You can do a "very" basic battery relocation by removing the existing battery and putting it in the trunk and then up front you then remove the existing positive battery cable that was off of the battery when in front down to the starter and leave it off of the car.
Now there is a wire that was a fusible link between the up front battery post and the junction block on the backside of the radiator support. This wire depending on how it was attached to the original battery positive post when the battery was up front is you need to take this section of fusible link off of the junction block and attach one end of it to the starter's battery cable post and then since it will not be long enough, extend the other end with at LEAST a 10 gauge wire to reach to the junction block on the backside of the radiator support.
Now you need to look at things and figure out your best plan of attack but your battery negative post when up front had a large cable off of it to the engine block or the alternator bracket or somewhere on the motor. This cable now need to be either reused and connected to another point or removed and not be on the vehicle. This cable may also have had a smaller wire off of it which should be a fusible link and the end of this smaller wire attached to either a bolt on the backside of the radiator support or a lip on the passenger fender.
Staying on point with this ground cable, you could then terminate the end of this big ground cable to a nut and bolt connection on the firewall while still leaving the other end attached where it was on the block, alternator bracket, or where ever. By making this connection you have now ties the engine, starter, alternator and engine sensors grounds to the body of the car.
Now looking at the car, you have a connection to get power from the starters positive post connection to the rest of the car like it was before and still protected like it was with the fusible link.
You now also have the engine, starter, alternator, engine sensors, and everything else grounded to the car body tied together.
What we lack now is connections to the battery.
This is where I can get into conflict with others but still be supported by anyone else that knows what happens when a wire or cable is unprotected and a short does occur.
Stay with me here but now you ahve your battery in the trunk with a positive post and a negative post that needs to be connected.
Lets start on the negative battery post. You now need to make up a cable to go from the negative battery post to a spot in the rear of the car and this cable needs to be at least the size of what was originally under the hood when the battery was up front. This connection to the car's metal in the back must be solid AND electrically sound and the cable when terminated should be nut and bolted to the metal and not be attached with metal self tap screws AND all paint and whatever must be removed so electrically this power flows easily. Hopefully on the front ground connection you have also done the same with a solid electrical connection and not had the cables terminal sitting on top of paint and junk.
Now we are getting close to being done. Here is where you need to get a new cable long enough to reach between the battery positive post and the starters battery stud and this cable needs to be large enough to limit voltage drops so that the motor still cranks over like it did before when the battery was up front.
Since we do not know what starter you have and what cranking pressures you are trying to over come with the engines pistons and cam timing, is lean towards bigger cabling.
Once you get the proper size cable and properly terminate it and connect it into the car, the car should fire up and run as it did before.
The rub here in my book doing it like above is there is NO protection on this big cable. Granted a LOT of vehicle companies do not do it now, more and more ARE doing things to protect this cable should an accident occur. Also more and more standards are being upgraded in the boating industry which also share a lot of 12V circuits we have in an automobile. Drag racing standard are also being upgrade to include vehicles that are electrically powered and not with combustion engines.
If you have ever experienced a short on a wire or cable and seen it melt it's insulation off you will start wondering, is there a way to prevent this and there is. Look at your house. What happens when a short occurs on a wire or cable or a light switch shorts out ?. A breaker or fuse opens up to stop the flow of electricity unless it's been put in the wrong way. Just like on your car already, you have fusible links to protect wiring as well as fuses in the fuse block to protect wiring. What happens if you take these things out and a short were to occur ?. PLENTY.
While I would love to say that I can route a battery cable from the battery all the way up front and have it protected from things I KNOW could cause issues like the wire going across a sharp metal edge or have it placed away from excessive heat and properly secured BUT I have NO idea what can happen in an accident. This cable could then become pinched and shorted to ground and without the proper parts your 1000 cranking amp battery will be dumping this current into the short and a small 4 gauge, 2 gauge, 1/0 wire and so on not designed for this amount of current will break down and more damage will occur than just at the point of the short.
I'll get on my chair or high horse here to say, a master on off switch requires someone to turn it on AND off. Fuses, circuit breakers and fusible links are designed open circuits and stop the flow of power ALL BY THEMSELVES. If you get in a bad enough accident, what happens if you cannot get to the master on/off switch to cut power ?. What happens if you are at a track following the rules with the relocated battery and a master on/off switch and have an accident and it takes a while before the track safety person gets there ?. What happens at a track and the car is so badly damaged that they cannot even find the switch to turn it off ?. With the proper wire protection such as fuses. circuit breakers and fusible links AT the source of power the electrical system is more properly protected.
Here is a VERY basic wiring diagram and the variable is how you might do the ground connections:
Here is also some pictures of a newer vehicle (Jaguar) with a trunk mounted battery and fusing on that cable that runs all the way up front to the engine starter and the rest of the electrical system:
Here is a car from Drag week that the cable shorted out and the interior filled with smoke while on a drive between tracks:
And an unprotected cable on a commercial rig:
Do what you want but I don't want to have this happen to any of my vehicles with a battery in the trunk.
Oh, and a mustang that he forgot to run a wire thought a grommet:
And another drawing I had from a while back:
Food for thought but don't forget to solidly mount the battery where ever it will be and properly vent it.
And I know you asked this before but on the master switches:
As far as the different disconnect's Jeg's offers (and I don't know which ones you have been looking at) is they do different things AND have different ratings on them. For example, they offer a disconnect model 74100 and it is for w/o alternator and is a two post disconnect switch. The ratings or specifications on it are 20A continuous and 125A surge. The 74102 is for with an alternator and it is a 4 post disconnect switch and the ratings or specifications for it are 300A continuous and 2000A surge. Just find out what you need there and what they offer and go with a higher than needed spec for added cushion or for supporting some future upgrades.
Jim
