For this reason, we recommend heavy-duty billet-type mounts and the use of gel cell-type batteries or full metal enclosures bolted to the solid parts of the chassis with grade five bolts and large fender washers if the battery is anywhere near the driver's compartment. This precaution also goes for trucks if there isn't a solid welded-in bulkhead between the trunk and the driver's compartment. Remember a battery can generate a sledgehammer-like blow of hundreds of pounds of force in a crash and you have to keep this in mind when you choose or build mounts.
Also, check the rules of whatever racing sanctioning body you might run under before starting a battery relocation project. The NHRA requires metal battery boxes vented to the outside of the car unless a dry cell-type battery is being used. The NHRA also requires an external accessible rear-mounted remote battery shutoff switch for trunk-mounted batteries. Most road racing associations require metal boxes unless the battery is a sealed dry cell type and has externally mounted cutoff switches marked with a triangular-standardized label.
Because of the hazard issues with normal lead-acid batteries-how they tend to emit flammable hydrogen gas and spew sulfuric acid-it's a good idea to go with a sealed racing dry cell battery. This also makes it easier to comply with a racing sanctioning body's rules for venting. Dry cell batteries have a dry paste instead of liquid sulfuric acid electrolyte, which is less likely to splash, spill around and burn you up like the blood of the beast in Alien. Dry cells don't emit Hindenburg burning hydrogen gas either. Hawker Energy, Braille and Odyssey make lightweight compact dry cell batteries and Optima makes full-size dry cell batteries. Most of these companies make sturdy billet or sheetmetal-type mounts suitable for proper mounting.
Lead-acid batteries are also very heavy. A typical stock battery for a four-cylinder compact weighs about 40 pounds and a battery that cranks over a large V-8 easily weighs about 50 pounds. A dry cell battery that can power your street car like an Odyssey 680 or a Braille B14115 weights only 11-15 pounds. Using one of these batteries is an easy way to instantly shave 30-40 pounds from your car as well as improve weight distribution. Smaller batteries like these aren't advisable if you're running a lot of lights, such as rally or night enduro racing or if you're into having large amounts of I.C.E. in your car. In this case the companies that sell dry cell racing batteries have units weighing about 20-25 pounds- half the weight of the stock batteries with superior power capability.
For cable, we recommend a copper conductor heavy-gauge wire since starter motors have the highest amperage draw of any electrical part in a car. We prefer welding cable as it has very low resistance. We use 0-gauge for truck-mounted batteries due to its low resistance. If you have a tough-to-spin, big high-compression V-8 motor, you might opt for the thicker even lower resistance 00- gauge wire. You can look at aircraft supply or marine supply houses for terminal ends and crimpers to terminate your cables properly. Dry cell battery makers also usually sell these items as accessories.
It is important to route the wire correctly. Remember that if the wire's insulation wears or chafes, you'll have a major short circuit and the potential for a really nasty electrical fire. In most cases we prefer to route the main power wire through the interior of the car so it's less likely to be damaged. It's important to secure the wire in many places using captive zip ties and/or rubber-lined adel clamps so that it can't rub and flop around. Also, use a circuit breaker like the type found in car audio, marine and aircraft supply stores or heavy-duty marine slow-blowing fuses to reduce the damage if something does cause the wire to short.
Relocating the battery is a good way to spend an afternoon that will give you a big gain in handling and can reduce overall weight for very little money.