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Old 11-04-2016, 06:10   #1
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Battery Size

I have an older (I mean 1983 old) Catalina 38 sailboat with 2 group 27 batteries. 665 cold cranking amps. I am wondering if it is ok to up the size to a group 31 with 700 CCA? Since the battery boxes will easily handle the size. 13 inch by 6.8.
Any harm done to the electrical system? This is one area I am very dull on.
I dunno much about these.


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Old 11-04-2016, 06:51   #2
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Battery Size

Tons of threads on boat batteries. Look for posts (or links to posts/websites) by Mainesail.

My 2 cents - 6 volt deep cycle batteries are much better than car batteries for boat applications, so you'll get more bang for the buck by switching to those rather than just upsizing your existing batteries. But upsizing your batteries won't hurt anything, either.

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Old 11-04-2016, 06:57   #3
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Re: Battery Size

Short answer: yes, you can substitute two Group 31 batteries for two Group 27s.

Since you mention cranking amps, we all may assume these are engine starting batteries?

Or perhaps they do double duty as starting batteries and house batteries? If the latter, you might want to look at "dual-purpose" Group 31s, or perhaps those labeled as "marine deep cycle" batteries. (Others will likely comment the label is an erroneous marketing term, but OTOH, these could be a slightly better choice for you if they also service house loads.)

Or... almost any reasonable battery pair will probably start your auxiliary engine.... so if these are both starting and house batteries, the comment about a pair of 6V golf cart batteries could be well worth a look. They would truly be "deep cycle" batteries (better for house loads) and will probably provide you enough cranking amps to start the engine. (Compare battery cranking amps to your engine minimum specs for cranking amps.)

As others have said, lots o' threads around here about batteries...

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Old 11-04-2016, 12:34   #4
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Re: Battery Size

No harm will be done by upsizing, but before you decide what to upsize to, you need to consider a number of issue - as previous posters have said. Questions to answer:

What do you use the batteries for? House/starting or both?
Are they normally connected together or is one house and one starter?
What sort of engine are you starting?
What electrical loads do you need to power? (fridge? electronics? etc)
How many Amp hours are the current batteries rated at?
(That's much more relevant than CCA for house loads).
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Old 18-04-2016, 08:18   #5
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Re: Battery Size

Battery size is not relevant. The Voltage 12v or 24 v is to suit your installation. Upping size increases ( or should) the capacity, i.e. ability to provide power for longer ( at the same load). Provided you have the room and the connecting posts are the correct fitting. and orientation ( placing of posts relative to the nd s of the battery0 probably not a concern on a boat as usually have spare length of cable not the nigardly ones on cars.
Don't forget to measure height of battery to make sure you have clearance above it.
Hope this helpful.
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Old 18-04-2016, 08:25   #6
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Re: Battery Size

as long as they fit the box you're fine. The more the merrier. watch for height issues!
"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard

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Old 18-04-2016, 09:41   #7
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Re: Battery Size

Originally Posted by Cheechako View Post
as long as they fit the box you're fine. The more the merrier. watch for height issues!
Yep. No harm in having more battery capacity. Don't mix battery types (chemistry) and it's best if they are the same brand, model and age.
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Old 18-04-2016, 09:47   #8
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Re: Battery Size

Dual purpose batteries are good for neither purpose. Don't do it!

Group 31s are rarely true deep cycle batteries like golf carts would be, but they do work. American Battery or US Battery makes good ones.

None of us was born an electrician. We all learned. You could start here to learn:

Electrical Systems 101 Electrical Systems 101

Unless you are a marina hopper who has someone else do all the work on your boat, with a 38 foot boat you should (actually MUST) spend some time to learn this stuff. Most of us did.

The systems on your boat are almost identical to the ones on ours.

Good luck.
Stu Jackson
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Cowichan Bay, BC, (Maple Bay Marina) SR/FK, M25, Rocna 10 (22#) (NZ model)
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Old 18-04-2016, 09:52   #9
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Re: Battery Size

As someone who used group 27's for far too many years, I have found the GC2 6V golf cart batteries to be superior and far less expensive. Added bonus, the 6V battery that WM sells (same OEM) for $180 ish is $89 at costco.

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