The original RX-8 battery had 305 cold cranking amps--totally inadequate and subsequently replaced with a 640 CCA model. If you have the original battery or just want a better designed battery, consider the Optima series.

The Optima Red Top Group 35 R battery (P/N 8020-164) is rated at 720 CCA and weighs 5 lbs. more than stock. Make sure the positive terminal is on the right side of the battery.

The Optima Yellow Top is designed for upgraded audio systems and other users who need increased deep cycle capabilities over the Red Top. The trade-off is less CCA (650) compared to the Red Top. Unfortunately I don't know if they make a reverse post version, which means you have to lengthen the terminal leads.

You will also need a 75/25/35 Optima adapter. The Optima battery is shorter than the OEM battery.

Step 1: Remove the battery cover.

Step 2: Use a 10mm socket to disconnect the battery terminals circled in red. Disconnect the negative terminal first, then the positive. Make sure you're not touching the car's chassis or other ground points when handling the positive terminal. I like to wear leather gloves as insulation.

Step 3: Use a 10mm deep socket to loosen the battery tie-downs circled in green. The tie-downs hook onto a plastic tab at the bottom so just loosen it enough to unhook from the tab.

Step 4: Remove the OEM battery using the nylon strap provided.

Step 5: Insert the Optima battery spacer. You don't need the bottom part (shown on the left) so remove and discard that part. There's supposed to be a white line that shows where the front edge of the battery should be to maintain proper airflow. I couldn't find it so I just placed it right in the middle.

Step 6: Drop the Optima battery down, making sure it's directly above the spacer.

Step 7: Reattach the tie-down hooks. The hook on the right points towards the front of the car. The left hook points towards the back of the car. Two hoses are in the way and makes it difficult to see. Use a flashlight and you should be able to locate the tab.

Step 8: Reattach the positive battery terminal, then the negative one. Put the battery cover back on.

Step 9: Start the car. The DSC light will blink since you disconnected power to the car. Turn the steering wheel all the way to the left, then all the way to the right. The DSC light should then turn off. You're done!