SOLAR BATTERIES..2 12 VOLT OR 2 6 VOLT?

I only have room for 2 group 31 batteries in my battery tray. I will be adding solar at some point. Do I go with 2 12 volt deep cycles or 2 6 volt DC batteries?

You could probably get a few more amp-hours by going with two good quality 6 volt batteries, but our Group 31 AGM 12 volt batteries have plenty of power (105 AH each.) If you've grown tired of maintaining your batteries, you might look into AGMs. More expensive for sure but well worth it IMO. Just make sure your charger has an AGM setting - they have a slightly different charging profile.

Kev

You would get more service from the 6 volts weather you go solar or not.>>>Dan

Kevin Means said:

You could probably get a few more amp-hours by going with two good quality 6 volt batteries, but our Group 31 AGM 12 volt batteries have plenty of power (105 AH each.) If you've grown tired of maintaining your batteries, you might look into AGMs. More expensive for sure but well worth it IMO. Just make sure your charger has an AGM setting - they have a slightly different charging profile.

Kev

Click to expand...

Kev,
What brand batteries do you have?

If one uses true 12 volt deep cycle batteries,  there is no issue. The 12 volt will give as good of service as the 6 volt batteries. The downside is the 12 volt cost more than the 6 volt. Look at the Trojan battery website and do some comparisons, then make the decision as to what is best for your needs and budget. One advantage of using 12 volt when you only have room for two is if one 12 volt goes bad for some reason,  you can take it out of the system and still function.  If one of the 6 volt goes bad, you're out of luck.

6 volt batteries have thicker plates (much like a deep cycle battery has thicker plates than a normal auto battery). They should generally last longer because of that.

12 volt deep cycle batteries,  not the dual purpose marine type,  also have thicker plates. The ONLY reason 6 volt batteries are recommended and used more is that they are cheaper because the factories produce more of them due to their higher demand.

So if a 6 vlt has 330 ah and you put two together it's 330 ah where as if you 2 12v at ...the max i can find is 228 ah = 456 ah. Isn't that better?

kdbgoat said:

12 volt deep cycle batteries,  not the dual purpose marine type,  also have thicker plates. The ONLY reason 6 volt batteries are recommended and used more is that they are cheaper because the factories produce more of them due to their higher demand.

Click to expand...

You believe that 6V deep cycle batteries (used primarily in golf carts) enjoy higher demand than comparable 12V batteries used throughout RVs, other trailers, marine, heavy equipment, aviation and some automotive applications? Can you validate this?

The plates in 6 Volt batteries by their nature are thicker. There are only 3 cells that will maintain comparable "real power" to 6 cells in a 12v battery (Each cell contains 11 plates either way). In order to store the same power, each cell will require more cathode & anode (plates) and therefore have to be thicker.

From our wholesale suppliers, 6 Volt batteries are actually slightly more expensive.

Amyken, our house-batteries are Napa brand, but they're manufactured by East Penn.

Kev

Having spent some time in the DIY EV world, it turns out that there is a thing called Peukert's law.  Has to do with discharge rates.  If your running a big inverter that is going to discharge the battery quickly, two 6volt batteries are better then two 12 volt batteries.  As widely held in the EV World lots of lower volt batteries will run longer then the same number of higher volt batteries due to this law.  On the other hand, if your discharge rates stay low I don't know that you will see any appreciable benefit one way or the other except for your budget.

Also if you look at the law you will see why AGM Batteries perform better as their multiplier is lower.

Kevin Means said:

Amyken, our house-batteries are Napa brand, but they're manufactured by East Penn.

Kev

Click to expand...

Kev...where would I out if my charger is AGM approved?

I would think the owners manual for your converter would indicate whether or not its got an AGM setting. Our manual does, and our Magnum converter/charger has a program setting in the menu for AGM Type 1 or AGM Type 2 batteries (Don't ask me what the differences are.)

Kev

I have to disagree with NickB. Voltage has nothing to do with plate thickness. Plates are made thicker or thinner for a variety of reasons, e.g.  to fit in the desired case size, to provide more or fewer charge cycles (deep cycling), to make them more rugged, or to install more plates in a given space and thus increase the max amp capability (CCA/MCA).  Modern battery design uses a standardized 2.1v cell and multiple cells are combined to get the desired voltage, i.e. 3 cells for 6v and 6 cells for 12v.

However, the "6v battery" we refer to here is nearly always the 6v "golf cart" battery which is designed as a rugged deep cycle that can withstand frequent charge/discharge cycles and provide sustained output at a moderate amp rate. It's an excellent choice for an RV house battery. 12v batteries come in a wider variety of designs for different applications, and a 12v starting battery packs more (and thus thinner) plates into each cell to get a higher amp rate capability, at the expense of fewer charge cycles and shorter duration of output. A 12v deep cycle, however, is similar to the 6v golf cart battery in internal design.

I suspect the 12v that NickB cited as cheaper than 6v are so-called "marine deep cycles", which are hybrid starting batteries. They have neither the deep cycling capability nor the ruggedness of the golf cart type. A 12v deep cycle of the same design as the 6v golf cart battery will cost substantially more than any marine 12v and usually somewhat more than the GC2 golf cart 6v as well. Compare the price of a Trojan 24TMX or 31XHS to any marine battery or GC2 6v.

To answer amyken's question:

Ignore the voltage and focus on the amp-hour capacity and number of charge cycles (deep cycle vs marine/auto). Then buy the most of each that you can get for your money.  If you can get a decent price on a quality 12v deep cycle, great. Use tow of them in size 30/31.  If not, check out the 6v GC2 6v's to see what you can get.

Two GC2 6v will produce about 230AH and something like 600+ full charge cycles. Two Group 31 12v deep cycles should be almost identical in AH and cycles. For maximum length of service, avoid 12v batteries designated "marine" or "marine/RV deep cycle". They will have about half the usable life of the true deep cycle types. Of course, they also cost quite a bit less.  If you don't mind replacing them more often, the lifetime cost may work out to be about the same for marine vs true deep cycles.

For maintenance free operation, consider AGM deep cycles instead of flooded cells.

So if a 6 vlt has 330 ah and you put two together it's 330 ah where as if you 2 12v at ...the max i can find is 228 ah = 456 ah. Isn't that better?

Click to expand...

Yes it is, but I don't know where you are going to find that 330AH 6v that fits in the space you described. The standard GC2 6v produces 220-230AH, not 330. A pair of flooded cell 12v deep cycle in the Group 31 size produce about 230-260AH. Look at the Trojan 31XHS, for example. It is 130AH (x2). The AGM version, the 31AGM is less AH (about 102 each) but maintenance free.


http://www.trojanbattery.com/pdf/TRJN0111_ProdSpecGuide.pdf

Gary RV Roamer said:

...A 12v deep cycle, however, is similar to the 6v golf cart battery in internal design...

Click to expand...

Similar, but different. I didn't say Voltage has anything to do with plate thickness, I said real power or available wattage does. To get the same amount of POWER(watts) out of less cells, you need more metal - plain and simple. That's where the difference lies. Larger cell with thicker plates storing more energy per cell in order to store the same amount of power overall. 

We got two 6v deep cycle at Costco last summer and they have been fine.  I thought the price was good.