My battery in the back is a cranking battery. It is 9yrs old and still going strong.I am going to replace it soon because of it’s age. It not only cranks the motor but also supplies voltage to the live well and the electronics.
What battery would you buy, cranking or duel purpose? My thoughts are to go with the duel purpose, however as you can see the cranking has done an excellent job.
cranking 
Short trips (less than 4 hours) cranking… Long trips = Dual Group 29
what motor are you starting with it? Is it hooked to an on board charger? The main difference between the different styles is the amount of surface area on the plates. Pure starting batteries have the most surface area for larger initial surge of amps. Dual purpose bateries have less surface area for a lower discharge rate meaning they will not give as large of an initial surge but will discharge longer. If a dual purpose has enough amps to start your motor you can get a longer run time on your livewell and electronics with that. If you have a large and or high compression motor you may need to stick with the starting battery.
Sorry, it is starting a Yamaha 40 hp. (very easy start.)The battery is a series 24 and will be replaced with a series 24.
I would go with the dual purpose but on the other side of it don’t change what isn’t broke. I think either way you will have good luck as long as you get a good battery and take care of it (check and maintain the water level and keep it charged over winter).
Just did the same on my 60 hp Merc. Went from a group 24 cranking to dual purpose because my application will benefit more from the longer run time than higher CCA. Different case with a motor requiring more juice, but mine didn’t need it.
I would get a dual purpose, or even add another battery. Put a group 24 dual purpose in for the electronics and livewells and all of that, then use the cranking battery for the motor. If you ever find yourself in a bind you can hook up the dual purpose in 2 minutes.
Don’t overlook a straight deep cycle. Lots of Amp Hours and designed to be recharged many times over. More CCA’s than that 40hp will ever need.
I have had the boat and motor since 05. A series 27 deep cycle is in the front for the trolling motor. No matter how that battery gets used, It has never lasted over 3 yrs. Yes sometimes it gets run down all the way! I always charge it after each use and maintain the charge through out the year. Just saying the cranking battery lasts and the deep cycle does not. The only difference is the deep cycle gets used a little harder.
I just replaced it last week. I’m guessing the battery had 50% life left.(3yrs old) I went with a series 29 this time.
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