# Batteries



## N7OQ (Jun 10, 2006)

I see posts about using 2 6 volt batteries in series. Are 2 6 volt batteries in series really better than 2 12 volt batteries in parallel ?

I was thinking about using 2 12 volt batteries with a Switch like the one I had in my boat that let you switch from battery 1 to battery 2 to both batteries or both off, anyone else use something like this?


----------



## Oregon_Camper (Sep 13, 2004)

Are you going to be doing a lot of dry camping? If so, then I would highly recommend moving to two 6v batteries. I have this setup in my Outback and I have enough battery power to last a family of 4 for about 4 days. Now, we do a lot of battery conservation, but I still get a lot more out of two 6v batteries then you would with two 12v's.

Take 20 mins and read the information in the link below. You will then have a lot more information to make your decision.

http://www.ccis.com/home/mnemeth/12volt/12volt.htm


----------



## HootBob (Apr 26, 2004)

Yes Bill 2 6 volt batteries was last longer then 2 12 volt batteries
Like JIm said if you are planning on doing alot of dry camping that would be the route to go

Don


----------



## Thor (Apr 7, 2004)

Don

I would have to disagree with you on that one. Batteries are not as straight forward as most people think. Here is some info on batteries that helped me make a decision.

Thanks the info was awesome.

Here is a summary of what I found.

any typical RV storage battery you get will be a series arrangement of cells. Each lead acid cell will contribute a bit over two volts so a nominal 12 volt battery will have six cells. Note that the energy you get from a battery, its capacity, depends upon the total energy of all its cells whether connected in series or parallel. The connection arrangement just balances the current and voltage components of this energy.

While a battery is a series arrangement of cells, a bank of batteries may be arranged with both series and parallel configurations. When this is done, careful attention needs to be paid to balancing the currents through each part of the battery bank and protecting against internal currents caused by failure.

Series arrangement means that voltage in each cell is added by connecting the negative side of one cell to the positive of the next.

A parallel arrangement connects all the positive sides of each cell together and all the negative sides of each cell together. This means the voltage is that of the lowest voltage cell but the current capability is that of all of the cells added together.

Why series?
Series arrangements means fewer cells which means each can contain more stuff for greater capacity or can be built more rugged in a given amount of space. A series arrangement also means that a weak cell does not sap the energy of the others and any current going through the battery will just pass through a weak cell. This fact, that any load or charge current goes through all cells equally, tends to keep all of the cells at an equal state. It also means that a failed cell does not cause internal circulating currents. The failed cell may add some resistance and, since it does not contribute any voltage, it will show as a reduced battery voltage.

Why parallel?
A parallel arrangement means that current is shared between batteries and this means that each battery is exercised less vigorously which may increase usable capacity. Since removing any single battery will not change the voltage, there is a redundancy in batteries that can be useful in the event of failure (if that failure is detected early enough). Parallel is also useful in low voltage systems because it may be easier to find batteries at needed supply voltages. A failed cell in a parallel bank will sap energy as the other cells try to charge it. This can cause heat and loss of water in the failed cell as well as lost current capacity.

Capacity and current drain
The usable current from a battery depends very much on how fast energy is taken from the battery. Slower energy draws usually mean more usable capacity from the battery. Energy draw is usually indicated by current because a battery's voltage is relatively fixed by the battery design. Energy draw is called power and is the product of voltage and current. Energy capacity is a product of power and time.

The capacity gained by reducing energy draw or current drain in a parallel configuration versus a series configuration can be determined by evaluating Peukert's Formula T = C / In where T is how long you can drain current I from a battery that has a capacity C and an internal resistance characteristic n. For the case of two six volt batteries in series versus two 12 volt batteries in parallel, the comparison is when the current changes by a factor of two (when voltage doubles, current halves and vice versa for the same amount of power). The formula would be

Tp - Ts = (C / In) - (C / (2I)n) = C(2n - 1) / (2n In) = Tp ( (2n - 1) / 2n )

{subscript p for parallel and s for serial, serial has twice the current of parallel, the LCD (2I)n which is where the 2n comes from in the Tp term to be able to subtract the fractions, percent change divides both sides by Tp and then multiplies by 100 - check the algebra yourself and let me know if you think you see an error!}

So percent change from the low current to high current times is (2n - 1) 2n . When this is calculated, the range for typical batteries means drain times will be from 71% (worst case worth purchasing where n=1.25) to 65% (best case, n=1.05) changed from parallel to serial. In other words, doubling current will reduce time of draw from 65% to 75%. For the 40 amp hours available (discharge to 80%) at a 5 amp load, the parallel configuration could provide 8 hours of usable battery while the serial configuration would provide maybe 6 hours. This would be balanced by the batteries in series probably have a bit more capacity to start with.

Factors to consider
Longevity or Load? - when you go looking for a battery, you need to consider the balance between maximum load and longevity. This is the deep cycle versus starting consideration. Your potential maximum load will definitely influence how you wire your battery bank and may be important in choosing a battery that is designed to provide the necessary current. Usually, a microwave through an inverter is about peak load in most RV uses and this means maybe 150 amps out of the battery for a few minutes. This is not a problem for most batteries but does mean you will need low gauge wires and good connections.

Size . The battery, including wires and connectors, will need to fit in the available compartment. Smaller batteries will fit easier but bigger batteries will contain more energy.

Weight . more capacity usually means more weight. But you need to consider the impact this weight will have on your rig depending upon where it is placed and you also need to consider your ability to maintain (lift, remove, shove around for access) the batteries.

Cost - The price of the battery is one of your primary clues about manufacturing trade-offs. For instance, Trojan sells three 6 volt batteries that are all the same size costing from $55 to $95 (see Uve's site). The higher prices get you more capacity, less internal resistance, and lower cycle ratings. The lesson in this case is that, for a given volume or physical size, higher price gets you more capacity but less life.

Availability . Many battery types are difficult to ship so you are limited by what you can find at outlets in your area or you need to add shipping charges to the cost.

Capacity . Be careful not to consider just an amp hour rating as a capacity measure. Capacity is always amps times volts times time, so multiply the amp hour rating times the battery voltage to get a comparable capacity number. Also be sure to use equivalent amp hour ratings. The twenty hour rating is usually a common specification.

Warranty . The warranty on the battery will often tell you something about what the manufacturer and the retailer think about the battery's longevity in typical service.

Charge cycle ratings . This indicates how many times you can expect a battery to be discharged to a certain point and then recharged before it starts to fail. It is a ruggedness rating more than a quality rating. Watch out to make sure you compare cycle ratings for equivalent discharge levels. 80% and 50% are perhaps most commonly used.

Battery Type . The primary classifications of concern are whether or not the battery is a sealed type that can be placed inside or whether it needs a specially vented cabinet. This gets into access for maintenance and other factors as well. The type of battery will also be a factor in charging and in usage considerations.

Complications
The batteries and how they are configured are only a part of the entire system in your RV low voltage system. A typical RV system does not push any particular extremes so the design does not need to give undue weight to any single factor. This complicates the choice because you have many different considerations to play against each other and none of them will likely have any significant impact on the total result for typical RV usage.

Cell failures are fairly uncommon with even modest care to use and charging so that difference between parallel and serial configurations is not significant in this regard. Plan on replacing your entire battery bank every five to ten years.

Current drains average fairly low and you should only use 20% to 50% of the battery capacity so the change in current between the two configurations for a capacity reason is also insignificant in most cases. If every last drop is important, you will likely already have solar cells or a generator.

Since all you are looking for is twelve volts, anything past two batteries in the bank will require some sort of parallel configuration. As you add batteries to the bank, your attention will need to be on battery matching and cabling concerns.

You are more likely to make a difference by attention to maintenance, good wiring and connections to reduce resistance losses, careful choice of loads for efficiency, a multi-stage intelligent battery charger, and a few lifestyle habits that can drastically impact energy use.

Make your choice
For the simple case of adding a bit of capacity to an existing RV system, the question of parallel or serial is more of convenience than anything else. The current loads are typically low and the demands do not usually stress the battery charge sufficient to make the current versus capacity consideration very important. Parallel might provide a bit more capacity and serial might be a bit more reliable but the benefits can be easily outweighed by smart battery choices.

Long term boondocking puts a premium on capacity, but those in this group also usually have means to keep their batteries at a reasonable charge point so, again, the differences between serial and parallel are minor.

Inverter driven heavy use is another matter. Here, the battery loads still average fairly low but there are high peak draws as when you warm a cup of coffee in the microwave. In this case, it is the wiring and connections that will be the major concern and not the battery bank configuration. [unless you go for a 24v or 48v inverter]

These considerations indicate that, unless you have some specific requirement to match, battery bank configuration choice is mostly one of convenience. The factors of cost, size, type, and availability will likely dominate the decision tree.

Thor


----------



## OutbackPM (Sep 14, 2005)

There was an article in Trailer Life on batteries and from what I remember is the 6 volt has a different constuction that allows deeper discharge while still maintaining a decent life.

Going less than 60% capacity before recharge limits the 12 volt life more than the 6 volt. I will try to find the artcle again to quote more specifically.


----------



## PDX_Doug (Nov 16, 2004)

That is a great run down, Thor, thanks!









Here, I was all set to convert my dual 12V's to dual 6V's. What I am seeing here, is that that is not such an automatic choice. I do believe that, because of their intended applications, the 6V battery designs and construction are more suited to deep discharge cycles. However, I guess I need to see what 12V options there are as well. I definately like the redundancy of parallel 12V's. If one battery fails, you are not dead in the water.









Good food for thought!

Happy Trails,
Doug


----------



## campmg (Dec 24, 2005)

Thanks for the great summary Thor. I've thought a lot about the dual 6 Volts but now will consider adding a 12V to the existing set up since I already have a new battery.


----------



## W4DRR (May 17, 2005)

PDX_Doug said:


> If one battery fails, you are not dead in the water.


Not to mention the fact that 12V batteries can be found at WalMart, in case of emergency. 6V golf-cart batteries on the other hand...

Bob


----------



## Scrib (Jun 28, 2005)

Walmart private-labels Johnson Controls batteries (Everstart) - same as Sears, Costco, Interstate, etc. The Optima brand is also owned by them.


----------



## Oregon_Camper (Sep 13, 2004)

Thors information is great, but I can tell you in the real world, my 6v setup will knock the socks off any dual 12v system.

At the Spring Rally, guys were recharging their 12v systems after one day...I went 3 days and went home with 1/2 a charge remaining.

Yes, 12v batteries can be bought anywhere, but if your battery does die (and really, how ofter could that be) then remember...you're CAMPING. You can live with no lights in the ceiling and only using a flashlight.


----------



## N7OQ (Jun 10, 2006)

Oregon_Camper said:


> Thors information is great, but I can tell you in the real world, my 6v setup will knock the socks off any dual 12v system.
> 
> At the Spring Rally, guys were recharging their 12v systems after one day...I went 3 days and went home with 1/2 a charge remaining.
> 
> Yes, 12v batteries can be bought anywhere, but if your battery does die (and really, how ofter could that be) then remember...you're CAMPING. You can live with no lights in the ceiling and only using a flashlight.


Well Thor sure has a lot of good info he must be a engineer







But I have to agree with you Oregon_Camper we are just camping so if it does fail than it won't just a big deal.

There must be something about the dual 6v system sense I hear about it in a lot of different circles.

Right now I will just stay with what I have. I have a new 24 series that came with the TT and a one year old 27 series battery. I can't hook these in parallel and I don't think that buying another 27 series battery would wotk with and older one. But I do have a generator so for now will use one at a time and just keep charging them. but I do mant to get a good working system going sense I do a lot of dry camping.

Thanks for all the good info


----------



## OutbackInfront (Jun 27, 2006)

Even if you get 2 new 12v batteries, they will appear equal at first. Over time one will deteriate more rapidly than the other which will further accelerate its failure. Been there, done that. Ask your dealers service department. They've seen it a bunch and I'm sure that's why they come with 2x6v and no option for 2x12v.

Regards,
Eric


----------



## W4DRR (May 17, 2005)

If you run a dual 12V system, the way to go is have a switch that selects one battery or the other, just like the propane tanks. Running both batteries simultaneously in parallel is just asking for trouble long-term.
I think this issue is clouded somewhat by the fact the typical 6V battery used has considerably more amp-hour capacity than a 24 group 12V battery (maybe double?)....not that is better by virtue of being 6V. Also, these batteries are more of an "industrial grade", meaning more rugged construction, and longer life....again not solely by virtue of being 6V.

Bob


----------



## California Jim (Dec 11, 2003)

Oregon_Camper said:


> Thors information is great, but I can tell you in the real world, my 6v setup will knock the socks off any dual 12v system.
> 
> At the Spring Rally, guys were recharging their 12v systems after one day...I went 3 days and went home with 1/2 a charge remaining.
> 
> Yes, 12v batteries can be bought anywhere, but if your battery does die (and really, how ofter could that be) then remember...you're CAMPING. You can live with no lights in the ceiling and only using a flashlight.


Agreed, and same here.


----------



## PDX_Doug (Nov 16, 2004)

Oregon_Camper said:


> if your battery does die (and really, how ofter could that be) then remember...you're CAMPING. You can live with no lights in the ceiling and only using a flashlight.


Yeah, and I could sleep on the ground in a tent too...

Sorry, been there, done that.
I didn't spend $20K+ to have to use a flashlight!









Happy Trails,
Doug


----------



## Thor (Apr 7, 2004)

I really think you need to consider exactly how much dry camping you do. If you are only going the weekend does it really matter?

When I did my system the bottom line came to $$$$$. I could get a dual 12v system cheaper than a dual 6v with the same spec's (Ie amp hours, reserve time, CCA). Other reasons why I went 12v

1 - $$$$$
2 - Solar charge controller I had was for 12v
3 - Can run single or dual mode on 12v
4 - Can use for my TV if the TV battery died out in the bush
5 - My home charger is for 12v deep cycle.

For me these factors outweigh how long the batteries will last. If you are dry camping for 5-10days, does it really matter if one set-up vs the other gets you 4 more hours????

Thor


----------



## campmg (Dec 24, 2005)

Well said Thor. Since I already have a new 12V, I'm better off adding another one without switching over to 2 6V's.


----------



## PDX_Doug (Nov 16, 2004)

Good points, Thor.









I'm thinking I will stick with my current dual 12V system, and just get better batteries. Adding a Perko battery management switch and a modest solar panel will probably give me all the advantages of a dual 6V system, and then some.









Happy Trails,
Doug


----------



## Scrib (Jun 28, 2005)

Thor said:


> I really think you need to consider exactly how much dry camping you do. If you are only going the weekend does it really matter?
> 
> When I did my system the bottom line came to $$$$$. I could get a dual 12v system cheaper than a dual 6v with the same spec's (Ie amp hours, reserve time, CCA). Other reasons why I went 12v
> 
> ...


1 - A Group 27 and a 6V golfcart battery are the same price ($70)
2 - 2 x 6V (in series) = 12V, it will work just fine
3 - right
4 - right, although you could jump your rig w/ 2 x 6V's
5 - will work just fine on 2 x 6V's in series

I agree with you though, that it's probably not that big a deal.


----------



## Thor (Apr 7, 2004)

Scrib

Question - if using a charger 12v and putting 2 6volts in series do you need to worry about the charger size? Normally charge my 12v batteries separately. I know I can charge them together but I do not wish to damage my charger.

Thor


----------



## Scrib (Jun 28, 2005)

I don't think you have to worry any more than you would for a single 12V - The 12volt Side of Life talks about "battery Amp-hour / 10" to calculate the maximum charge-rate (i.e. 220Ah battery = 22A). Now, I'm guessing that your (and most) charger is outputing way less than 22A - so it is going to take longer to charge 220Ah. Maybe the same as charging 2 x 12V? I don't know.


----------



## Scooter (May 9, 2005)

I have read each and every reply to this thread, I neither agree nor disagree with any reply. 
My only personal response here is I only dry camp, I've run Both types of systems 12v and 6v (and yes they were of combined equal specs) in their applicable configurations, and all theory aside , under real world applications the 6v's last longer. Tried and true. So I submit perhaps the overall contributing factor to this revelation could simply be the quality of each battery manufacurer themselves. The trojan T125'S 6v's are awesome.

Thor - on the lighter side - I submit, 4 additional hours of use dry camping on the 5th or 6th day when it's windy, cold and rainy can indeed make considerable difference.







Peace all.


----------

