When relocating the battery to the boot (or tray in my case) is a short cable from the neg battery terminal to the body sufficient grounding for a CA18 for starting etc? I'm not sure if I should run a neg cable to the starter from the battery or if I can get away with only 1 pos cable running to the engine bay and use a second earth to the block. I'm interested to see what others have done and your experiences. Cheers.

the pros all say to run thick cables up to the engine (starter and block)

you can get away with earthing the body, if you also connect the block to the body, for a while. It will be difficult to diagnose when problems occur

1200s only have a small gauge wire grounding the body (suitable for 35A draw). The high current the starter draws goes through the negative cable to the engine block. If instead you fasten the Neg cable to the body and run a cable from the body to the block, it will work, but is susceptible to earth problems when the inevitable slight corrosion occurs. The 1200 has very few starter connection problems even after 40 years because they run the cable directly, and it is still recommended. Just think of all the lighting issues that have been caused by body grounding, then imagine that happening with the starter

Cool, thanks for the info. I think both cables is the way to go, was just curious if anyone had gotten away with the short earth.

I have relocated the battery underneath the tray and run a very thick positive cable to the starter. For the Negative I have run a short battery cable from the Battery Neg post to an unused bolt hole under the tray (I think it was one of the mount points for the original fuel tank. I have a smallish ground/earth lead from the CA block to the firewall. So far so good!

I have done the same as cadet1200 for my A series, but I used a positive jump block bolted to my strut tower in the engine bay.

1. Positive cable from battery under tray to jump block on strut tower
2. Positive cable from jump block to starter
3. Negative cable from battery under tray to tray body
4. Negative cable from engine block to body earth

I used all really thick cables that came with my battery relocation kit I bought from Summit racing.

When I relocated my battery to the trunk I did the following:

1. Positioned the plastic battery box over the unibody frame on the passenger side of the car (gas tank fill side).

2. Fabricated metal frame tray to hold battery in plastic box. Drilled though tray, box and chassis. Bolted down. Drilled another hole outside of box. Bolted battery ground to that one with a metal tab on bottom of car for a jumper cable ground connection.

3. Installed kill switch on back of car and ran positive cable through it and then up to front of car. Positive isolator block mounted to strut tower. Positive cable to starter and positive wires to fuse boxes (I have two).

4. Welded large nut to frame. Ground cable from engine block to frame as well as ground cable from starter to frame.

5. Alternator return charging wires routed directly back to the battery in trunk. (True Kill Switch)

Cables were #2 in size. The setup gives me good ground connections.

After you bolt the cable to the chassis body, how are you preventing rust? Do you spray paint right over the bolt?

No paint but also no rust. I don't drive the car when the weather is wet. With a lock washer the bolt is snug. A good wire brushing when ever it is removed.
Really no concerns.

Howdy,

My 2 cents worth.

Relocating the battery to the boot is a cool idea.

Things to remember:

Standard lead acid batteries give off Hydrogen gas when being
charged, so they are a fire hazard if placed in a non-vented
confined space.

I = V/R (Current = Volts divided by Resistance)

All cables have resistance, the longer the cable the higher
the resistance.

The bigger the diameter of the conductor in the cable,
the lower the resistance per metre will be.

So I would recommend 50mm squared cable from the battery positive to the starter motor solenoid.

50mm squared cable from the battery negative to the body of the car as close to the battery as possible.
The cars Body will be of lower resistance than running an other cable
because of the cross sectional area of the Metal in the body.

50mm squared cable from the motor block to the Body of the car as short as possible.

For modern engines it is also very important the ECU is earthed well to the body.

The positive feed to the ECU should also be large-ish
(6mm squared) to minimise volt drop to the ECU during engine
cranking.

All connection to the body need to be on bare metal as paint
is an insulator and will increase the joints resistance.
So carefully drill your mounting hole in the body,
then remove the paint around the hole so the cable lug attaches
to bare metal as you want the electrical current to travel through the lug and not the bolt.
then once attached, paint over the lug, bolt and body.

Just so you know,
I've been in the Electrical game both Commercial and Automotive
for the last 35 years and my cars all work well.

Cheers,

Geoff

ps. sealed gel batteries are a good idea and are safer as they do not need venting and will not spill in the event of a roll over . . .
but they will not take as much abuse, eg. long periods of cranking or being left discharged as a standard lead/acid battery will.

Thanks guys for the awesome feedback. It's good to hear what others are running, and thanks Geoff for your input that gives me a pretty clear idea of what I'm going to need to do. I'll be sure to post up how I have put mine together once it's done.