Around Christmas time, I relocated my car's battery to the boot. I'm writing this guide to help other people trying to the same to do it safely and with less hassle.
It makes a reasonable difference to weight distribution. If you are building a corner-balanced race vehicle, this is a good move. The reason I did it was for the other advantage. I live in the tropics, and a hard drive on a hot day gets under-bonnet temperatures to dizzying heights, and my last calcium battery went from new to completely cooked in under a year. If you're doing track work in the southern states, you'll still probably cook the battery after a while. The new battery has been running for 7 months now, and still performs like new, while the old battery required a charge every second day by this point in its life cycle. Also, you can use any battery you like when it's in the boot, and a commodore battery is identical to a 206 GTi battery, but taller and $40 cheaper.
The parts list is bigger than you'd think for this job:
5M + 1M of decent 2 gauge or good quality 4 gauge copper cable. The reason quality is important is that the American Wire Gauge only measures the outside of the insulation, not the copper inside. Cheap cables are all plastic. For the 206 GTi, we're looking for a minimum of around 32mm^2 cross sectional area of copper to be safe. The 4 gauge cable I used has around 35mm^2 cross section of copper, and is about the thickest cable you'll get through the firewall without taking out a hole saw.
A sealed and vented battery box. The cheapy battery boxes that the big auto reatilers sell are NOT ventilated, NOT sealed, and will NOT be legal to use in QLD or (I assume) most other states. Even if you have an Optima battery which is sealed, the law is badly written and it still isn't legal. Since sealed battery boxes go for around $70 more than the non-sealed ones, I opted to just seal and vent a cheapy battery box myself.
A battery clamp or strap. This is a legal requirement, and also a good idea to stop the battery from warping the box with its weight when cornering hard.
A modification plate. You need to get one after doing this, as the car won't be legal to drive on the road (in QLD at least) until you do.
A cable distrobution block for either 4 gauge-4 gauge or 4 gauge-4x8 gauge. The 206 GTi's original battery cable is in fact 2 separate 8 gauge cables, one to the alternator, one to the engine fuse box. A friend of mine bought one of those "amplifier wiring kit" ripoffs because he got it on special for less than the copper value and wanted the digital voltmeter from it. So I ended up with the rest of the kit for free. Part of this was a distrobution block for a 4 gauge cable to 4 separate 8 gauge cables, which nicely joined the 4 gauge long battery cable to the two short 8 gauge cables under the bonnet.
Some heat shrink insulation tubing. We're dealing with a battery that can pump out more amps than a cheap arc welder here, so lets not short anything out.
2 crimp-on or screw-down battery terminals. The original Peugeot battery terminals are crimp down ones which only just fit the 8 gauge original cabling. Trying to reuse these will be a huge pain, and not worth the $10 you can get a decent pair of new brass ones for.
A large crimp-on washer grounding point. We need the negative connection to join the body of the car, and the best way to do this is to attach the ground to a large nearby bolt to the chassis (not just sheet metal work).
And as always when working on Peugeot's, a set of torx bits, hex sockets, and a driver. You need to remove the glovebox, a rear seat mounting bolt, and some interior trim.
Power drill. Unless your forarms are like popeye's and you have a very strong hand drill.
The easiest place to start is wherever you want your battery to be located in the car. I chose behind the rear right seat, next to my subwoofer and amplifier (by pushing this all into the corner of the boot, I can still get my bike in the back with the left rear 60% seat section folded down).
First we need to remove the spare wheel and its holder. We'll be drilling holes to mount the battery box next, and we don't want to drill through our spare wheel. For legality, the battery box must be mounted to the steel of the car by screws or bolts. For some reason it's illegal to attach the battery box with a strap, but the battery can use a strap instead of a clamp.
Step 1: Mounting The Battery Box Line up the battery box where you want it to mount, and drill through it into the steel below, making sure to check where the drill will come out on the other side. Do this for all of the mounting screws for the box and the battery clamp (if you're using one instead of a strap). Then drill another hole, 5mm-10mm diameter from the edge of the box down through to under the car. This will take the the ventilation and drainage tube and be sealed up with silicone.
After drilling the holes, give them a quick spray with some spray paint. This will help to stop any exposed steel from rusting. Put the self-tapping steel screws through next, mounting the box in place.
Step 2: Grounding. Attach the battery terminals to the 5M and 1M cables, and the washer grounding point to the other end of the 1M cable. Remove the closest rear seat bolt, scratch off the paint beneath the head, and then replace the bolt with the grounding washer in place. The seat mounting points go straight to chassis for strength, which also means for a clean ground connection.
Step 3: Cable Routing. Now for the hard part, running the 5M positive connection cable. In the backmost of the boot, right next to where the spare wheel carrier can be lowered from, is a black plastic conduit. This carries a bunch of cables already, and has enough spare room for our cable to join it. This runs across the back of the boot, and we want to run our cable to the left side of the boot when facing the direction of travel (passenger side for Australia). Under the carpet on the left side of the boot is a cable gutter with enough space to fit our big cable into to safely run it forward. Follow this cable gutter under the rear plastic trim (which houses the rear seat arm rests), where it gains a plastic clip-down lid to keep the cables still inside.
The cable gutter then goes under the carpet right next to the left hand door opening. If you pull off the rubber door seal (it'll slide straight back on), you'll see some little plastic clips which hold the edge of the carpet to the bodywork. Use a flat blade screwdriver to disconnect the clips, and fold the carpet in toward the seat. If you then peer back under the door sil toward your feet, you'll find another, even larger cable gutter, where the cables are secured using the bottom of the plastic clips which hold the carpet. Secure your new cable in the spare section of these clips to keep it from moving around. Continue running the cable up through these clips until you reach the area under the glovebox.
Open the glovebox and look inside for the tiny black torx screws which hold it in place. Remove all the screws and lift the glovebox up and back, and remove the cable to the glovebox light. Then you can pull the glovebox down toward the floor and out of the dash.
Now we can see a large bunch of cables feeding through a sealed joint into the engine bay. Near this (down to the right) is a little hole that looks like it could go through, but it doesn't. It's blocked with about 4mm thick of plastic, which can easily be drilled out, leaving a hole just big enough for our 4 gauge cable to feed through.
Inside the engine bay, you'll immediately see your cable sticking out from behind the left hand strut tower and next to the brake booster.
Remove the original battery box, cut off the original battery positive terminal (you can leave the ground or remove it), and strip the cables. Use the distribution block to connect your new cable to the two original power cables. Mount your distribution block to the firewall wtih screws, or just use cable ties to secure it to the top transmission mount, which is right under where the old battery box used to be.
The cable feeding into the top of the transmission mount here is the original negative terminal, unneeded but still connected, so it has been tucked in here to keep it out of the way.
Now put the new battery in your boot, connect it up and start the car. Turn off the car and start it again a few times, and then grab your power cable. If the cable is warm or hot, go and buy a thicker one and redo it all. If you get a cable with over 32mm^2 of copper, you should be fine.
Step 4: Dream. Look into the engine bay. Notice that you can see the exhaust manifold and downpipe from the side now. Notice you can see the intake system in its entirity. Notice that there's a perfect T3/T4 turbocharger-sized gap right between them where the battery box used to be. Try to resist the temptation.
Note that I also have the intake removed back to the throttle body in this picture.
I hope this guide proves to be helpfull for others looking to relocate their 206's battery. Following the same instructions can help with running a large power cable for an amplifier in the boot also. The same firewall hole could be useful to know of if you're installing a wideband O2 sensor, fog lights, or one of those A'pexi AFC things.