How long will a Mazda RX-7 FD run with a new battery and a bad alternator?

The exact time or distance your FD RX-7 will run on a new battery with a bad alternator varies depending on whether the alternator has failed completely or is just putting out a weak charge.

In one reported case with a 1993 RX-7, an owner was able to drive for 3 to 4 months on a new battery after their alternator's internal voltage regulator was diagnosed as bad before the battery finally died ². This suggests the alternator was still providing a partial charge.

However, if your alternator has suffered a total failure and is providing absolutely zero charge, the reserve capacity of the battery will drain very quickly. For reference, an owner of an older 1979 RX-7 reported driving only about 20 miles on a new battery with a faulty charging system before the car failed to restart ¹.

Recommended Next Steps (Difficulty: Easy)

Relying on battery reserve power alone will eventually leave you stranded. To properly address the issue, I recommend the following:

1. Verify the Alternator: The FD RX-7 alternator features an internal voltage regulator. If you suspect it is failing, remove the unit and have it load-tested at a local auto parts store, or perform the specific alternator checks outlined in the Factory Service Manual (FSM) ².
2. Check for Wiring Shorts: If you are experiencing sudden dead batteries and your car is lowered, inspect the driver's side (US) front fender well. It is common for the tire to rub through the fender liner and damage the wiring harness, which causes significant electrical issues and parasitic drains ⁴.
3. Inspect Grounds and Accessories: Ensure your engine and chassis grounds are secure. Additionally, if you have non-stock wiring such as elaborate audio installations or turbo timers, verify they are properly wired, as these are common sources of electrical drain ².