6V house batteries in series VS new 12v single

[inspectr]

I have had my Dufour 35 Classic for nearly 10 years.

The boat has a single dedicated 12V starting battery, replaced around 3 years ago and going strong

The house batteries are a pair of 6 Volters, in series, been there from before I bought the boat in 2008. Working perfectly until now

Last month, locking up the boat for a driving holiday in tasmania I forgot to turn off the battery breakers, and when I went back to the boat the house set was flat, engine battery started the engine and seemed to charge OK.
Kill the engine, ten minutes later the battery meter shows 11.0V and alarms.

Fair enough, 10 year old batteries are probably dead, and 10 years has got to be acceptable.

Two days later, needing to replace some corroded wiring on some the pumps (another story, why do boat builders use domestic un-tinned copper wiring in bilge and freshwater pumps, and then run them in inaccessible places with the wiring brace tied together with cable ties) went back to the boat, ran the engine for 2.5 hours and the batteries are 90% charged and running at 12.5V. I am going to replace the batteries anyway.

My question, what is the advantage / disadvantage of 2 x 6 Volters, VS one decent sized 12V current technology battery as a replacement - obviously, there is plenty of room in the battery recess, and the only complication may be the need to replace the cables??

[boatbod]

6v "golf cart" batteries are cost effective and rated for deep cycle applications. Engine starting batteries are designed to produce lots of cranking amps but they don't have as good amp-hour rating or tolerance for deep discharge.

Small sailboat diesels can be effectively started by either type of battery, so it's perfectly ok for the entire vessel to be run from deep cycle batteries. Larger diesels require more cranking amps, thus on larger vessels the type of battery becomes more important.

[TeddyDiver]

Originally Posted by inspectr

My question, what is the advantage / disadvantage of 2 x 6 Volters, VS one decent sized 12V current technology battery as a replacement - obviously, there is plenty of room in the battery recess, and the only complication may be the need to replace the cables??

Advantage 2x6v:
-large selection of deep cycle batteries
-lighter to move vs single 12v
-better cooling of the cells vs 12v battery (6cells), thou 2 or 4v batteries would be more ideal.

Advantage single 12v:
-cheaper..

BR Teddy

[Crew of Turning]

There are many debates in this forum regarding batteries. There may not be any consensus yet, but the cost/benefit analysis still leans towards the flooded golf cart style GC2 6 volt battery. Serviced and used correctly they deliver both service and economy .

[ontherocks83]

If you only have room for 2 6v's as your house bank, then the only risk is if one battery is defective you have lost your entire house bank.

If you put two 12v batteries you have a back up in case one dies and can still run your house, however they will have less capacity and are not true deep cycle.

My preference is 6v's for house with a back up start battery as long as you can have 4 or more 6v's. (redundancy for failure) If you can't fit 4 6v's then you have to weigh the benefit to risk ratio of either: redundant but lower capacity non deep cycle 12v's OR non redundant (no back up) higher capacity deep cycle 6v's

[rwidman]

Originally Posted by TeddyDiver

Advantage 2x6v:
-large selection of deep cycle batteries
-lighter to move vs single 12v
-better cooling of the cells vs 12v battery (6cells), thou 2 or 4v batteries would be more ideal.

Advantage single 12v:
-cheaper..

BR Teddy

For engine starting, a single 12 volt battery is also simpler. Most of us can get by just fine with a single battery for engine starting.

For house loads, it depends on the loads. Some folks have very modest house loads while others have every electronic and electrical appliance that can be put on a boat. In most cases though, the 6 volt true deep cycle batteries are probably better.

[zstine]

if I understand, The PO is replacing the HOUSE bank, currently two 6V.

The BIG difference is Amp Hour capacity. Two 6V batteries will have over 200Ahr compared to 110Ahr for a single 12V. Cost per Amp Hr is mch less for 6V, which are available for $85 each at Sam's club in the USA... Yes, one single 12V (maybe $100) will be less, but you are getting ~half the capacity.. so 12V is actually more $$ for 'the size' if size = Amp Hrs.

[sotavento]

Originally Posted by inspectr

I have had my Dufour 35 Classic for nearly 10 years.

The boat has a single dedicated 12V starting battery, replaced around 3 years ago and going strong

The house batteries are a pair of 6 Volters, in series, been there from before I bought the boat in 2008. Working perfectly until now

Last month, locking up the boat for a driving holiday in tasmania I forgot to turn off the battery breakers, and when I went back to the boat the house set was flat, engine battery started the engine and seemed to charge OK.
Kill the engine, ten minutes later the battery meter shows 11.0V and alarms.

Fair enough, 10 year old batteries are probably dead, and 10 years has got to be acceptable.

Two days later, needing to replace some corroded wiring on some the pumps (another story, why do boat builders use domestic un-tinned copper wiring in bilge and freshwater pumps, and then run them in inaccessible places with the wiring brace tied together with cable ties) went back to the boat, ran the engine for 2.5 hours and the batteries are 90% charged and running at 12.5V. I am going to replace the batteries anyway.

My question, what is the advantage / disadvantage of 2 x 6 Volters, VS one decent sized 12V current technology battery as a replacement - obviously, there is plenty of room in the battery recess, and the only complication may be the need to replace the cables??

6V batteries a I consider are better than 12V, but normally they are toller which in some cases as mine they do not fit.

[a64pilot]

Originally Posted by inspectr

My question, what is the advantage / disadvantage of 2 x 6 Volters, VS one decent sized 12V current technology battery as a replacement - obviously, there is plenty of room in the battery recess, and the only complication may be the need to replace the cables??

Hump one of those Monster 12V batteries down the dock into the cockpit, down the companionway and into the battery box once, and you will understand clearly.
But it boils down to two reasons.
1. With only a couple of exceptions of the higher quality batteries, 12 V batteries are not true deep cycle batteries and are not as suitable for house banks as deep cycle batteries are, labels lie
2. Usually you just cannot get an AH cheaper than you can in a golf cart battery and the cost savings can be significant

[ranger58sb]

Originally Posted by zstine

The BIG difference is Amp Hour capacity. Two 6V batteries will have over 200Ahr compared to 110Ahr for a single 12V.

If that single 12V battery was an 8D, it'd be rated at approx 245 Ah.

But it would weigh a lot more than individual 6V batteries.

-Chris

[leftbrainstuff]

Couple of pros and cons

2 x 6V in series
1) your options are mostly limited to good quality deep cycle batteries. Mostly flooded lead acid. Cheap, reliable and proven technology.
2) two batteries are more fun to get in and out of the boat. As opposed to one large 12V beast.

1 x 12V
1) Lots of junk products on the market, many of which are not deep cycle suitable. Lots of marketing bs around dual role starting / deep cycle claims.
2) May be cheaper than 2 x 6V. Either because of volume discounts to the reseller or because of an inferior quality battery. Weight is one test but the addition of alloying elements like antimony in deep cycle batteries is often absent rebranded start batteries.
3) Will fit in places where 2 x 6V wont
4) less cabling
5) there are some more interesting technology options.

Here's some more detailed advice that will let you choose wisely.

http://www.mpoweruk.com/leadacid.htm

[rwidman]

Originally Posted by zstine

........The BIG difference is Amp Hour capacity. Two 6V batteries will have over 200Ahr compared to 110Ahr for a single 12V. Cost per Amp Hr is mch less for 6V, which are available for $85 each at Sam's club in the USA... Yes, one single 12V (maybe $100) will be less, but you are getting ~half the capacity.. so 12V is actually more $$ for 'the size' if size = Amp Hrs.

But two 12 volt batteries will have the same power as two 6 volt batteries.

Battery power (AH) is pretty closely related to the amount of lead.

[leftbrainstuff]

Originally Posted by rwidman

But two 12 volt batteries will have the same power as two 6 volt batteries.

Battery power (AH) is pretty closely related to the amount of lead.

Be careful of the math people. Two 6V in series does not magically give you double the Amp hours.

They need to be parallel connected to double Amp hours and your group voltage remains at 6V.

[rgleason]

2 x 12v. 74ah = 148 ah
Yet 2 trojan 6v batteries are 224 ah at 12vdc in series. Those are the T104 batteries I believe.

[ranger58sb]

Originally Posted by rgleason

2 x 12v. 74ah = 148 ah

Must be small 12V batteries. What are those, G27s?

Two 12V 8Ds would be about 490 Ah.

My only point is that there are 12V batteries, and then there are 12V batteries. Depends on which one uses for calculations.

And then some of those 12V batteries are optimized for starting (MCAs/CCAs), while some are more forgiving of house loads.

-Chris