Does Size Matter?

When I bought a 95 Ski Sanger DLX20 equipped with a 350 Mag Tournament Ski TBI, it came with a Group24 size battery that needed replacing. I bought another G24 battery with the 550 CCA recommended by the Merc Service Manual. Now that battery is on its last legs. Does the physical size of the battery matter? I don't know what size battery came with the boat from the factory. There's room for a larger battery, but is there any advantage to buying one?

Many thanks for yr help, PW

bigger means heavier and that may not be desirable on a ski boat. The min battery rating is only for the engine...if you use accessories, you will need to factor that into the battery size as well. The other thing to check is your battery cable(s) - size and condition...

How long have your batteries lasted?? the batteries I've seen in this century usually fail 'hard'...depending upon the brand 4-5 years is about the extent of it...

Finally, marine batteries are usually constructed with thicker plates than auto batteries...and you typically get what you pay for...

makomark said:

bigger means heavier and that may not be desirable on a ski boat. The min battery rating is only for the engine...if you use accessories, you will need to factor that into the battery size as well. The other thing to check is your battery cable(s) - size and condition...

How long have your batteries lasted?? the batteries I've seen in this century usually fail 'hard'...depending upon the brand 4-5 years is about the extent of it...

Finally, marine batteries are usually constructed with thicker plates than auto batteries...and you typically get what you pay for...

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Thanks for your reply. Should have made it clear that I use marine starting batteries in both my ski and pontoon boats. Don't run accessories on my boat batteries. The ski boat is used for skiing/wakeboarding and NOT partying. (Hate those barges with DJ sound systems onboard.) I bring a boombox on the pontoon with it's own battery power. I usually get 4-5 seasons out of a boat battery before it dies. The pontoon batteries last a little longer because there's very little stress on them starting a 1984 Evinrude 30.

..."I usually get 4-5 seasons out of a boat battery before it dies."

That's pretty typical. Not sure why a car/ truck battery lasts far longer.

Jeff

The advantage of larger battery is that it doesn't have to work as hard to do the job. Your Group 24 is fine and a proper size for the engine and the boat since you're pretty much only running the engine and maybe lights occasionally. However, a larger battery will (generally speaking) last longer because you're not discharging it as much to do the same work. Group 31 is about the largest you would want as any bigger becomes harder to handle. A proper group 31 is around 70lbs.
I urge you to buy top quality. Deka, Odyssey and Trojan are the names to look for. I would not pay the extra for AGM type or Gel Cell, as these are often three times the price as regular lead-acid flooded and don't really last that much longer.

Considered the same question (have ski boat, 350 mag, barge sound system).

I just bought late this season a AGM G24 Odyssey (never installed). Is the G31 a larger battery in width/length (fit battery tray/fiberglass landing)? Would it be strange to have a starter and a house battery two different battery groups? Planning on a BlueSea ACR next season. Thanks

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sab said:

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Would it be strange to have a starter and a house battery two different battery groups?
Not at all.
Many cruisers are set up like that.
You would have a cranking battery on MBSS #1 and a Deep Cycle battery bank on MBSS #2.
Manual battery bank selection would be made via an MBSS (main battery selector switch).

Keep the battery chemistry the same and you won't have any charging issues.

Planning on a BlueSea ACR next season.
That would also be good!
Look at the BlueSea 7622 ML also.

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Without diverting the OP's thread too much, to answer the size question sab presents, the short answer is yes. The mansplaining answer is that you should really know what you need to power and for how long in order to size the house bank properly. There are plenty of calculators out there on the web to help you make the determination, but it boils down to how much you're willing to spend in order to run the lights, stereo, pumps, toilet, fridge, fish finder, radar, TV, hair dryer, George Foreman grill, etc etc.
The goal is to be able to operate all these devices without being in danger of flattening the start battery so that you can't get the engine running.
Good batteries are heavy. Buy by weight, not cranking amps or some other useless number. A group 24 start battery by Deka weighs in at about 45 lbs and the deep cycle group 31 weighs 60 lbs. So if you want two for your house loads that's 160 lbs of weight plus cabling. Another option is golf cart batteries, which are monsters. two of those 6V batteries weigh in at a staggering 134 lbs. That is heavy but you'd get roughly 16 hours at 25A draw.

Thanks Rick. According to what I found on-line
Group 24 (G24): 10.25 L x 6.8125 W x 8.875 H, 70-85 Amp hours (AH)
Group 31 (G31): 13 L x 6.8125 W x 9.4375 H, 95-125 AH

Need a new battery tray for length. Batteries/tray sit on a fiberglass molded (G24) flat landing. Never look at how the trays are mounted but assume an extra 3" of tray plastic length is O'K. I'm also assuming the cables and the battery tender can all be reused for a G31 AGM starter w/ Bluesea 7622 ML ACR. Its a summer lake boat so I can get away with murder.

O2- sorry posted same time...understood

Save your money on the AGM and buy regular lead-acid flooded. The only advantage of AGM is that you can stand it on end or lay it on its side. If you aren't gonna do that then there's no need for the extra cost. For starting battery you don't want to use deep cycle type. But, if you only use one battery for the whole boat, then consider a dual purpose type that has the open grid plates of a start battery but spaced with more distance between them. Deep cycle batteries don't have holes in the plates at all.
Not surprising, but there are regulations regarding marine battery installation. Some light reading there for ya. Basically says make sure they are securely installed. A box isn't required but it's a nice to have as it keeps the terminals away from possible contact.

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sab said:

Thanks Rick. According to what I found on-line
Group 24 (G24): 10.25 L x 6.8125 W x 8.875 H, 70-85 Amp hours (AH)
Group 31 (G31): 13 L x 6.8125 W x 9.4375 H, 95-125 AH

Need a new battery tray for length. Batteries/tray sit on a fiberglass molded (G24) flat landing. Never look at how the trays are mounted but assume an extra 3" of tray plastic length is O'K.
Regardless of the battery's physical size, you will want a dedicated starting battery for #1, and a Deep Cycle battery for #2.
The construction of each is different, and is specific for it's use.

The cranking battery is designed for a short term heavy amp load, whereas the Deep Cycle is designed for lower but longer duration amp loads.
If your #2 bank is to be used for the HLBB (house load battery bank), Deep Cycles will serve you best.

I'm also assuming the cables and the battery tender can all be reused for a G31 AGM starter w/ Bluesea 7622 ML ACR.
The 7622 ML is a 500 amp ACR with ML (magnetic latch) capability.
It will serve as an ACR, and will also have the ability to allow you to select and use the #2 bank, but it will bring in the SLBB (start load batt bank) during engine cranking.
In other words, if you use the ML feature, you will want to use heavy battery cables for it.


Its a summer lake boat so I can get away with murder.
Explain please!

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Bank 1- (driver side of boat) I run a cranking battery- NAPA Marine grade 625 CCA standard size what ever that is. I guess GR 24

Bank 2- (passenger side of boat) I run two (2) 900 amp Napa deep cycle marine batteries in parallel (1800 amps) for my sound system only, connected with 2 gauge 1600 copper strand welding cable to audio amplifier. These are much bigger and heavier so they may be the GR 31 or one size bigger

I have a 95 amp battery isolator. Most stock Mercruiser charging is ~ 55-65 amp alternator.

I use 4 gauge wire to all battery and ground connections. I also replaced the alternator 10 gauge output wire with 4 gauge batter cable.

I use a battery isolator (device with DIODES) as it charges both at the same time but the batteries do not "talk" to each other. The only thing in common is engine ground.

Been running this for over 5 years. I can crank the tunes all day and never worry about starting the engine and never have to turn a switch from one bank to the other.

I also have a microprocessor controlled two bank battery charger (6 amp max per bank) which plugs in AC for when at the dock to keep all batteries up to full charge.

Batteries are standard chemistry, lead acid wet.

thanks for the input all. I'm really tired of moving cushions to get to the battery selector switch. I'm also not always onboard when "they" leave the dock so I was really hoping in the future for no touch battery management

There's some debate online about Battery Isolators vs Automatic Charging Relays (ACR's) but regarding G24 vs G31 battery size I think my questions got answered

Thanks
Steve

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sab said:

I'm really tired of moving cushions to get to the battery selector switch.
In my opinion, an MBSS should be located where it is easy to access, not only for easy battery management, but also for an emergency 12 volt shut down should the need arise.

I'm also not always onboard when "they" leave the dock so I was really hoping in the future for no touch battery management
The 7622 ML will solve that issue.

There's some debate online about Battery Isolators vs Automatic Charging Relays (ACR's)
Anyone who believes that an Isolator is a good or even better option, is point blank incorrect!

but regarding G24 vs G31 battery size I think my questions got answered

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RicardoMarine said:

A true Deep Cycle battery capacity is measured in Amp Hours, not Cold Cranking Amps.
A total of 1,800 amps would represent CCA, of which is rather meaningless in terms or powering auxiliary "house load" items such as lights, stereo, amplifier, etc.

A run-of-mill 55 or 65 amp marine alternator will cut back on the charge rate rather quickly. You might see 55 amps for a very short duration only.

Battery isolators (devices using DIODES) are old school by today's standards. They also offer an inherent .7 voltage loss.

I would suggest that you stick to your plan and go with an ACR or a VSR ....... that would be a much better choice.

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What is meaningless? Any post from you.

Amp/hrs for a engine cranking battery? try finding that on a battery label............Virtually all batteries are labeled by CCA. If you get that info then that is a bonus

There is no correlation between the CCA and Ah. Therefore you can’t convert it directly to get the exact value. However, you can get a rule of thumb value for most batteries, even if it’s not an exact value, depending on the battery.

The rule of thumb for converting CCA to Ah is dividing it by a 7.25 constant.



For instance, if your battery is marked with a 900 CCA, it represents 124 Ah. A battery of this rating should last for 25 hours while producing power of 4 amps.




Millions of "run of the mill" 55-65 amp Marine alternators are currently in service. 99% of all small pleasure boats with Mercruiser have exactly these. So another stupid comment of no value...

Battery isolators may be old school but they are simple and they work and a .7 amp loss is not worth mentioning!
ACR and VSR may be newer technology but they can fail as they have relays and sensing circuits...

Another note- There are only a few battery manufacturers so many brands are simple label changes.