Bosch M Series Battery: When selecting a marine battery, you need to calculate the power requirements for engine starting, as well as the current draw of your on board electrical accessories. For light moderate duty, low amp draw requirement, choose from the Bosch M3 or M5 BDP dual purpose battery ranges, specifically designed to handle both starting and light cycling. For heavy duty cycling, choose from the Bosch deep cycle M4 or M6 battery ranges. They provide enough cranking amperage to start your engine and the most reserve power to keep your accessories running longer. You may find you need more than one battery to satisfy your boat power requirements. In this instance, you may utilise one battery as an engine starter, and another as a deep cycle to maintain your equipment.