Keep the cells filled to the split ring (except sealed maintenance-free batteries) but not any higher. You’ll know when they’re filled by the almond-shaped “eye” reflection produced when the water reaches the level of the split ring. Water levels have to be topped up periodically say, every six months. Use distilled water ONLY.
Keep the battery terminals and clamps clean. Use a wire brush to clean the inside of the clamps and outside of battery posts as necessary, typically every three months or so. If you don’t, one day the car will start just fine and you’ll leave home to pick up a paper and when you try to start the car it won’t even click let alone crank over. The number one cause is build-up between the battery posts and the clamps, and this is true with any car, not just RX7s. Undo the terminals, clean them up with a wire brush (or better yet, a battery post cleaner) re-attach and you’re good to go.
You can keep this build-up from forming in the first place by smearing the terminals with dielectric grease or “plastic spray,” which keeps the battery fumes from coming in contact with the terminals and corroding them.