OEM Miata batteries are absorption glass mat.. Here's the WestCo replacement for it. Acid is suspended and immobilized in a fiberglass mesh between the plates. Small, light, lasts much longer than normal batteries, and they're safe, but they don't hold a hell of a lot of power. Since it's in the trunk, you don't want the acid of a normal battery leaking all over your stuff... and when a urban assault vehicle rear-ends you and crushes it, free acid flying everywhere or exploding on you wouldn't be good.

They require special treatment for charging, especially from a dead state.. low current (<= 1A) for long periods of time (18-24hrs); if you blast it with 20A like it's a regular battery you'll destroy it.