The D31-series batteries are the largest made by Optima. The only differences are in the type of electrical connections and the color. There is no difference in the internal construction or performance of these three batteries; they are electrically identical. The D31A has standard automotive (“SAE”) posts for regular automotive-type cable end clamps. (The D31A does not have threaded studs.) The D31T has 3/8″ threaded stainless steel studs for flat connectors held on by provided wing nuts. (The D31T does not have SAE posts.) The D31M has both SAE posts and 5/16″ threaded stainless steel studs with wing nuts. The D31-series batteries are deep cycle batteries and they ALSO function superbly as starter batteries. In fact, they are Optima’s largest starter batteries, providing the most powerful Optima battery choice for either starting or deep cycle use. Multiple D31-series Optima batteries can be connected in series or in parallel for high amperage needs.