Advantages of 2 x 6v versus 1 x 12v?

[ElectronicFur]

I am planning a small 12v off-grid solar system to provide power and I'm looking at deep cycle batteries.

What I'd like to know is why 2 x 6v is often recommended for such installations, rather than just a 1 x 12v battery. I saw this

I assumed that it might be because the 6v batteries are better spec, but this doesn't seem to be the case always.

I could for example taking the Rolls 4000 series, I could buy two 6v s290 220Ah batteries, or one 12v T12250 200Ah battery.

The 6v battery has 19 smaller and thinner plates per cell compared to the one 12v battery which 9 larger and thicker plates per cell.

http://rollsbattery.com/wp-content/uploads/specsheets/T12250.pdf

http://rollsbattery.com/wp-content/uploads/specsheets/S-290.pdf

So in this case does it not make sense to go for the 1 x 12v option, with the only downside being it is heavier to handle as it's 57kg, and price-wise it slightly more per Ah?

Thanks,
EF

[Sunking]

You can use either 2 6-volt batteries in series, or 1 12 volt battery. Makes no difference if comparing Apples to Apples quality wise. What you are missing or not understanding is Battery Quality, and you only want to use a Single String and avoid PARALLEL STRINGS. Right off the bat you are comparing an Apple to an Orange.

Example lets say you require a 12 volt 400 AH battery and want to use Rolls battery. Try to find a 12 volt 400 AH battery. You cannot find one. All you are going to find is a 12 volt 200 AH T12 250 right? However no problem finding a 6-volt 400 AH or a Rolls S530. So what if you needed a 2400 AH battery. Forget 12, 8, 6, or even 4 volt batteries. You are looking for a 2-volt battery like a Rolls 2YS31PS a 2 volt 2400 AH battery.

However you are comparing an Apple to an Orange. A "S" model is not equal quality to a "3000" model.

Why is that way. Because there is a direct relationship to weight, size, quality, and watt hour capacity. For lead acid batteries the energy density is roughly 40-60 watt hours per Kg. So a 6 volt 400 AH battery has a capacity of 6 volts x 400 Amp Hours = 2400 watt hours. That means it will weigh around 50 Kg. Also means if there was a 12 volt 400 AH battery, it would weigh 100 Kg or more. Now imagine a 12 volt 2400 AH battery. How in the hell do you plan on handling a 1400 Kg battery? It would be much easier handling a 2 volt 2400 AH 130 Kg battery right?

The other thing you are over looking in Rolls is the model series and quality of each series.

The "S" models are the low end 2-year product line.
3000 series is a 4 year battery
4000 series is a 5 year battery
5000 series is a 10 year battery.

Look at the weight of same size capacity. The 10 year battery is a lot heavier than the 2 year battery. You get what you pay for and what you are missing is the WEIGHT

But guess what? The cheaper batteries (S Model in Rolls) are the most expensive batteries you can buy. The 5000 series are the most expensive but least expensive at the same time.

The S models will cost you around $150/Kwh. The 5000 serries will cost you $250/Kwh. That means the S models cost you 3 times as much as the 5000 series. Go figure it out. You get what you pay for.

EDIT NOTE:

Forget AH, it means nothing. Use Watt Hours for all calculations including cost and energy. WATT HOURS = BATTERY VOLTAGE x AMP HOURS

[ElectronicFur]

Thanks Sunking, but now I am more confused.

According to the Rolls website, both the 6v S290 battery and the 12v T12250 are 4000 series batteries, with 7 year warrantee. This is why I assumed I was comparing apples. They don't even mention a 3000 series, or that the S Model are a different cheaper line: http://rollsbattery.com/uploads/pdfs/products/17.pdf

Weight of the s290 is 27 kg dry, 31 kg wet, and the T12250 is 44 kg dry, 57 kg wet.

So two s290's are heavier but they have a lot more plates per cell, which are smaller thinner plates. So I'm still not sure if this is better or worse than one T12250, as what I gleaned from reading various threads on this forum, is that plate size and thickness also matters.

With regards to using 6v instead of 12v I can see your point if you need to store a lot of Watt Hours. But for my small setup one T12250, or two S290 will suffice.

[joerossjr]

You can use either 2 6-volt batteries in series, or 1 12 volt battery. Makes no difference if comparing Apples to Apples quality wise. What you are missing or not understanding is Battery Quality, and you only want to use a Single String and avoid PARALLEL STRINGS. Right off the bat you are comparing an Apple to an Orange.

Example lets say you require a 12 volt 400 AH battery and want to use Rolls battery. Try to find a 12 volt 400 AH battery. You cannot find one. All you are going to find is a 12 volt 200 AH T12 250 right? However no problem finding a 6-volt 400 AH or a Rolls S530. So what if you needed a 2400 AH battery. Forget 12, 8, 6, or even 4 volt batteries. You are looking for a 2-volt battery like a Rolls 2YS31PS a 2 volt 2400 AH battery.

However you are comparing an Apple to an Orange. A "S" model is not equal quality to a "3000" model.

Why is that way. Because there is a direct relationship to weight, size, quality, and watt hour capacity. For lead acid batteries the energy density is roughly 40-60 watt hours per Kg. So a 6 volt 400 AH battery has a capacity of 6 volts x 400 Amp Hours = 2400 watt hours. That means it will weigh around 50 Kg. Also means if there was a 12 volt 400 AH battery, it would weigh 100 Kg or more. Now imagine a 12 volt 2400 AH battery. How in the hell do you plan on handling a 1400 Kg battery? It would be much easier handling a 2 volt 2400 AH 130 Kg battery right?

The other thing you are over looking in Rolls is the model series and quality of each series.

The "S" models are the low end 2-year product line.
3000 series is a 4 year battery
4000 series is a 5 year battery
5000 series is a 10 year battery.

Look at the weight of same size capacity. The 10 year battery is a lot heavier than the 2 year battery. You get what you pay for and what you are missing is the WEIGHT

But guess what? The cheaper batteries (S Model in Rolls) are the most expensive batteries you can buy. The 5000 series are the most expensive but least expensive at the same time.

The S models will cost you around $150/Kwh. The 5000 serries will cost you $250/Kwh. That means the S models cost you 3 times as much as the 5000 series. Go figure it out. You get what you pay for.

EDIT NOTE:

Forget AH, it means nothing. Use Watt Hours for all calculations including cost and energy. WATT HOURS = BATTERY VOLTAGE X AMP HOURS

^^
This guy seems legit.

[solarix]

The reason 6V batteries are preferred is because batteries are more easily tested when wired in series. Get a bunch of current flowing (either charging or discharging) and measure the voltage across each battery to compare how uniform they read. A bad battery will be obvious then. When wired in parallel - its hard to tell which battery is bad. Plus, with 6V batts you have less caps to deal with when watering.

[Sunking]

The reason 6V batteries are preferred is because batteries are more easily tested when wired in series.

Huh? How many cells are required to make a 12 volt battery?

The answer is 6 every time right?. The reason for using 2, 4, 6, and 12 batteries is so you can implement only using 1 series string of batteries vs parallel strings to make a given capacity. That keeps the fill caps to a minimum as you said.

Largest 12 volt battery you can find depends on the manufacture, but 200 AH is about as large as they come. If you need 400 AH would require 2-parallel strings at 12 volts. To use a single string would require you to use a lower voltage battery like 2, 4, or 6 volts. They make 6-volt batteries up to about 500 AH depending on manufactures. Above 500 AH requires using lower voltages of 2 and 4 volts. Largest AH I know of are 2-Volt 6000 AH submarine batteries. Otherwise 2-volt 4000 AH made by Exide and C&D.

It comes down to if you need a 500 AH battery, then buy 500 AH batteries, not 250 AH batteries in parallel. That will be either a 4 or 6 volt battery. Only time you want to parallel FLA batteries is when you need redundancy like taking a string off-line for maintenance or testing, or required to make the AH capacity of greater than 4000 AH.

Only exception is if you use 2-volt cells. There you can use as many in parallel as you want because you parallel cells, not stings. See below Alternate

[ElectronicFur]

I dont need 500Ah, as explained in the 3rd post, for my small setup one T12250, or two S290 will suffice, so I'm still confused about which is better

[Sunking]

I dont need 500Ah, as explained in the 3rd post, for my small setup one T12250, or two S290 will suffice, so I'm still confused about which is better

Sorry but I thought I made that clear. Guess not. My bad.

The S290 is a "S" series 2 year warranty battery.
The T12 250 is a 4000 series 5 year warranty battery. It has a lot more lead in it, thus weighs more..

So which is better?

The one with a a 2 year warranty? Or the one with a 5 year warranty?

I don't know, you have to tell me. My professional best guess of 36 years is the battery with more lead in it last longer.

[ElectronicFur]

Thanks Sunking, but now I am more confused.

According to the Rolls website, both the 6v S290 battery and the 12v T12250 are 4000 series batteries, with 7 year warrantee. This is why I assumed I was comparing apples. They don't even mention a 3000 series, or that the S Model are a different cheaper line: http://rollsbattery.com/uploads/pdfs/products/17.pdf

Weight of the s290 is 27 kg dry, 31 kg wet, and the T12250 is 44 kg dry, 57 kg wet.

So two s290's are heavier but they have a lot more plates per cell, which are smaller thinner plates. So I'm still not sure if this is better or worse than one T12250, as what I gleaned from reading various threads on this forum, is that plate size and thickness also matters.

With regards to using 6v instead of 12v I can see your point if you need to store a lot of Watt Hours. But for my small setup one T12250, or two S290 will suffice.

Hi Sunking, I'm guessing you didn't see my other post, in the time it was awaiting moderator approval, see above. Rolls say the S290 is a 4000 series, and two of them weigh more than the T12250, but they have thinner, smaller, and more numerous plates per cell, hence my confusion.

[inetdog]

If you have two FLA batteries that are the same voltage and approximate AH rating the one with more, thinner, plates is cranking battery (= Starting, Lighting, Ignition or SLI.)
The one with thicker plates will be a hybrid or deep cycle battery.
Just a general rule. There may be exceptions.
The construction of AGM and GEL batteries is different.

[ElectronicFur]

If you have two FLA batteries that are the same voltage and approximate AH rating the one with more, thinner, plates is cranking battery (= Starting, Lighting, Ignition or SLI.)
The one with thicker plates will be a hybrid or deep cycle battery.
Just a general rule. There may be exceptions.
The construction of AGM and GEL batteries is different.

But these are Deep Cycle batteries from Rolls, specifically for solar, so I'm assuming that is not the case.

[Sunking]

Hi Sunking, I'm guessing you didn't see my other post, in the time it was awaiting moderator approval, see above. Rolls say the S290 is a 4000 series, and two of them weigh more than the T12250, but they have thinner, smaller, and more numerous plates per cell, hence my confusion.

You are correct, I did not see the post, so I went back and took a look. Inetdog hit on the difference.

The S290 is a 6-volt Golf Cart battery with 6-volt 220 AH capacity. Look at the Case Designation = GC2 or Golf Cart-2 which refers to the demensions and use. Note it uses 19 plates. What this does or means it is a hybrid battery. It is designed to provide higher discharge rates. These are intended for Golf Carts, not solar but can be used with solar.

The T1250 is a 12-volt 200 AH battery so it is smaller capacity but uses 9 larger plates. It is a true Deep Cycle in that since. It internal Resistance is higher than the S290, thus not as good as delivering high currents. Its advantage is can be discharged deeper and will last longer.

Bottom line here the S290 is higher capacity (220 vs 200 at the 20 hour discharge rate, and can deliver higher discharge currents.

Knowing this if I were you I would look at a Trojan T-105RE a true 6-volt 225 AH Deep Cuycle battery that cost a bit less and actually a better product than the two you are looking at in Rolls product line.

[ElectronicFur]

Thanks Sunking, I'm surprised the S290 is a Golf Cart battery when it's part of the 4000 series listed under Solar. Bit confusing for newbies like me, I'm glad I looked at the plate config now and asked about it, having read up on things on here.

I actually ended up looking at the Rolls rather than the Trojan's because in another thread you stated that the best battery was the one with the longer warranty, and the Rolls 4000 series have 7 years, compared to the 5 years of the Trojan Premium.

I was actually looking at the Trojan T-105RE yesterday, but couldn't find any info about the plate configuration. I noticed they mention the plates are Carbon coated now on the Premium batteries. Does that make a big difference on things in your experience?

Where I am the two 6V T-105RE works out about the same price as a 12V T1250, so I'd get more Ah, but less warranty. You think the T-105RE's are the ones to go for?

[Sunking]

Thanks Sunking, I'm surprised the S290 is a Golf Cart battery when it's part of the 4000 series listed under Solar.

Actually you will see the same exact batteries in Motive Power, Marine, and Rail Road.

I actually ended up looking at the Rolls rather than the Trojan's because in another thread you stated that the best battery was the one with the longer warranty, and the Rolls 4000 series have 7 years, compared to the 5 years of the Trojan Premium.

As a general statement that is true, and I stand by it.

I was actually looking at the Trojan T-105RE yesterday, but couldn't find any info about the plate configuration. I noticed they mention the plates are Carbon coated now on the Premium batteries. Does that make a big difference on things in your experience?

Where I am the two 6V T-105RE works out about the same price as a 12V T1250, so I'd get more Ah, but less warranty. You think the T-105RE's are the ones to go for?

Trojan does not go into detail about plate configuration. The T-105 is the Golf Cart Battery which means it is a hybrid, and the T-105RE is a true Deep Cycle battery. Look at the weight and you will see the difference. The T-105RE is 5 pounds heavier with more lead in it.

Myself personally I have more experience with Trojan. I don't think you could go wrong with either Trojan or Rolls. Out of personal bias I would probable go with Trojan T-105RE if it fit my purpose. The reason for that is the T2 Carbon Technology has proven itself being able to handle Partial State of Charge aka PSOC, where is all other Pb batteries suffer operating in PSOC.

In solar systems, batteries stay in PSOC range and never really ever reach 100% SOC. Leaving lead acid batteries less than 100% charged up is the killer of lead acid batteries. The T2 Carbon is showing to work fairly well at minimizing the effect of lead sulfate crystals hardening on the plates aka sulfation. Additionally Trojan warranty claim service IMO works a lot better. Trojan has a lot of dealers here in the USA. So if you do have a warranty claim is just a lot easier and faster to deal with.

Also keep in mind Rolls target markets are Marine and Rail Roads, and Heavy Equipment. That is part of what confused you. Go look and Click on Rolls Marine, Rail Road, and Motive Power batteries. They are the exact same as Renewable Energy batteries. All Rolls did is when RE became a market just made a category called RE and uses the same batteries.Rolls bread is buttered by the Marine market.

Now Trojan was guitly of that too when RE market opened up. But Trojan, Rolls, and all others discovered a huge problem when they started going after the RE market. All of them got clobbered with warranty claims from RE customers. What all the battery manufactures soon discovered is batteries do not get fully charged when solar is used and work in a PSOC range. The problem was two-fold.

1. The manufactures charging voltage recommendations were too low for Solar. Worked fine when used with AC chargers because with AC you got all the time in the world to go through all 3-Stage Charging. In Solar you do not. So first thing they did was raise charging voltage set points for Solar Customers to help speed up the process by forcing the controller to stay in Constant Current mode longer from sun rise to sun set.

2. Trojan took an extra step. The found a way to deal with PSOC and developed a new line of batteries just for RE, and also upgraded to their Industrial line. So far the other manufactures have only changed charging recommendations.

As a final thought if you want the best battery out there I would say that battery is Trojan Industrial line or Fork Lift Batteries which again are Industrial. Problem is they are much larger than you are looking for.