Batteries are linked as main and back-up. Need to be able to start the boat and also run stereo, fridge and electrical when anchored. Also need back-up starting power if main battery is drained. Concerned that a deep cycle battery in the number two spot won't give me back up starting power. Also don't know if I can mix cranking, dual, deep cycle in this configuration. Boat is a Crownline 275CCR.
You can, as long as they are the same type, flooded,gel or AGM ,on my boat I have 3 batteries 2 group 27 deep cycle/starting batteries @850 mca each, and 1 group 24 @ 1000 mca, and I can last 3 days without a charge with my fridge on and limited use of lights, I pack my fridge full (full fridge keeps colder longer with less cycles) I might run the engine for a short time not only to chare the batteries but also for hot water,now the thing to help battery longevity is make sure you chech the water in them ,I do every month,and only use distilled water,and I never leave them on the boat over the winter,and my batteries are 10 years old , good luck :-)
What myles said.
I like AGM dual purpose batteries for small boats. Leave the "dedicated" deep-cycle batts for big cruisers.
I would prefer AGM but they are EXPENSIVE, and require a different type of charger if you want them to last, so he would have all those added costs, I had the same setup on my 26ft and it worked great as well, a little tight for room in the engine room, but worked great, we could sit on one of the Thousand Islands for 3/4 days with out running for power, as now they don't allow generators,and running of engines, when your on the Islands, to keep your batteries charged, cheers :-)
Its Gel cells that require special chargers and voltage regulators; not AGMS. AGMs dont self-discharge (hardly), are maintenance free and cant spill acid. They also pack more power in a smaller size. They last twice as long as flooded cell conventional batteries in our experience. All of this makes them a better buy...in our opinion.
Actually Kevin each type requires a specific charging voltages, and mixing battery types can result in under/ over-charging, also gel and AGM self discharge at the rate of 3% per month, flooded dual purpose and deep cycle flooded are at a rate of 6% per month, and a AGM group 24 has 735 mca, and the group 27 is 955 mca, while the flooded dual purpose is 685mca, while the deep cycle flooded is 625mca, my point was if you don't want to start or go the the expense of changing all the batteries to the same type and the charger for it stick with the ones you got.
Your right about not mixing battery types for ease of charging etc. My point was you do not need special chargers or a special voltage regulator for AGM batts: the same that work for flooded electrolyte work for AGM.
Now, Gel Cel or Lithium Ion batteries do requires special charging equipment
Personally (and without turning this into a brand name plug) I have a pair of AGMS that have been is use for 8 years now. They are stored 6 mos per year and self-discharge at less than 2-percent per month. I don't even bother charging them over the winter. Just reinstall in the boat and they have been good to go. Average terminal voltage each April, according to my log, is 12.4 volts. I have never gotten more than 4 years out of a conventional battery without degraded performance. So right there, the cost is justified. Add in no need to check electrolyte levels, run a charger,no acid to spill,..... well, they are tough to beat.
Your mileage may vary.
So what you are saying is this,I have a customer come into my shop and I put it to them like this , spend $1,000.00 on batteries, or spend $200.00 which do you think the customer will say to go with, and I can guarantee you where they will tell me to go with the first choice, I give them the option but tell them the second choice first and tell them to check the water at least once a season and if they don't feel comfortable doing it I will do it for them, and it always turns in my favour as most appreciate the little added touch and they will always come to me for parts and service, because I'm honest and truthfull about what they need and when they need to do it and most of them come back because of that.I agree that AGM batteries are good but not worth the money and most people FREAK at the price of them, I don't carry them because they DON"T sell because of the price, no matter what the benefits are.
I 'm not trying to tell you how to run your business. I'm not telling you you are wrong. There is usually more than one way to do something right. And this is a perfect example. I am just sharing my experience.
For what its worth a quick google search just revealed prices for Group 31 AGMS from name brand manufacturers ( basically size of a group 24 flooded cell) at around $230 each.
This conversation was very helpful. I was having the same argument with myself between flooded cell and AGM. In the end, I opted for the no maintenance option. I purchased 2 dual purpose AGM batteries from Bass Pro (made by Deka) for $182 each. I didn't think that was a bad price for AGM. Plan to install them this weekend. Thanks for the information.