Why 2 x 6V battery over a single 12V?
Rybren
Solar Expert Posts: 351 ✭✭
I often see recommendations to use multiple 6V batteries in series rather than a single 12V (or multiple) batteries to build the battery bank, but I've never seen a good explanation for why (aside from the possible cost).
For example, if I wanted a 200Ah 12V bank, I could use 2 Crown CR200 6V batteries - $149ea locally or a single Crown CR210 12V - $349 (they don't have a 12V 200Ah)
Aside from the cost, what is the advantage to using the 6V batteries? There'd be a cost to connect them in series, and I'd have to check & test two batteries. What am I missing?(or perhaps I've misunderstood and there is no advantage
)
I'm not ready to buy anything yet (still working on the AIM) but I am trying to improve my knowledge and basic understanding.
Thanks
For example, if I wanted a 200Ah 12V bank, I could use 2 Crown CR200 6V batteries - $149ea locally or a single Crown CR210 12V - $349 (they don't have a 12V 200Ah)
Aside from the cost, what is the advantage to using the 6V batteries? There'd be a cost to connect them in series, and I'd have to check & test two batteries. What am I missing?(or perhaps I've misunderstood and there is no advantage
)I'm not ready to buy anything yet (still working on the AIM) but I am trying to improve my knowledge and basic understanding.
Thanks
Comments
This might help clear things up for you:
http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power
It's about 12 Volt vs. 24 Volt vs. 48 Volt systems, but the basic principal is the same: having a single string of batteries rather than parallel strings of equivalent power.
So with the 6 Volt batteries vs. 12 Volt, putting two 200 Amp hour 6 Volt batteries in series is preferable to putting two 100 Amp hour 12 Volt batteries in parallel.
Here is a thread that goes "oh too deeply for sane people" into why I, and others, suggest trying for a single string of batteries, and never more than ~3 strings in parallel unless you have no other options:
Series rule of thumb
Note, there are people with lots of parallel strings of batteries and very happy with them... However, in my humble opinion, lots of paralleled batteries require a fair amount of extra costs (fuses/breakers per string), work (more cells to check/add water), and monitoring (a DC Current Clamp Meter/DMM measuring current sharing per string and voltage per cell/battery when under heavy charge/discharge), etc...
For folks outside the US/major population centers, their battery choices are severely limited and they have few other options than lots of paralleled connections.
-Bill
In the battery you mention there would be no difference in buying one 12 v or two 6 v, unless the weight ( 131 LB ) of one makes it hard to handle. It is true you have to have a connector cable for the 6 V. Most that talk about a 12 V are talking about a " Wal-mart Marine / Deep Cycle " battery that will NOT hold up under Solar Charging and Discharge.
BC04 brings up another key point--True Deep Cycle vs "Marine" vs SLI (Starting, Lighting, Ignition) type car batteries. Marine and SLI batteries are not very good at supporting deep cycle (typical off grid power) systems. This is not a 6volt/12volt battery issue--it is the internal design and external wiring/cabling/maintenance issues that tend to favor a single string of high AH batteries vs a lot of "higher voltage" batteries connected in parallel.
Physical weight of battery bank is a limiting factor for many people. Not everyone has a concrete loading area and a setup for a crane/forklift/pallet jack to move 2,000 lb batteries around. The "solution" is usually finding lower voltage (6 volt, 4 volt, 2 volt cells) with high AH ratings that are still reasonable weight to move around by hand (150-300 lbs maximum for a couple people) and connect in series. And raise the bank voltage (12 to 24 to 48 volt) to keep the AH size down (i.e., a 220 AH battery bank at 48 volts has 4x the energy storage of a 12 volt 220 AH battery bank--and is much "easier" to support than an 880 AH @ 12 volt battery bank with the 4x higher current needed to support charging/loads).
Read up on Batteries--Lots of information, and opinion (sometimes the old way of doing things still make sense, and others are actually dead wrong):
Battery FAQ
www.batteryfaq.org
After I read the above--I am about convinced that lead acid storage batteries do not work for solar applications
Obviously they do, and are one of the best choices (cost, reliability, ruggedness, ability to recycle, etc.) of battery chemistry out there.
-Bill
Guys thanks for the quick and informative replies.
Blackcherry04 hit on what I was trying to get at - is there an advantage to using 2 x 6v versus a single 12V to obtain the same amount of capacity (200Ah in my example above). I guess that the short answer is that there isn't a performance advantage.
Thanks
A single 12 volt @ 220 AH battery would weigh ~2x as much as a single 6 volt @ 220 AH battery--So it is your ability to move the battery(ies) around that would be the limiting factor here (and total costs).
-Bill
Thanks Bill.
I guess that I was reading too much into all of those battery threads, FAQs, etc.
Yep. The other practical difference comes when the AH requirement is so high that you cannot find a reasonable 12 volt battery to supply it and would have to go to putting 2 or more 12 volt batteries in parallel. That would be a big performance problem, in terms of equal sharing of charge and discharge current, etc. So by making the bank up of a series string of higher amp hour, lower voltage batteries you avoid the need to parallel wire.
This is a different region of size than you are currently operating in.
An additional point, is that when/if one cell fails, the fewer the number of additional cells that would need to be tossed when replacing that battery, the better.
Some larger batteries have removeable cells that are bolted together, which also helps. IMHO, the fewest number of cells per battery, the better. 2 V batteries are very nice.
This is a bit beyond the original question, but just belaboring the point, while throwing in a few misspelled words.
old thread, but the previous poster alludes to the redundancy issue. when off the grid running 12VDC draws, if one cell goes bad in one member in a serial 2X6V setup, you're 12VDC powerless. if one cell goes bad in one member in a paralleled 2X12V setup, you've still got some power available at 12VDC. i'm assuming you're in the field somewhere, and repairing or replacing the failed 6VDC battery/cell isn't an option.
There are even other options too... We had one person here with a large 48 volt battery bank made from 2 volt cells in series.
He had one 2 volt cell fail. Just took it out of the string and reprogrammed the charge controller one cell lower until he could get a replacement.
Parallel strings are a bit of a pain for maintenance. Many more cells to check, more wiring to keep clean. Should have a fuse/breaker per string (more expense), more cells--therefore a greater chance that "something" will fail, etc...
My own personal recommendation is to have backups... Generally, that would be a fuel driven genset (or two). But, if you need a backup battery bank--Then going high voltage with 2 volt cells, or limiting the number of parallel strings to 2-3 parallel strings maximum--both to reduce maintenance/things that can fail/expenses, and because current sharing for Voltage Sources (e.g., batteries) can be poor.
Get a "cheap" (or expensive) DC current clamp meter (as well as a hydrometer for flooded cell batteries plus a Battery Monitor of some sort) and use it to monitor shared current (during heavy charging and discharging) to ensure that the batteries are properly sharing the loads (once a week to once a month).
-Bill
re: backups, yes, always. i have a small RV with 2 large 12V AGMs for coach power, and a small generator as a backup. i've been toying with the idea of upgrading the whole package with newer technology for years, but every time i travel and use what i have, and it works flawlessly, it winds up on the back burner until the next trip. and there's no guarantee the RV will outlast whatever i add, so that slows the thought process down a bit, too. my luck, i'd spend a few thousand on it, trick out the off grid power systems, and the engine would blow up the next day.
and the engine would blow up the next day.
well then you will have a tricked out PV system for the new coach...8)
KID #51B 4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
West Chilcotin, BC, Canada
i could look at it that way, i suppose.:p
It may also be easier to replace a failed cell depending on how the 6v string is constructed.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
Second system 1890W 3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
What is the application? Are you talking about 12v, 24v or 48v or other systems?
What are your expectations of the system in terms of daily performance and lifespan?
Marc
Tony
2 x 12v =12 cells, 2 x 6v =6 cells for about the same overall weight and capacity.
Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
a) with more physically separate cells, one can often catch a problem, replace/omit that cell and get greater life from the entire battery system
b) more cells is more failure points, reducing average life of the entire battery system
I am available for custom hardware/firmware development
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A
solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,
Also, in an RV ( mobile environment ) an open cell is sudden and unexpected.