6v vs 12v

[80srockandrum]

Looking at possibly switching batteries over from 12v to 6v. Advantages? Disadvantages?

 

[Jkoht]

Two 6v batteries would have increased amp hours for longer run time on 12v appliances, but you'd also have additional weight. Also you'd have to know how to wire them properly so that they also produce 12v and can be charged by the converter. Generally you'd also have to have two of the exact same battery to ensure that one doesn't draw down or charge unevenly compared to the other.

 

[docj]

It's really pretty easy and is done all the time. I have four 6-volt batteries in my motorhome bay. Connected end to end a pair provides 12 volts. The biggest issue I can think of is do you have the space for them?

 

[John From Detroit]

Technically there are no 6 volt batteries in an RV they are all 12 volt but.. I know that when they are not in the RV they are six volt WHY

-{6v}+-{6v}+=12 volts at this point.

The most common 6V is the GC-2 or Golf Car 2 (Takes 2 to make 12 volts is a good way to think of them) This is a True DEEP CYCLE battery. about 220 amp hours 110 usable. (C/20 rate)

Now there is a GC-12. also a True deep cycle but not as much.

Most 12 volt batteries used in RV/s are MARINE/deep cycle. pay attnetion to how I typed MARINE and deep cycle.. Note the use of lower case for Deep Cycle.

Now 200 amp hours of 12 volt Marine/Deep cycle (Rounding for ease) Weigh the same as 200 Amp Hours of GC-2.. but .. you can only use about 25% of the power (or 50 amp horus to compare to the GC2 that would be 55)

And they may well cost more.

WHY: Economy of scale The GC-2 is the most "popular" Lead Acid battery.. Every car on the planet it seems has to have a unique battery so the battery makers need to retool daily or more.. 6 volt line never retools.

 

[Mark_K5LXP]

... True DEEP CYCLE battery. about 220 amp hours 110 usable. ...

What happened to the "unusable" Ah?

Mark B.
Albuquerque, NM

 

[rbrdriver]

I know they are pricey but if you can afford it consider 12 volt deep cycle lithium batteries. They weigh less, you don't have to worry about venting them, and you can mount them in any position (even upside down). Of course you also need a different converter/charger designed for lithiums.

 

[Gary RV_Wizard]

There is no inherent advantage in 6v vs 12v, but thanks to the popularity of golf cars, good quality 6v deep cycles are widely available at reasonable prices. Comparable capacity & quality 12v deep cycles are a specialty item that typically costs 30%-50% more than a pair of 6v golf car battery, so you can usually save money on a cost per amp-hour basis. However, if you are accustomed to buying lower-priced marine/rv hybrid "deep cycles", there isn't much upfront price difference either. The GC2 golf car 6v's will usually have a longer useful life, though. I cover the pros and cons of 6v vs 12v in my article on RV Battery Choices - see RVForum - Choosing a battery

 

[John From Detroit]

What happened to the "unusable" Ah?

Mark B.
Albuquerque, NM

That is a good question Mike. every time you cycle a battery (run it down and recharge it) it gives up a bit of it's life.
What happens to the other half of the power in a DEEP CYCLE is it helps the battery last longer. Run it below 50% full and the battery ages faster, also the voltage goes down to the point where it's not all that good at lighting up your life or running fridge, water pump, et-al.

Now a DEEP CYCLE may recover from occsional OH C!@$! level discharge. (Below 50%)

A MARINE/deep cycle might (less likely) And the cut off is 75% soc or about

A Starting battery taken down to 50%. Replace it

 

[Mark_K5LXP]

Now a DEEP CYCLE may recover from occsional OH C!@$! level discharge. (Below 50%)

So are the manufacturer data sheets lying (say the T105) when it says they can deliver 700 or so cycles down to 20% SOC? Some batteries spec 10% or even 0% SOC.

Not trying to be snarky. The reality is deep cycle batteries do NOT have some magic threshold where they are "damaged less". All lead acid batteries are failing from the day they're made, just that less used batteries fail for different reasons than hard used ones. When running the data, the difference in service life for a T105 that's cycled 40% vs 80% is 3%. When you throw in all the other circumstances and conditions that also affect performance and service life you'd be hard pressed to realize 3%. There is a throughput efficiency strategy that involves DOD, cycle life and calendar life but that's for applications where batteries are being cycled daily, month after month, year after year. There is no RV use that will meet that profile. Trojan has a tech paper on this topic: https://www.trojanbattery.com/pdf/RE_IEC_61427_Standard.pdf

The "50% rule" has no basis in RV (or most any other) storage battery service. All it does is promote the idea of buying more battery than you need or using the ones you have less, only to take unused life to the recycler when they ultimately crap out for any one of a dozen different reasons, none of which will be DOD.

Mark B.
Albuquerque, NM

 

[Gary RV_Wizard]

You are absolutely right, Mark, but the "50% rule" is a merely rule of thumb, an easy-to-understand crutch for people who can't be bothered to read battery tech papers or measure everything accurately. No different than a 55 mph speed limit or a roasting a chicken for 20 minutes/lb. A rule of thumb can be useful (practical) without being precise or reliable in every situation. When we give answers here, we seldom have anyway to judge whether the recipient is a geek or techo-phobe who won't deal with precise formulas or detailed analysis.

I'll grant that the so-called "50% rule" is vastly over-hyped on the internet and too often given as gospel rather than guideline. Irritates me all the time!

 

[John From Detroit]

All lead acid batteries are failing from the day they're made, j

On this I agree but there are chemical differences and physical differences so that TRUE Deep cycle (As opposed To Marine Deepcycle) Last longer at discharge levels below.. Now the "Minimum state of charge" is a trade off. The deeper the greater the damage.. But for most GC-2's the "Knee" the spot at which the damage increases faster and faster) seems to be about 50%
Still.. as I said. I've run 'em down to near zero and had 'em recover.. Never had a Starting battery do that.

 

[Fastlane Tracks]

I have used 6V in my 5th wheel and always liked the service I have received but always had a issue with acid vapor creating corrosion around wiring and surrounding metal items caused by the charging process.
I then learned about a better way. Gel cell batteries eliminates the corrosion problems and doesn’t require a specific charging system.
I now use optima yellow top batteries in both my 5th wheel and my coach . I have found that they produce a little bit more voltage and don’t have as an aggressive voltage drop curve as a lead acid battery, more like a Lipo battery
Here is a picture of what I have In my coach now they have 900 cold cranking amps per battery times 5batteries that’s 4500 cold cranking amps total, 2 for chassis and 3 for house
When boondocking I need all amps I can get to make sure I don’t run out of power during the night to keep my CPAP running so I don’t stop breathing at night (gen doesn’t have automatic start )

 

[Mark_K5LXP]

The 50% Rule has little credibility as a rule of thumb because it offers very little in terms of a practical difference over ignoring it entirely. But whatever, "it can't hurt". But somehow this rule of thumb has mutated from optimizing battery life to a dire consequence of don't ever cross that line or you'll 'kill your batteries'. That's where I throw the flag and call bullcrap, it's just not true. Gary's right, few people are going to study and become battery application engineers just to go camping and use a battery, and would just like a little simple advice to follow. Telling folks they're going to kill their batteries using them as specified by the manufacturer is wrong. The discussion should center around other more important facets of battery use like charging, storage and maintenance which are by far the more predominant factors in battery life.

Mark B.
Albuquerque, NM

 

[Mark_K5LXP]

for most GC-2's the "Knee" the spot at which the damage increases faster and faster) seems to be about 50%

Clarification - "damage" is a condition where performance or service life is adversely affected. Using a battery within it's specified range does not damage it because you will realize all of it's rated life. >50%, up to 80% DOD (100% of Ah) in most cases is within the specified range.

I get it - according to the oft-cited cycle life chart the line droops somewhere around 40-50% DOD. But cycles don't run your stuff. Amp hours do and when you factor the ever increasing Ah delivered over those deeper cycles the mix of many shallow cycles vs fewer deep cycles becomes nearly a wash.

I too have seen the reference in manufacturer literature that suggests "for best life limit discharges to less than 50%" (not mentioning the charge efficiency penalty in doing so) but lets look at that from the other direction - what happens if I don't? Let's pick the worst operating point on the chart, what are the dire consequences? About a 10% reduction in service life. So for all the anguish about "never" going below 50% and using those "unusable" Ah is all for, at best, a 10% improvement. Which of course is predicated on actually using all the cycles available at shallow discharges, which is difficult if not impossible to do. So "saving" the battery by using it artificially conservatively ultimately results in taking unused Ah to the recycler when they ultimately die. One can set arbitrary DOD limits for a number of legitimate reasons like operating reserve, charge time constraints, solar capacity, what have you but practically extending battery life isn't among them. I buy batteries to run my stuff and the last thing I'll be doing is watching my battery monitor and shutting stuff off for the sake of 10%. Want a rule of thumb that can make a difference? Make sure your batteries get and stay charged as much as you can. That's not a contradiction- discharge your batteries as often and as deeply as you need to, but recharge as completely and as often as you can, and between outings make sure they stay that way. Sounds simple but more batteries are killed by improper charging or neglect than crossing over the 'unusable' Ah line.

Mark B.
Albuquerque, NM

 

[Gary RV_Wizard]

But for most GC-2's the "Knee" the spot at which the damage increases faster and faster) seems to be about 50%

Not in any of the graphs I've seen - it's a gentle parabola curve without any obvious knee. Obviously it varies by battery type and design, but I think the differences are modest. Attached is an example of a depth-of-discharge plot for a deep cycle battery. I've seen other test plots that have steeper curves, but the knee in those is more like at 10% than 50%. That's not a practical limit for RV use.

The impact of a deep discharge shows up more quickly on a starting (SLI) battery simply because it has fewer life cycles to begin with. If using a starting battery in an RV house application, the recommendation for maximum life should be something like max 10% discharge, cause 50% would already be eating heavily into its life cycle count. On the other hand, a true deep cycle design like a golf car battery is going to survive many 50+% discharges.

Counting cycles is a laboratory measure anyway - not something an RV owner can do because the battery is always cycling to some degree.Even if disconnected (self-discharge).

 

[Babe2201]

I am new to RVing but from what I have read and listened to the 6V golf car batteries tend to have a few more Amp hours than marine 12V deep cycle. One disadvantage I see in a 2 battery system is that if down the road one fails you have no power.

 

[solarman]

I am new to RVing but from what I have read and listened to the 6V golf car batteries tend to have a few more Amp hours than marine 12V deep cycle. One disadvantage I see in a 2 battery system is that if down the road one fails you have no power.

1) best advantage of 6V over 12 is the total cost of ownership, 6V deep cycle have higher lifespan, generally more capacity for the size and their cost is attractive vs a similar size 12V.

2) if a single battery fails then yes, you are sol.. in either scenario, any failure will leave you with no power..

 

[Babe2201]

1) best advantage of 6V over 12 is the total cost of ownership, 6V deep cycle have higher lifespan, generally more capacity for the size and their cost is attractive vs a similar size 12V.

2) if a single battery fails then yes, you are sol.. in either scenario, any failure will leave you with no power..

Correct me if I am wrong but with 2 12V batteries if one fails the other one will give you power

 

[solarman]

to shed some light on the 50% DOD "rule"

1) The "50% rule" comes from fixed-base solar, where recurring total daily loads are known and more or less fixed. That's why I mention it and use it in my trade, because it's a good starting estimator.

2) There is a sweet spot in most lead-acid DOD vs. lifecycle charts where 50% usually results in the best number of cycles.

3) Years of solar designers and estimators playing with the numbers, including TCO over time
has distilled down into the "50% rule" as the standard estimator to start with as a default battery bank size , because under sizing and/or over sizing to get a lower daily DoD can cost more over time, again total cost of ownership... trade off between cost and usability

4) Years of Internet parroting of poorly understood principles of solar sytem design has resulted in a meme that 50% is a hard and fast rule. It isn't a rule, It never was. It's a guide used by estimators.

 

[solarman]

Correct me if I am wrong but with 2 12V batteries if one fails the other one will give you power

if they are wired in parallel and one fails open circuit, then yes, you will still have power.
if one fails short and they are fused, the shorting current should blow the fuse and you still have power.
if the shorting current is insufficient to blow the fuse, then you will lose power rapidly.
if one fails short and you are not fused, you will have no power and a bad hair day with lots
of smoke/fire/acid/screaming.. LOL ( an exaggeration but a rare possibility )..

6V is usually deemed "best back for the buck" capacity vs cost vs lifespan is the prime mover here.