Battery move to boot

[bobfather]

I have seen that some owners have chosen to terminate the negative lead to the lower chassis bolt behind the fuel tank. As I currently have my fuel tank out I was considering fitting this negative lead in readiness. Are there any downsides to terminating there rather than bringing the negative back to the earth point at the front?

[Polly Grigora]

The only problem I can think of is being unable to check the connection without removing the tank

[Belle427]

When I did mine I created a new one under the car at the back so it could be accessed easily.
Put some rust proofing over the top of it, should never corrode really if the car is just dry use.
I suppose belt and braces is to run a new one up front to the engine block, I didn’t want to run 2 cables for neatness sake.

[Polly Grigora]

There is a test you could carry out before fitting the tank

Remove fuel pump fuse or its relay

Connect volt meter between battery negative and starter motor body or very close to it

Crank engine over and read the voltage drop

0.25 volts or less voltage drop = It's a goer

You could also carry out the same test on the battery to starter motor positive cable

0.75 volts or less voltage drop = It's a goer

[bobfather]

Polly Grigora said:

There is a test you could carry out before fitting the tank

Remove fuel pump fuse or its relay

Connect volt meter between battery negative and starter motor body or very close to it

Crank engine over and read the voltage drop

0.25 volts or less voltage drop = It's a goer

You could also carry out the same test on the battery to starter motor positive cable

0.75 volts or less voltage drop = It's a goer

The chassis is a pretty big hunk of conductive steel, I would be surprised if there was any measurable loss by adding the additional distance. The chassis would surely have more conductivity than the same length of 7mm2 copper wire. I was thinking more about potential for corrosion issues reducing connective surfaces and the possibility of a hot spot near the fuel tank. As pointed out above, access would be a consideration, I may favour the upper mounting bolts for that as the tank can be eased back for access to those. I just need to confirm that they are not too close to the fuel tank wall

[Polly Grigora]

bobfather said:

Polly Grigora said:

There is a test you could carry out before fitting the tank

Remove fuel pump fuse or its relay

Connect volt meter between battery negative and starter motor body or very close to it

Crank engine over and read the voltage drop

0.25 volts or less voltage drop = It's a goer

You could also carry out the same test on the battery to starter motor positive cable

0.75 volts or less voltage drop = It's a goer

The chassis is a pretty big hunk of conductive steel, I would be surprised if there was any measurable loss by adding the additional distance. The chassis would surely have more conductivity than the same length of 7mm2 copper wire. I was thinking more about potential for corrosion issues reducing connective surfaces and the possibility of a hot spot near the fuel tank. As pointed out above, access would be a consideration, I may favour the upper mounting bolts for that as the tank can be eased back for access to those. I just need to confirm that they are not too close to the fuel tank wall

You did ask

[bobfather]

Polly Grigora said:

You did ask

Indeed I think I was really asking if others had grounded the neutral at the rear of the chassis. I'm more concerned that those bolts are corroded, I could easily clean them up but corrosion would reoccur

[Polly Grigora]

bobfather said:

Polly Grigora said:

You did ask

Indeed I think I was really asking if others had grounded the neutral at the rear of the chassis. I'm more concerned that those bolts are corroded, I could easily clean them up but corrosion would reoccur

Ok then

Was comparing the resistance of copper and steel yesterday, came to the conclusion that you would have to crush your steel chassis with a huge press to be able to then calculate the cross sectional area of steel used which could then be used to determine if a run of copper cable would be better than using the chassis for the return path

bobfather said:

The chassis is a pretty big hunk of conductive steel, I would be surprised if there was any measurable loss by adding the additional distance. The chassis would surely have more conductivity than the same length of 7mm2 copper wire

7mm²???

40mm² flexi looks up to the job

https://www.autoelectricsupplies.co.uk/product/752...

[bobfather]

Polly Grigora said:

7mm²???

40mm² flexi looks up to the job

https://www.autoelectricsupplies.co.uk/product/752...

Oops, 70mm2 was where I was going. Too early on a Sunday morning. I also wonder whether 70mm2 is overkill, I don't fancy getting the positive cable all the way through to the starter, my plan is to go under the passenger side carpet to an isolation switch in the passenger foot well where the fuse box take-off can be made, then over the tunnel and into the front wing void and out to the starter at some convenient point. That way it's protected from the elements and from hot engine surfaces

[Polly Grigora]

Good idea

5 meters of 35mm² @ 250 amps starter current draw passes the volt drop test

Voltage drop calculator takes into account the same length of cable being used for a return run, this is why a 2.5 meter length is used for the calculation

Yes 70mm² is over-kill, 40mm² or 50mm² is heavy enough

Edited by Polly Grigora on Monday 28th March 17:09

[Steve_D]

When I did my chassis I welded a bolt in place ready to do my boot battery earth.
The upper chassis bolts behind the tank are not suitable as there is no clearance which is why the factory used button head bolts there.
You can use the lower chassis bolts but these are not ideal as they will suffer damp and corrosion issues in the same way the front chassis earth point suffers.
If you can do it ,or get it done, a welded bolt on the chassis is your best option.

Route your battery live all the way to the starter. Use the existing starter cable to provide the power back to the fuse/relay box.

Steve

[bobfather]

Polly Grigora said:

Good idea

Yes 70mm² is over-kill, 40mm² or 50mm² is heavy enough

Many thanks, that'll be easier

[s p a c e m a n]

Yeah, big fat cable under the car to the starter motor, no need to go anywhere near the fuse box.

[bobfather]

s p a c e m a n said:

Yeah, big fat cable under the car to the starter motor, no need to go anywhere near the fuse box.

I'm assuming access is a bit awkward around the gearbox & bell housing if I want it strapped to the upper ladder rail

[Belle427]

bobfather said:

s p a c e m a n said:

Yeah, big fat cable under the car to the starter motor, no need to go anywhere near the fuse box.

I'm assuming access is a bit awkward around the gearbox & bell housing if I want it strapped to the upper ladder rail

I ran some 20 mm electrical conduit along the top rail, used 50 mm squared cable in it.
Needed to drop the exhaust off to gain access properly.

[bobfather]

Belle427 said:

I ran some 20 mm electrical conduit along the top rail, used 50 mm squared cable in it.
Needed to drop the exhaust off to gain access properly.

UPVC conduit?

[Belle427]

bobfather said:

Belle427 said:

I ran some 20 mm electrical conduit along the top rail, used 50 mm squared cable in it.
Needed to drop the exhaust off to gain access properly.

UPVC conduit?

The plastic convoluted type.

[Polly Grigora]

Fibreglass sleeving made for the job Bob



https://www.ebay.co.uk/itm/184374770519?hash=item2...

[bobfather]

Definitely going to use the lower diff cage body bolts for the negative termination. I've removed both bolts and they're spotless, zero corrosion on the head and threads. I'm going to link to the drivers side bolt and add a daisy-chain link to the passenger side bolt using 50mm2 cable.

Now to work my positive cable route. Simple but awkward under car route versus complex but easy route through the car.

Edited by bobfather on Thursday 31st March 19:29

[Belle427]

It’s easy with the exhaust out of the way, would be pretty difficult to run it through the cabin neatly I think.
Here’s where I took the cables into the boot on the drivers side.

Edited by Belle427 on Saturday 2nd April 07:52