Left Weight Right Weight Battery Location Front-to-Rear Weight Percentage Crossweight
Front Weight 686 627 Left rear of trunk 52/48 50.4%
Rear Weight 646 567

Although the weight bias is more to the heavier left side of the car, it's still much better than stock. The crossweight percentage is even better but this can be deceiving, as it could easily be zeroed out with some slight adjustments of the coilovers. The right rear of the trunk is the best place for the battery.

Sometimes you can't fit the battery in the trunk or you want to keep the car's polar moment of inertia low for quicker transitional handling. To visualize polar moment of inertia, imagine trying to twist a set of dumbbells back and forth from the handle. Sort of tough, right? Now imagine trying to twist the same weight in your hand if it was a compact sphere. This is much easier. A car, with its mass located closer to its center, will respond to steering input faster. So if you want to reduce your polar moment of inertia more than improving your overall weight distribution, you'll want to place the battery within the car's wheelbase.

Let's see what happens when the battery is placed behind the passenger seat.

Left Weight Right Weight Battery Location Front-to-Rear Weight Percentage Crossweight
Front Weight 687 651 Behind front passenger seat 53/47 50.4%
Rear Weight 622 567

The left-to-right wheel weights are considerably improved and the front-to-rear weight distribution is still improved by an impressive 1 percent-equivalent to about a 5-inch engine setback. Most drivers can feel a 1 percent change in weight distribution.

Placing the battery behind the driver seat resulted in these weights.

Left Weight Right Weight Battery Location Front-to-Rear Weight Percentage Crossweight
Front Weight 703 631 Behind driver seat 53/47 49.7%
Rear Weight 625 568

This method wasn't as good but it's still definitely way better than in the stock position. Now you can see that relocating the battery is as good as most expensive fabrication procedures. What's stopping you from doing it?

How It's Done
Many companies make and sell battery relocation kits. You can simply purchase and use one of these. There is a caveat though. Many kits, even those offered by famous and reputable companies, use flimsy stamped steel or thick-formed wire battery mounts and brittle plastic boxes for their kits. In an accident, the heavy battery can easily deform or break these mounts and bust out of a fragile battery box becoming a 40- to 50-pound missile filled with acid hurtling around the inside of the car. This sort of thinking can kill you instantly if it hits you in the wrong spot. A loose battery can strike you with melon-crushing force. We've seen this happen several times in racing accidents.