Marine dual-purpose and boat battery questions

I’m looking for suggestions on what would be the best fit for replacing my boat starting battery? I have a Glastron 185 Fish-Ski with a 3.0L Volvo Penta in it. I have mostly standard electronics in it like a stereo, Depth finder, live well, lights, etc. nothing to big or over the top.
The previous owner had a car battery as the starting battery. Believe it or not it lasted about 6 years. I believe that I probably should go with a Marine dual-purpose battery, and it looks like I should even focus on a AGM design battery. I know Optimus is one good brand, but I’d like to hear some pros and cons of Optimus and other brands as well.

Also as I am going from a 12v trolling to a 24v system. I have a 2 bank battery charger. Now that the deep cycles are wired in series, do I put only one charger wire on one deep cycle battery and the other on the new dual purpose battery?

I would recommend finding out what the CCA are for your motor.

Then look for the biggest deep cycle that will easily start your motor. All the major brand deep cycles claim to be dual purpose now.

Wire each of your 2 batteries in the 24v system like they are standalone 12v batteries. You will use both banks of your 2 bank charger to charge that system.

Mr Sticks wrote:

Also as I am going from a 12v trolling to a 24v system. I have a 2 bank battery charger. Now that the deep cycles are wired in series, do I put only one charger wire on one deep cycle battery and the other on the new dual purpose battery?

You will need a charging bank for every battery in your boat. I very highly recommend you have 3 batteries. One for starting and electronics and the pair for your trolling motor only.

If you use one battery in the series for starting you’ll have one battery that will die long before the other. Also you really don’t want to risk draining your starting battery with a trolling motor.

I would use standard deep cycle group 29,30 or 31 for the 24 volt trolling motor (those group batteries are all the same rating). Use your 2 bank charger for them. Get a duel starting/deep cycle and a 1 bank charger for starting and other electronics. If you are going from 12 volt to 24 volt trolling motor you should replace with matched batteries ( both the same size and age). Most batteries are made by 3 company’s so you don’t have to spend $500.00 on a battery. I’ve had good luck with Fleet Farm Road Runner batteries.

Thanks, I did just get 2 matching 12v deep cycle Road Runners from Fleet farm, so they match now. I did cave into a closeout dual purpose battery from Menards, but it is only a group 24. Looks like I could have easily fit a group 27 battery into the holder in the boat as space isn’t really an issue for me. Not sure if I should return it or wait and see.

Return it and get the biggest battery that will fit.

Question, is bigger always better for a starting battery? Not joking, curious. I’m all in for bigger is better for trolling and electronics for obvious reasons, but curious if necessary for main motor starting battery…i suppose if you have short or little run time it could be an issue but if you have an alternator, it should keep your starting battery good pending how much you run it. Anyone have a dead starting battery caused by too many starts? vs bad battery? Curious.

Regarding chargers, as long as you are using similar battery types (AGM/Flooded vs Gel) you can get away with a 3 bank charger. That’s the route I’d go vs 1 2-bank and 1 1-bank charger.

Starting batteries normally have all of the electronics and accessories, so yes, bigger is better unless you enjoy having to jump start yourself off of your trolling motor batteries.