Why would you want to move a perfectly good battery out from under the hood to somewhere else in the car?
Well, there are a couple of reasons. First of all the environment under the hood subjects your battery to some extremes that don’t do a lot for its ability to hold a charge or extend its useful life. Under the hood it gets hot from the engine and radiator as well as the occasional water and mud thrown up from the road. Secondly, they tend to weigh a lot, forty pounds or more, and are not usually placed in the best location from a weight balance point of view. The only exception I can think of was the air cooled VW Beetle where it was under the back seat. That kept it close to the rear engine but out of the engine compartment which just did not have the room for it. Other than rusting out the floor from the splashed corrosive fluid that was in the battery, it turned out to be a good place for it. It kept it out of a harsh environment, and also kept the weight low and close to the center of the car inside the wheel base.
Our front engine cars will usually have the battery mounted up high in the engine compartment and often ahead of the front axle. Knowing that the best design for handling would be to concentrate as much of the car’s mass as low as possible and centered between the axles, this factory mounting spot is definitely not the ideal. This has prompted many a race car builder and even home mechanic to relocate the battery to a more optimal location.
Such a relocation in a race car is subject to sanctioning body rules that ensure it is done in a safe manner that protects the driver and the car while it is competing on the track. The backyard mechanics should take note of these rules as they perform their own battery relocation so the their good idea doesn’t turn into a nightmare.
There are some relocation kits available from after market suppliers such as Summit and Jegs that give you a head start with most of the components you will need. They usually come with a battery box made of plastic, a set of large diameter cables for the positive and negative battery leads, some hardware to facilitate mounting the battery inside the box, and maybe some zip ties. This is a great start and very economical, but there are a few other components you had better plan to add. Before we get to those let’s think about the new location for the battery. You should plan on changing to a “gel” battery that not only doesn’t need water added, but has nothing to spill if not kept in an upright position. The Optima brand battery is a good example. While you are at it, be sure to get a battery with plenty of cold cranking amps. You will be pushing the current to the starter over a lot longer distance and having 800 to 1000 cranking amp capacity will not be overkill, especially if you have a high compression engine to turn over in the winter. So where do you mount this lead weight? Often the spare tire well is the most convenient. That usually means relocating or eliminating the spare tire. Getting rid of that weight might not be a bad idea anyway. When I did just that with my Mitsubishi Eclipse GSX I added a repair kit to my “trunk”. It included a tire plugging kit, a small air compressor that would run off my battery, and a can of flat fix for when I was really desperate. Scout out the location and try to keep it as close to the ideal of being inside the wheel base and low. In a race car you don’t have to deal with the daily driver world of carrying passengers, etc. so you can pick a spot in the cabin area (as long as the sanctioning body permits) that is close to the ideal location for mass. In your street car you will have to compromise and keep it in the trunk area. If you want to keep your spare tire you can often find a spot behind the rear seat and between the shock/strut towers that will do nicely. It might be a nice time to integrate a strut tower brace with the battery box. It all depends on your budget.Some things will be required that you must fit into your budget. Things like a circuit breaker (or fuse) for the positive battery lead. This should be mounted within 15″ or less of the battery terminal. This will protect you from a short to ground with that long positive lead you will run from the battery’s new location back to the engine compartment. If the positive lead every gets chaffed or cut in an accident you want that circuit breaker to trip before a fire occurs. You will also need an insulated terminal block. When you look at your battery in the factory location you will notice that there are several wires going directly to the positive terminal. With the battery relocated you will need a terminal block in the engine compartment to tie them all together. This is not the time to cut a block of wood, screw it to the fender, and bolt all the cables to it. High end automotive audio shops will have things like insulated terminal blocks and 100 amp (minimum) circuit breakers. You can also look on the Internet. A side benefit of having the circuit breaker is that it is a quick disconnect of the battery for when you are working on the electrical system of your car.
Another must is to cover the positive cable with split plastic sheath for the whole length. This is to protect against chaffing. Yes, I know the cable already has insulation, but you need all the protection you can get. It is much cheaper than having your car catch fire. Get some red electrical tape and wrap the sheath from one end to the other. That will identify this as the positive lead.
I found the battery box supplied in the Summit kit way too large, so I picked up a better sized one from Auto Zone. In my installation I had to make it shorter so that it would fit in the spare tire well and still cover the battery. I also had to fashion a solid mount for the battery and the battery box. Batteries weigh up and you don’t want them wandering around on their own. Make sure the metal parts of your battery mount are insulated and take the time to slip red and black insulated covers over the positive and negative terminal clamps. Batteries can explode and you NEVER want to allow anything to short those two terminals.
Run the positive cable to the engine compartment through the car’s interior. Routing it outside along the frame is a bad idea. You do not want to expose the positive cable to an environment where it could be cut or impacted by road debris. I find that most cars have a nice cable path close to the door sills and under the carpet. Use your head and do everything to keep it protected. When you reach the firewall you have a couple of choices as to how you get it into the engine compartment. You may luck out and find that there is all ready an insulated access point through the firewall. If not you have to decide the best place to make your own. If it means you must drill a hole you have to make sure that you have a way to cover the edges of that hole so that NOTHING can chafe the cable. There are firewall pass-through terminals made that are designed so that they completely seal and insulate the hole and provide terminals for the cable to bolt to on both sides of the firewall. I have always been lucky to find a well insulated factory pass-through on the cars that I’ve done this modification on.
Pick your ground connection point very carefully and do it well. First you will need to find a spot for the negative cable in the back close to where you have the battery. Make certain that it has all the paint off the spot where you attach the cable. You can also use a star washer under the terminal to assure a good mechanical metal to metal connection. Then go back to your engine compartment and add some ground cables there. I make mine from 8 gauge cable from a stereo shop, cut them to length, and solder on terminal ends. Then I use heat shrinkable tubing to cover the cable insulation and as much of the terminal connector as is reasonable leaving only the part you bolt down exposed. This ensures that moisture and air is kept away from the connection that is soldered. Do this to any connections you make and you won’t be plagued by corrosion in these spots. Corrosion means electrical resistance and trouble. You want to have a good ground between the engine block and the frame, the frame and the transmission. On cars with O2 sensors you want to be sure that the exhaust system also has a good ground back to the chassis.
If you plan to take your car to a track you may find that you will be required to have a battery cut-off switch installed and accessable from the outside of the car. Be sure to set this up so that it disconnects the alternator as well as the battery. When done properly it should kill the engine while it is running and not just take the battery out of the electrical loop. Some sanctioning bodies will also require more than a plastic battery box. They will want to see a solidly mounted metal box that has a vent tube going from the box to the outside of the car (Yes, I know that the gel batteries have nothing to vent. Get over it.). In any case you are responsible for finding out the rules.
If this is just to make your street ride handle better and give yourself the experience of a challenging, but satisfying modification you don’t necessarily have to have the cut-off switch.
Leverage the Internet and see what others have done, keeping in mind the safety tips I have highlighted. Take the time to think out the steps to make this project successful. When you are done you should have an installation that looks so professional it could have come that way from the factory.