Can I use a half-size battery in a Mazda RX-7 FD?

Yes, you can absolutely use a half-size or compact battery in an FD RX-7. This is a very common modification to free up engine bay space for custom intercoolers (like a V-mount or large SMIC) or to reduce weight.

Difficulty: Easy to Moderate (Depending on mounting solutions)

Here are the most proven compact battery options and what you need to know before making the switch:

Top Compact Battery Options

• Odyssey PC925 (or Hawker 925): This is one of the most highly recommended AGM/dry-cell options. It provides 470 CCA, is half the size and weight of a standard battery, and is very tolerant of charging system variations. Some owners have reported these lasting 8 to 10 years in an FD ². The Odyssey PC680 is an even smaller alternative but has less capacity.
• Optima 51R: This is a slightly narrower battery than the factory Group 35. It offers a good balance of size and cranking power (450 CCA for the deep cycle version) ².
• Miata AGM Batteries (Westco 12V31M / Deka): These are direct replacements for the NB Miata and fit the FD well without cutting. However, their reserve capacity is quite low. They are not recommended if you have aftermarket gauges, heavy audio equipment, or if the car frequently sits unused ².
• Lithium-Iron (Shorai LFX36L3-BS12): If you are looking for extreme weight savings, this battery weighs under 5 lbs and delivers 540 CCA. Note that Lithium-Iron batteries require a proprietary Shorai charger ².

Mounting Requirements

You cannot safely secure a half-size battery using the stock battery box and tie-downs. You will need an aftermarket battery tray (such as the TurboJeff tray, which is highly regarded for the 51R) or a specific bracket assembly ². For the Odyssey batteries, specific billet mounts are available from suppliers like Summit Racing (e.g., part ODY-HDB680 or ODY-HDB925).

Important Considerations

• Heat Stress: The lifespan of any battery located in the FD's engine compartment will be substantially reduced due to extreme heat stress ².
• Parasitic Drain & Tenders: Smaller batteries lack the reserve capacity of an OEM-sized battery. If your FD sits for more than a few days, the factory alarm system and normal parasitic draw will drain it enough to cause starting issues. Keeping the car on a battery tender is highly recommended and can double the battery's useful life ².
• Cranking Retries: If your car is heavily flooded, having starting issues, or is fresh off a rebuild and requires tuning, a smaller battery gives you significantly fewer cranking retries before going dead ².