Deep cycle/dual purpose/start battery

[mr300z87]

As we approach boating season I was thinking of replacing 1 of 3 batteries on our 90 310 Sundancer twin 7.4L inboards. We used her all last season with the 3 unknown batteries that came in her when we bought her in 11/21. She currently has 2 battery banks 1 consisting of 2 group 27 marine start batteries. This bank start the starboard engine and handles most of the house. Not sure why the PO used start batteries. Bank 2 is currently a single group 31 dual purpose (DP) and it starts port engine and thats about it that i know of other than the memory for the radio. Yesterday I read 12.9 volt on all 3 they have been disconnected all winter no charger. Since this an older boat without much technology I plan to stick with standard lead acid batteries. I normally operate the boat on bank 1 either battery 1 or 2 (alternating between them), and obviously bank 2 on. When just hanging at the dock only bank 1 is on. We are day cruisers with maybe an over night on the hook although we have not done that yet, I do not have generator.

Battery specs in the Sea Ray owner manual I have are group 27 600 CCA and 156 min reserve. All the group 27 batteries I looked at both DC and DP 650 CCA and 190 reserve, so what is the difference?

Since I do not know the age of the 2 on bank 1 and the bank 2 battery has a 4/19 date. My thought is based on the Sea Ray spec either 3 DP or DC group 27s is what it should have and plan is to replace 1 battery per year so they all so they all don't die at the same time and to ease the $ pain. Also thinking I could move the group 31 DP and a new group 27 DC as bank 1 and put one of the start batteries on bank 2 since it only needs to start the port engine. I would think the house benefit from the reserve of the DC type batteries.

I'm hoping the experts can help me better understand the difference between the 3 types available, what all the specs mean, as well as the best type for my application. As always any information is greatly appreciated.

 

[airshot]

In my opinion, if it were my boat, I would have a group 24 marine starting battery for each engine. Then I would have a group 31 deep cycle as a house battery. If you use a lot of battery power ( house battery) then I might consider using two group 27's as house batteries. 2 bank charger for starting batteries and a deep cycle charger for the house battery so everything can be charged while moored. This way each battery has its own job to do and own charger to keep full. Deep cycle batteries do better when charged on a dedicated charger, rather than the alternator on your engine. Many different ways to skin a cat, your boat your call.

 

[dingbat]

I run Grp. 27 RV deep cycles for both start and house positions for their storage capacity and their ability to tolerate being (deep) discharged repeatedly. Deep discharge a starting battery once or twice and you’re done.

A dual purpose battery is a compromise between a starting battery and a deep cycle. Does neither function particularly well

 

[mr300z87]

Thanks for the replies. Airshot, I would love to set it up the way you explaine as it totally makes sense and I don't understand why SR didn't build that way. However that would take considerable time and probably expense to separate the circuits and untangle Sea Ray's 30 plus year old wiring. The boat still has its original pro mariner 3 bank 30amp charger, If it ain't broke don't fix it. Dingbat, assuming the group 27 DC have the umph to start 7.4L which they should, that is the way I will go at least for bank 1 (starboard engine and house). I could save a few $s on bank 2 and just get a group 24 start battery. I will look more into what items if any are powered by bank 2.

I wanted to add, this is a cruiser, our general usage consists of start engines cruise to hang out spot, hang for couple hours, then start engine and head back to dock where she is connected to shore power. Power draws while at anchor are minimal and consist of the radio, the refrigerator, and an occasional flush of the vacflush toilet.

Part of the reason for this thread, on my previous boat 1990 250 Sundancer with single 7.4L I had 1 battery bank consisting of 1 27 group DC and 1 group 24 start battery. The problem I had was the DC battery(even brand new) would not turn over the engine and I would have to keep the battery switch in both position to start the engine. I figured it was something with the wiring even though I replaced all the main 00 gauge battery wires and the switch. Lazy me never spent the time to track it down.

I got about a month before launch to sort this out. Looking forward to our 2nd season on our old Dancer Seahorse at a new marina after 6 seasons at the old one. We hated to leave but they had a NO dog policy and we want to bring our 2 Aussies with us.

Enjoy Sunday
Mike

 

[dingbat]

I could save a few $s on bank 2 and just get a group 24 start battery.

Not really......
While you may save a couple of bucks upfront, the life expectancy of a marine starting battery is about half that of a properly maintained deep cycle.
I run Interstate DC27 that comes with a full 2 year warranty.

 

[airshot]

When it comes to boating....saving a few dollars now, will cost you more dollars down the road....proven everyday by new boat owners!!!