The 8D marine battery is a very popular BCI group size. It can be overwhelming as you attempt to compare and decide what is the best brand and style for your application. Generally speaking these batteries will range between 225 amp hour (Ah) and 255Ah, 1200 cold cranking amps (CCA) and 1400 CCA and 430 reserve capacity (RC) and 550 RC. It depends on whether it is a thick plate deep cycle or thin plate stop/start battery or a dual purpose combination. We prefer the AGM Deep Cycle battery. The dimensions should be roughly 20.75 x 10.75 x 10.0 inches.