrote:


>I have a typical nose heavy compact wagon and want to move the battery
>from the engine compartment, to the rear of the vehicle. I have two
>tech details to work out.

>1. What size of batt cable to use. I will be using tinned marine grade
>wire, from Ancor Marine. The run will be no more than 15' long. Engine
>size is 2.5 litre, alternator is 60 amp.

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:
....
>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,

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:

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

AZ Nomad wrote:

> 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'.

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?..<grin>

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.

On Sat, 04 Feb 2006 21:00:32 GMT, AZ Nomad >
wrote:

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

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

"Jim" > wrote in message
nk.net...
> I have a typical nose heavy compact wagon and want to move the battery
> from the engine compartment, to the rear of the vehicle. I have two
> tech details to work out.

I have used
www.custombatterycables.com

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:

bowgus wrote:
> 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'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.