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Old 04-02-2018, 16:57   #1
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Group 31 or 2-6v for max starting capability

My boat had 2- 6v batteries in series as the start batteries for the Perkins 4-236. They are long dead and gone. Should I re-install a pair of 6Vs or can I do better re cranking juice with a single group 31
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Old 04-02-2018, 18:22   #2
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Re: Group 31 or 2-6v for max starting capability

Either will suffice. Both have advantages and disadvantages. No doubt others will express opinions but it really is a matter of preference as there is no single beat option.
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Old 04-02-2018, 18:42   #3
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Re: Group 31 or 2-6v for max starting capability

If the battery is solely used for engine starting, there is little advantage to deep cycle capabilities. I'd use the group 31 or even a 27 unless you operate in very low temperatures. Either would be less expensive than the GC batts, would weigh less and easily start your medium-small marine diesel.

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Old 04-02-2018, 18:44   #4
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Re: Group 31 or 2-6v for max starting capability

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Originally Posted by Rowglide View Post
My boat had 2- 6v batteries in series as the start batteries for the Perkins 4-236. They are long dead and gone. Should I re-install a pair of 6Vs or can I do better re cranking juice with a single group 31
Thanks
Jim
Depends what 6v batteries you had. If GC2, a pair of them will carry almost twice the capacity of a single group 31.

Typically a group 31 will not fit where a 6v battery was before without considerable rework.

As Jim Cate points out upthread any of these choices are overkill for just an engine starting battery. A group 24 will get the job done.
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Old 04-02-2018, 19:33   #5
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Re: Group 31 or 2-6v for max starting capability

Thanks
Just want to be sure I have more than enough Amps to crank engine as long as necessary in a worst case scenario (Cold Weather, Cold engine, lost prime...)without relying on using the house batteries.
Had a tough time with an old westerbeke 4-107 in a previous boat with an undersized battery... so I'd rather overkill where I can.
There is plenty of room for a group 31 in the battery space.
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Old 04-02-2018, 19:42   #6
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Re: Group 31 or 2-6v for max starting capability

the 6v are deep cycle, not ment for starting. get a starting battery for starting.

for exaple if you look at the t105 data sheet. they don't even list a CCA for them. it's blank.
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Old 05-02-2018, 03:40   #7
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Re: Group 31 or 2-6v for max starting capability

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Originally Posted by Rowglide View Post
Thanks
Just want to be sure I have more than enough Amps to crank engine as long as necessary in a worst case scenario (Cold Weather, Cold engine, lost prime...)without relying on using the house batteries.
Had a tough time with an old westerbeke 4-107 in a previous boat with an undersized battery... so I'd rather overkill where I can.
There is plenty of room for a group 31 in the battery space.

Shop for start batteries and compare CCA/MCA for several. All G31 start batteries aren't created equal. FWIW, Odyssey's PC-2150 (AGM) is one with really high cranking amps, possibly the highest out there... Not inexpensive, but our oldest bank lasted 12 seasons as dual house/start batteries. There are probably several good options...

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Old 05-02-2018, 04:00   #8
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Re: Group 31 or 2-6v for max starting capability

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FWIW, Odyssey's PC-2150 (AGM) is one with really high cranking amps, possibly the highest out there... Not inexpensive, but our oldest bank lasted 12 seasons as dual house/start batteries. There are probably several good options... -Chris
Eye watering is the way I would describe the price this side of the pond. I have gone back to using an ordinary Varta car engine battery rather than pay the Redflash Price again. Didn't notice any difference in starting.

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Old 05-02-2018, 08:48   #9
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Re: Group 31 or 2-6v for max starting capability

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Eye watering is the way I would describe the price this side of the pond.

Yep, and not really necessary for most sailboat engines. I just mentioned it more as an example of a battery offering high cranking amps.

A single PC-2150 would start our big diesels, though... whereas not all single G31s would do that... so in our case it offered choices of 1 battery versus more batteries in a start bank.

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Old 05-02-2018, 09:05   #10
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Re: Group 31 or 2-6v for max starting capability

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If the battery is solely used for engine starting, there is little advantage to deep cycle capabilities. I'd use the group 31 or even a 27 unless you operate in very low temperatures. Either would be less expensive than the GC batts, would weigh less and easily start your medium-small marine diesel.

Jim
I agree with this. If you sail without motoring for some time or leave the boat on a mooring, you may opt for an AGM start battery of the right size as they hold a charge longer than FLA and it may be convenient to lay it on its side.
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Old 05-02-2018, 09:08   #11
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Re: Group 31 or 2-6v for max starting capability

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Eye watering is the way I would describe the price this side of the pond. I have gone back to using an ordinary Varta car engine battery rather than pay the Redflash Price again. Didn't notice any difference in starting.

Pete
A great deal depends on the size of the battery cabling, their length, the charge regimen and the starter draw. Optimize all those and you can use a smaller battery for a start battery. Even a Group 24 with 2/0 ga. cabling to the starter will turn it a few dozen times without complaint from a full or nearly full state of charge.
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Old 05-02-2018, 10:27   #12
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Re: Group 31 or 2-6v for max starting capability

Quote: "Had a tough time with an old westerbeke 4-107 in a previous boat with an undersized battery... so I'd rather overkill where I can."

Fair enuff, but what makes you say that the batt was undersized? Was it t'other way about: That the engine was not a willing starter?

One of our members has contributed a nicely quantified test report on this matter of juice-use for starting. Look it up in the archives. The conclusion is that juice-use for a start is so little, and the replenishment time when charging from the engine's alternator is so short that it is perfectly doable and safe to start OFF THE HOUSE BATTERIES, i.e. with an engine in good condition you don't need a brute of a starter battery at all. As Jim sez: In your case, IF the engine is in good nick, a 27 series FLA will be more than adequate.

The upshot of that is that if your engine is does NOT start with alacrity every single time you hit the button - after having followed procedure - then it is not the battery you should be blaming and looking at, but the engine and its associated fuel, aspiration and electrical systems

In TP, which is only a baby and had a poor electrical system when she came to us, I start off the house batts. They are now 31 series deep cycle FLA batts. For house functions I use Batt#1 on odd dates and #2 on even dates. That means that if the "in use" battery is getting low, I can start of the "idle" batt and soon have both the "active" and the "idle" batt up to snuff again.

TP
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Old 05-02-2018, 10:58   #13
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Re: Group 31 or 2-6v for max starting capability

I have four AGMs and a Start Battery. The four AGMs are connected thru blocking diodes to two battery switches. The Start Battery is connected to a Battery switch, no diode. The outputs of the three Battery switches are all connected to a common bus feeding the House, the anchor winch and the start circuits. The arrangement allows me to put any battery arrangement on line at any time. The start battery is charged by solar power. The AGMs are kept charged at dock by a charger through two dividers. At sea the alternator charges the AGMs through the same dividers. Works great with no issues.
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Old 05-02-2018, 13:01   #14
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Re: Group 31 or 2-6v for max starting capability

Based on all research I have ever seen 2 GC2 batteries will outlast and out power any other type of battery including AGM. in the same installation footprint. In the US they go for approximately $100. each at an automotive distributer. I have used 2 in series to start a Detroit 71 series in our old work boat for years. The down sides I see are they are heavy and worst issue is they are water hogs and require frequent maintenance.
The group 31 is the US industry standard for over the road trucks and can be purchased at most commercial truck resources for less than $80. That is cheaper than a group 34 battery at the automotive store.!
The last one we installed was 3 years ago in a 50 HP diesel farm tractor. It has not had an issue with hydration or of starting the diesel.
My future is pointed to the common group 31 as the batteries in my future. They do the job at less than have the cost and essentially NO maintenance. So they do not last as long as GC2's but have none of the headaches at less than 1/2 the cost.
I use group 31 AGM house batteries primarily because the do not out-gas much on cycling and to me safer in operation.
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