Immy your guide is great. To help the OP I'll share some thoughts.
Battery to boot conversion is very easy. Here are some ways of going about it.
1. Secure the battery! You don't want a battery moving about in your car. For the very best solution without spending money on 'motorsport tax' I would highly recommend heading to eBay and buying a 'marine battery box'. These are designed for hobby boats (canal to speed boats). Made for batteries and you can find one the right size for your needs. Mount the box to your shell securely. If you can't nut and bolt then consider riv nuts or a thick metal strap of some kind. Use your common sense.
2. ground.
The battery needs a good earth. Clean the metal up. Earth it. Done. However.... Don't forget that on some cars the engine and or the loom will earth through the same place as the original battery does. So keep them earth's or stuff won't work!
3. Get the right gauge wire... This is simple but very important and is often over looked. Your battery kicks out lots of electricitys... If you try and feed the live down a wire that's too thin then you risk over heating the cable and burning it out. The result can be a fire and all kinds of death type situations. So get the right guage!!! your supplier can help with this. (See edit)
4. How to run the cables... Earth.
With your battery in the boot simply earth the - terminal of the battery to a clean earth on the chassis. Again, use an appropriate gauge. Best solution is to use the OEM earth cable (assuming it only earth's the battery and as a result would have no use in the engine bay once the battery has been moved). Secure earth strap with a bolt.
6. How to run cables, live.
The + is the cable mentioned in 3. This needs to run to where the battery + used to go.* Connecting this new cable to the OEM location can be tricky as it's a very very thick wire to solder. Consider a brand new crimped connecter to where the original battery cable went. Your supplier can crimp on a ring terminal for you (because it'll be a thick guage wire it'd be a specialist tool needed to crimp a terminal on you see)...
Make sure you insulate well.
Fuse: I've seen lots of factory cars with batterys in the boot and I've seen lots of high end conversions. I've done lots myself. None have had a fuse. One can't hurt. A fuse is never a bad idea. But I honestly don't see the need as long as your cable is the correct size.
5. Jump start!!!!
This is important to think about. Especially if your boot is opened via a solinoid.... If your battery is flat. How will you jump start. It's worth thinking about.
6... Amps, voltage drop and the right battery.
I don't want to go too deep into this. Mainly because I'm still learning about all this myself. BUT the basics are this... The longer the run of wire from your battery live to your starter motor, the more volts will be lost on the way there. As a result your engine will crank slower...
I'm about to be corrected if I'm wrong and if I am i want to be told I'm wrong BUT as I understand it, a battery with higher amps per hour will push it's voltage more efficiently and stop the voltage drop being so extreme.
In practice I've found that moving my battery to the boot means a slower crank of the engine but it's barely noticeable and has never caused me issues with daily driving.
* On most cars the battery + feeds the OEM power distribution block and main engine bay fuse box. These are usually the same part in the engine bay. The box will often inside large amp fuses and some relays... This is certainly the case on a S13 and 14. On a r33 its a tad different as the main fuses are in the boot with the battery and then the relays are in the engine bay at the end of a battery (in boot) to engine bay wire.... Shows the many different ways it can work! Lol sorry to confuse things if I've waffled too much
I hope that helps.
Long reply as I'm bored and can't sleep lol
Oh edit: supplier. I really recommend vehicle wiring products. Based near Derby in the UK. Very good to deal with and carry everything you need. Their website isn't bad but they do more than just what's on there and they are good to deal with on the phone. Very good company.
