There was mistake on two of the diagrams ( "Battery in the trunk" and "Battery in the trunk with master cut-off switch" ). A 125A fuse with 4 gauge cable should be used instead of 50A and 8 gauge. Thanks to Vonore TN for pointing that out.
This question gets asked from time to time. I rarely read good answers. The following is a compilation of tips to move the battery to the trunk.
Basic stock diagram which has been simplified for sake of explanation
Here are some tips to do this:
1.
DO NOT do (like some) and run a large gauge cable directly from the battery, through the floor and to the starter. Hack jobs are done like that and it is a recipe for a fire. Buy a solenoid kit (
Summit # SUM-G1750 or
similar) and mount it in the trunk and use it as a switch to let current flow to the starter when you start the car. The solenoid will disengage and cut current off as soon as you release the ignition key after the engine starts. With this setup, the positive battery cable is only HOT when the key is turned to "START". To control this solenoid, you have to buy a small 30 amp relay ($5 at most auto parts store). This relay is energized by the wire that used to go to the starter solenoid. Make sure you use grommets where any cables go through sheet metal. Very important. I cannot stress that enough.
2. Do Not "Jump" the starter solenoid from the battery cable. This
picture shows this common setup (similar to the one shown by Buffhomer who posts on the Corral.net). With 1993 and later Mustangs that is not a good idea since they use permanent magnets in the starter motor. The starter motor pinion could stick to the flywheel after your release the ignition key. Not good. So follow the
proper setup.
3. If you go racing, some tracks or sanctioning bodies may require a Master cut-off switch because you moved the battery. That makes the diagram a little more complicated. See this
diagram for the setup. You have to use a "four terminal" master cut-off switch (
Taylor #1033 or
Moroso 74102) and an alternator cut-off switch (
Painless #50105) to cut-off the juice from the alternator AT THE SAME TIME as you cut the battery. If you do not do this, the voltage will spike (18+V) and damage the ECU or ignition. This happens because the battery acts like a stabilizer in the electrical system. Without it, the alternator will try to compensate and Boom. Bye bye ignition and ECU. Very expensive mistake. The Painless #50105 must be switched via a relay so that you do not drain the battery when the car is not running. Find a circuit that is HOT in START and RUN and tap from it to energize the relay. By the way, the Moroso switch is a little on the overkill side with a 300A continuous rating. The terminals are also designed for 1/2" lugs. That is larger than standard (3/8").
4. For #3 above, this is not well documented on the internet. I suggest you follow my recommendations.
5. Breakers: Some people try to get away from using a trunk mounted solenoid by using a large amp breaker on the + cable. These have become very popular over the years because of the big sound systems. I have never tried one. But keep this in mind: the starter motor needs a LOT of current when it is very cold outside. So the current draw could exceed the capabilities of a 150A~200A breaker. You could end up with a breaker that trips all the time. Not fun. I recommend you use a solenoid. MAD Electrical is another source of kits for remote battery installation.
6. These diagrams are for reference. The key to avoid shocks, shorts and fires is proper wiring and good connections. Large gauge cables should be soldered to their terminals (crimping is not enough). Use flux to clean the copper and use good heat shrink tubing. The good heat shrink tubing will produce a "goop" that seals the terminal when it shrinks. The goop oozes out and solidifies. Very good stuff and it is sold by Napa auto parts stores. The stuff at Lowes or Home Depot is designed for home electrical and is not designed to handle road salt environment.
7. Proper grounding is supremely important. If not done properly you will get strange behavior from electronic components in the car such as radio, ignition, etc… DO NOT simply scratch the paint off and drill a hole to bolt the negative to the chassis. That is not a good connection. Trust me. The proper way is to WELD a 3/8" nut to the chassis and bolt the negative cable to that. One in the engine compartment and one in the trunk area. Once the cable is bolted, cover the weld, cable terminal and bolt with dielectric grease to prevent corrosion. Very important to avoid corrosion. If anything replace the stupid flat RED ground strap that Ford uses to ground the motor. What a piece of junk! It is bolted to the frame using a self tapping screw. Also junk. You should use a good 4Gauge cable and bolt it to a welded nut or to the sway bar bracket. This strap left me stranded in the middle of Ohio one Saturday, 4 years ago. The terminal on the red strap tends to crack if overtorqued. Junk!
8. I am very anal about fuses. I fuse everything that goes through sheet metal (except the main battery cables, that task is handled by the solenoid). Fuses are cheap, don’t skimp on them.
9. Most sanctioning bodies require that the battery be placed in a box. Prepare accordingly and make sure you use the same 3/8" threaded ROD to bolt the box AND the battery through the floor (that is required by NHRA anyway). Use a large thick washer under the floor to prevent the battery from pulling through the floor in an accident. Rods with a J hook at the end are not usually accepted at the track. By the way, I do crash testing for the Big Three and I'm telling you that a 30lbs battery can do some serious damage if it comes loose in a crash.
10. 1Gauge battery cable is fine to handle most jobs. 2 Gauge might be a bit small. 1/0Gauge is overkill, but if you like to over design by all means please do so. Just keep in mind that larger cables require larger connectors and terminals and they are not always easy to find.
11. Welding cable is not designed to live in the harsh environment under a car (at least not the stuff I have seen around here). Buy proper cable from Taylor or other. Those cables have the correct sheating to withstand the heat and cold. For the positive cable at the starter motor, make a small bracket to hold the cable to one of the transmission bolts (using tie straps). This will strain relief the cable and not put the connector on the starter under constant load when the motor bounces or torques.
12. With enough patience and a fish tape, you can use existing holes in the trunk to route the cables and wires to the starter. It's a good idea to cover the wire bundle with
convolute or protective plastic tubing. Again, use grommets to prevent a short.
Parts needed:
Battery box (sealed)
Wiring kit for battery relocation
Summit SUM-G1750 or MAD Electrical or similar Remote Solenoid kit
General purpose relay
For racing you ALSO need:
Painless #50105 Alternator cut-off switch (solenoid)
Taylor 1033 or Moroso 74102 master cut-off switch
As you can see, this is a lot of work.
Disclaimer: The result of your project is your responsibility. I only offer recommendations. Someday I will do a complete write-up with pictures.
Relocating a battery to the trunk
Linear Mode
