GuffinsBay a deep cycle wet cell battery (the most common type of battery) is not made to hold up to the sudden need for power like starting a motor. Deep cycle batteries are made for the long constant draw of power like a trolling motor uses. A starting battery is made for that sudden burst of power to start a motor. That is why your deep cycle used for a starting battery died so fast. If you use an AGM or Gel battery it can be used for both starting and trolling.