Battery
One of the most important components of your electrical system will be your battery. Buy one big enough for the job. If you don’t plan on using your car often either buy a battery big enough to start the car after lying for a month or keep it regularly charged. The battery should be placed in a location where it can be securely mounted while placing it as far as possible from hot areas like exhaust pipes - batteries die real quick if you cook them!
It also pays to position it so you can get jumper cables or a battery charger on it without having to remove half the car first - sooner or later your car battery will either go flat or you will be asked to help some one else whose battery has, and it is not much fun if you have to take a wheel off (invariably in the dark and in the rain after you forgot to turn the lights off when you went into the movie theatre three hours ago) (of course you have 90 watt dip bulbs so the battery is really dead!) to get at that battery terminal - fine for a race car but not so hot in a road car... of course you can always add a couple of remote contacts.
By the way, my car is injected with a standard Toyota EFI system and has been jump started and used to jump start other cars and never suffered a problem through voltage spikes. I always hook up with the engine off, but doubt it makes much difference!
Additionally, if you intend to corner hard it pays to orientate the battery so the filler holes are in a line fore and aft. As they sit in the middle of the battery this means the acid has to slop further to escape and start eating your panel work. I have the plates barely covered as any higher the electrolyte slops out when I scream around corners.
It is a good idea to place them as low as possible too, as they are obviously heavy, so you may as well have them contribute to lowering the centre of gravity as opposed to raising it! Some prefer to place them in the boot due to under bonnet clearance or appearance considerations, or to use its weight to counter balance the driver etc. If you decide to put it in the boot or car interior etc it should really be placed in an enclosed battery box that is externally vented. You need to think about where the acid will go if you roll too - I for one would not like to be trapped upside down with what little remaining hair I have lying in a pool of battery acid!
If you have a long run between the battery and the engine or dash etc it is a very good idea to use a single very heavy cable between the battery and the starter motor, and don’t forget a good earth either. As shown above a really heavy wire is not a bad idea. The last thing you want is a sluggish starter motor because there is only 10 Volts getting to it. You must also ensure that the wire is heavily insulated. This is because it will not be fused and the last thing you want is the insulation wearing through and the wire contacting the body... Some people use arc welder hand piece wire as it is heavy duty and well insulated.
Now, since you have power permanently connected to the positive terminal on the starter motor, why not use this point as the starting point for the power to the rest of the car’s electrics? Saves having heaps of wires trundling all the way back to the battery and face it, this big heavy cable is just sitting there unused all of the time once the motor is running.
A battery master off switch (as per racing regulations) with the power off and the key removed makes a pretty effective immobiliser - especially if you have it mounted under the car so the only evidence is a small hole in the inside carpet and floor pan to put a key through. It is hard to get better or cheaper protection than that. Only thing is, remember to have a bypass for those computer memories - it is easy enough to have a little switch that will allow them to be isolated with the rest of the car via the main power switch if you want to go racing and be safe.
