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Old 10-08-2019, 05:49   #1
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12v vs. 2x6v Batteries for Sailboat

I have an older sailboat with a very simple electrical system. My house batteries are shot and need replacing. I would like some guidance on getting new batteries.

The environment for these batteries is simple: I have a small diesel motor with a 35 amp alternator. Presently, I have two group 24 12v batteries, in parallel, as my house batteries, which I normally use all the time, including starting - the motor does not require much power to start. I also have a small, 12v, garden tractor-type battery that is isolated with a off-1-both-2 switch as a backup starting battery.

My battery requirements are not huge. The boat does not have any heavy load items such as a microwave or even radar. On long sailing runs I run a small chart plotter, depth sounder, knot meter, VHF and AIS receiver. I rarely sail at night requiring lights. On anchor we typically run a few lights and often have a cell phone or two charging. When cruising, we usually move at least every other day providing sufficient charging time. I don't want to have to do a daily charge of the batteries with an idling motor as it takes too long and the motor is too noisy.

Given that both of the group 24 batteries are bad (one is totally shot and is not presently wired into the circuit, the other will only maintain 12.4v), I have an opportunity to replace both at the same time. I am considering getting two Duracel SLIGC110 batteries, which will provide more amps (12v=2x75 vs. 6vx2=215) and presumably have more cycle life in them than the group 24s. Either solution costs about $160. (Note that my battery box, which I will check, should fit the 6v batteries, but it is too small to fit group 27 batteries.)

My question relates to charging and management of two six volt batteries. I have no special battery maintenance or load balancing electronics and do not want to equip the boat with them. Also, I have the issue of a small charging capacity - 35 amps. Will two six volt batteries charge sufficiently in this environment? Does it really make any difference other than how much time is required to fully charge the batteries? What issues am I unaware of and likely to run into?
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Old 10-08-2019, 06:12   #2
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12v vs. 2x6v Batteries for Sailboat

The only significance between 6V “Golf cart” batteries and any relatively inexpensive 12V batteries is the 6V are deep cycle and the 12V are not.

When you hook two 6V batteries together in series, you have a 12V battery, there is absolutely no difference, no special equipment etc required, no change at all.
See lead acid batteries are made from individual 2V cells. A 12V battery has six cells in series making 12V, 6V batteries have three cells. So connecting two 6V batteries together you have a 12V battery. One advantage is it’s easier to carry than one large 12V.
But the real advantage is that being deep cycle batteries, they will last a lot longer doing what you want to do.
Don’t let anyone talk you into an AGM battery though, they way you use a battery they won’t last as long and will in my opinion be a waste of money
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Old 10-08-2019, 06:12   #3
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Re: 12v vs. 2x6v Batteries for Sailboat

Go with (2) 6v GC batteries.
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Old 10-08-2019, 06:23   #4
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Re: 12v vs. 2x6v Batteries for Sailboat

While you can buy a 12 v deep cycle battery, it’s a lot easier to lift 2 6 v batteries. Generally, the latter will have a greater capacity. Check out the weight and capacity ratings of a 4D or 8D 12 v deep cycle vs any 6v battery to understand the differences.
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Old 10-08-2019, 06:27   #5
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Re: 12v vs. 2x6v Batteries for Sailboat

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Originally Posted by a64pilot View Post
The only significance between 6V “Golf cart” batteries and any relatively inexpensive 12V batteries is the 6V are deep cycle and the 12V are not.

When you hook two 6V batteries together in series, you have a 12V battery, there is absolutely no difference, no special equipment etc required, no change at all.
See lead acid batteries are made from individual 2V cells. A 12V battery has six cells in series making 12V, 6V batteries have three cells. So connecting two 6V batteries together you have a 12V battery. One advantage is it’s easier to carry than one large 12V.
But the real advantage is that being deep cycle batteries, they will last a lot longer doing what you want to do.
Don’t let anyone talk you into an AGM battery though, they way you use a battery they won’t last as long and will in my opinion be a waste of money



What he said


For sure, go with the golf cart batteries!


These are true deep cycle batts. The added capacity will further extend their life, as the same amount of usage will give a shallower discharge. This is a no-brainer.


Be careful NOT to think that a basic 35 amp alternator will charge them properly, however. Lead batteries need a real full charge on a regular basis and will sulfate and lose capacity and die pretty soon if they don't get that.



You will need external multistage regulation to get a decent charge from the alternator, or maybe you can add a solar panel?




To answer your other question: There is no disadvantage whatsoever to using two 6v batts in series, vs. two 12v batts in parallel. On the contrary -- the two 6v batts have half the number of cells, which are twice the size. This is inherently more durable. Plus cell balance is much more likely to stay good with such a setup.
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Old 10-08-2019, 06:48   #6
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Re: 12v vs. 2x6v Batteries for Sailboat

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Originally Posted by Dockhead View Post
What he said


For sure, go with the golf cart batteries!


These are true deep cycle batts. The added capacity will further extend their life, as the same amount of usage will give a shallower discharge. This is a no-brainer.


Be careful NOT to think that a basic 35 amp alternator will charge them properly, however. Lead batteries need a real full charge on a regular basis and will sulfate and lose capacity and die pretty soon if they don't get that.



You will need external multistage regulation to get a decent charge from the alternator, or maybe you can add a solar panel?




To answer your other question: There is no disadvantage whatsoever to using two 6v batts in series, vs. two 12v batts in parallel. On the contrary -- the two 6v batts have half the number of cells, which are twice the size. This is inherently more durable. Plus cell balance is much more likely to stay good with such a setup.
And they are easy to get in and out.
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Old 10-08-2019, 06:54   #7
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Re: 12v vs. 2x6v Batteries for Sailboat

A group 24 is not a deep cycle. Some may have a sticking claiming that. But it’s still not. Get the 6v.

Make sure the backup battery is big enough. To start the engine. (Test it every so often)
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Old 10-08-2019, 07:20   #8
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Re: 12v vs. 2x6v Batteries for Sailboat

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Originally Posted by Homer Shannon View Post
Either solution costs about $160. (Note that my battery box, which I will check, should fit the 6v batteries, but it is too small to fit group 27 batteries.)
Well then the 2-6V batteries are better long term solution "in theory". But whether they will be better in real life is really going to be about your charging. Your charging by "moving every couple days" isn't really getting you batteries fully charged. In the end I doubt for you it is going to make any difference in use or long term cost which batteries you get and you should just get what is easier for you to do.
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Old 10-08-2019, 08:23   #9
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Re: 12v vs. 2x6v Batteries for Sailboat

Sounds like I should go with the two 6v batteries. I'll double check the space in the box - the 6v batteries are slightly wider - and get them installed. I noticed that the weight of two 6v batteries is 30 lbs more than two deep cycle G24 12v batteries. That is not significant to the boat, but it indicates that there is a lot more lead n the 6v battery than in the 12v battery.

Thanks for the input.
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Old 10-08-2019, 08:45   #10
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Re: 12v vs. 2x6v Batteries for Sailboat

Yeah, most 6V batteries have thicker plates and last longer, especially in a heavy draw down situation like golf carts. Compare ratings for storage though. Possibly longer life and higher storage capacity may not necessarily go hand in hand. Although probably do.
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Old 10-08-2019, 09:10   #11
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Re: 12v vs. 2x6v Batteries for Sailboat

Go with the golf cart batteries. You need to accept that they will not be charged fully when underway and that is totally OK, most people use them this way.

One of the advantages of having two 12V batteries is that when one fails down the road, you still have one providing 12V for starting, etc. If one of the golf cart batteries fails, your entire system is down. Most people who use golf cart batteries have 4x, so they can absorb a single battery failure. However, finding good deep cycle 12V batteries at reasonable cost is nearly impossible. Thus, go with golf cart batteries.

If you want redundancy in this setup your cheapest option may be a car starter kit or a small lithium/AGM battery. Your tractor battery is probably shot as well.

I used to have a 12V starting battery and it would always go bad after a few months (undercharging, sometimes used as a house battery, etc.). Then I switched to an AGM 35 AHr battery that I always keep charged via solar. It does start my 18 Hp motor (1000W starter), even if it hesitates sometimes.
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Old 10-08-2019, 09:13   #12
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Re: 12v vs. 2x6v Batteries for Sailboat

The Duracell 6 volts are made by deka/east penn, great price at sams club and are good true deep cycle batteries. Way to go it you have the room.

I just switched from 2 4ds to 4 6 volts. Increased ah, easier to deal with

Good luck
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Old 10-08-2019, 09:35   #13
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Re: 12v vs. 2x6v Batteries for Sailboat

I have found that the GC batts require much more water than the GP 27 12v I replaced, they out gas more so require more ventilation and they are to tall to fit in the 12 V bat box. Before purchase check for fitment, or be prepared to construct a new battery area. Another advantage of the GC batts is that they can be mounted a distance from each other for weight distribution. Since I only used 2 I located a two batt box specifically designed for the GC's. I would run AGM's if ventilation is an issue.
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Old 10-08-2019, 09:54   #14
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Re: 12v vs. 2x6v Batteries for Sailboat

The only problem with two 6 volt batteries is that you have 2 more connectors to corrode.
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Old 10-08-2019, 09:57   #15
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Re: 12v vs. 2x6v Batteries for Sailboat

Those EPM-Deka Duracells are by far **the best** deep cycling value on the planet, especially at $160 per pair, but even at a bit over $200. Compare to Trojans, usually quite a bit higher.

Just make **sure** you're buying the FLA not sealed version.

Charging requirements are no different, PSOC abuse (getting to true 100% Full fewer than say 30-50% of cycles) will hurt 12V units just as much as 6V ones.

The only caveat is that regular equalizing would be a good idea to extend longevity, especially in PSOC conditions, and that is best done at nominal 6V individually.

Hardly anyone bothers with that ideal however, so perhaps set that reco aside.

For Deka FLA your charger should put out about 14.7V for normal charging, and 15.3-15.75V for equalizing (ideally 7.875 V per unit).

As low as 15V for equalizing might be "good enough", same with 14.4V for Absorb.

Deka's spec for holding Absorb, definition of true 100% SoC Full is :

"Current change over 1 hour period of less than 0.1A"

If that is difficult to implement, use trailing amps dropping to 0.005C, or about 1A for that 200A bank.

Float spec is 13.5V, certainly no higher than 14.1V.
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