6- vs 12-volt

jsboettcherjsboettcher Registered Users Posts: 3

I have read multiple treads on 6- vs 12-volt batteries, and everyone goes too in-depth for some of the questions. I have a couple questions, and I will try and frame them in a direct and simple way.

 I have a simple weekend cabin in the woods, and I operate on solar. I have two independent banks of thirteen 12-volt batteries that are (87AH), for a total of 1131 AH. These are charged with 900 watts of solar panels.

My two banks of batteries are near the end of their life. I need to replace them. I am debating 12-volt batteries, or 6-volt.

My loads are five LED light bulbs, a refrigerator (1.5A @110v) and a flat screen TV (0.9 A @ 110v)

I am NOT worried about weight, cells to fill, nor space.  Considering performance and the ability to last longer between chargings, will the 6-volt batteries provide better performance than the 12-volt?

I am looking at CROWN 6-volt batteries that have 205AH rating.

I am looking at connecting the 6-volt batteries in series (to get to 12 volt), and then in parallel to get the best usage of the AH. I am looking to make each bank 1230 AH.


What will offer the best performance and longevity, this 6-volt set up, or twelve 12-volt batteries?



Comments

  • BB.BB. Super Moderators, Administrators Posts: 31,063 admin

    Welcome to the forum JS,

    If the choice is to make a very large battery bank from "smaller" batteries, I would prefer two 6 volt @ ~200 AH batteries in series to make a 12 volt battery, then paralleling those--Vs just getting 12 volt @ 100-200+ AH in parallel.

    The simple reasons--A) 6 volt @ 200 AH batteries are very common "Golf Cart" batteries and tend to be cheaper and a bit more rugged (larger and possibly thicker plates) that an equivalent 12 volt @ 100 AH battery. 3x higher amperage cells in series for 6 volt, vis 6 smaller cells in series for 12 volts--Both battery store the same amount of energy (6v*200AH=12v*100AH=1,200 WH of storage).

    B) It is very easy to use a voltmeter and measure the voltage across each 6 volt battery to see the basic health of the battery bank. You will find batteries with high or low voltage, and can look deeper to see what is "wrong". With a bunch of 12 volt cells in parallel--They all have the same voltage and you have to do other things to find weak/failing batteries (measure specific gravity, use a DC current clamp meter to see how charging/discharging current is being shared, disconnecting a lead to each battery and measure the disconnected/resting voltage).

    I am not a fan of making a battery bank out of 5-6 parallel strings of batteries--And I would be looking at something else (larger AH batteries or cells) and trying to get the number of battery strings down to 1-3 strings in parallel as a maximum (save on wiring connections and things that can go wrong).

    I would also be suggesting looking at your loads and energy needs. Battery banks over ~800 AH, it is usually a good idea to step up to the next higher battery bus voltage... I.e., a 12 volt @ 1,200 battery bank becomes a 24 volt @ 600 AH battery bank (now 4x 6 volt @ 200 AH in series for 24 volts, and 3x parallel strings for 600 AH at 24 volt battery bank). And you will be down to 3 parallel strings.

    Inverter wiring is now 1/2 the size, and voltage drop is less of an issue with 24 volts vs 12 volts. If you are using solar chargers (not sure--cabin in the woods may mean too much shading for solar), solar chargers are rated at current output... i.e., 60 amps at 12 volt = ~720 Watt array and 60 amps at 24 volts = ~1,440 Watt array support (same controller, just higher bus voltage).

    You talk about time between charges and health of batteries? You run a genset to recharge your bank?

    If so, this gets into you want to recharge your battery bank back over 90% state of charge, and come back within a month to run the genset again. FLA batteries typically can go only ~1 month between charging or they begin to sulfate and "die".

    If you have a float solar array (1% to 2% rate of charge) and some decent sun (and cool weather), that may be able to float your battery bank between visits (recharge with genset before you leave, then float on solar).

    By the way, how long did this bank last, and did it meet your needs? Would a propane refrigerator work, and allow you to reduce your electrical needs? For a weekend/seasonal cabin, sometimes propane makes more economic sense than a battery/solar/genset system.

    I will stop here--I am not sure how far you want to go and review your present system and design.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jsboettcherjsboettcher Registered Users Posts: 3

    I have a 900 watts of solar panels charging one bank, and 600 watts charging the other bank. They are also managed by a MPPT charge controller, so they are being managed daily.

    I can but the 6-volt batteries for $80.00 new. the 12-volt are $130.00 for 100AH


  • BB.BB. Super Moderators, Administrators Posts: 31,063 admin

    Yep, 6 volt is better value:

    • 2x 6 volt @ 200 AH in series gives 12 volt @ 200 AH for $120 for the pair
    • 2x 12 volt @ 100 AH in parallel gives 12 volt @ 200 AH for $260 for the pair

    Just an "extra jumper" to series connect the two 6 volt batteries in 12 volt string -- Also, now 1x series string (6 volt batts) vs 2x parallel (12v batts). I am a big fan of reducing parallel strings. With 6 volt @ 200 AH batteries, you will have 1/2 the number of parallel connected strings.

    Just to give you an idea of what a "balanced" system would look like... A 12 volt @ 1,200 AH battery bank would suggest a 3kWatt maximum inverter--But for 12 volt battery banks, it is difficult reliably get more than ~1,800 Watts because of 12 volt bus/wiring voltage drops.

    We recommend 5% rate of charge minimum, and 10% to 13%+ for solar charging. 5% for weekend/summer cabin. 10%+ for full time off grid usage (Note that 1,200 AH @ 12 volt battery bank is the same storage capacity as 600 AH @ 24 volt battery bank--So the Array calculations are the same (p=v*I, 2xV and 1/2xI -- equals same power):

    • 1,200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 1,230 Watt minimum array
    • 1,200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,260 Watt nominal array
    • 1,200 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.15 rate of charge = 2,938 Watt array "cost effective" maximum

    In general, more solar panels, less generator usage. Deep Cycle flooded cell lead acid battery mfg. typically recommend 10% minimum rate of charge for best battery life.

    In general, weekend cabin batteries are generally going to age out before they cycle out of life. Avoiding a bunch of deep cycle to 20% state of charge, storing batteries >75% state of charge, and keeping batteries cool (for every 10C/18F over 25C/75F, batteries will age 2x faster--1/2 life).

    Cheap golf cart batteries typically last 3-5 years. And good quality may last 7-8 years (assuming something else does not go "wrong").

    Not sure where the cabin is located, but assuming not used in deep winter, you will probably get 4-5 hours of sun minimum per day (assuming you are up north). A 1,230 Watt system would output (on average):

    • 1,230 Watt array * 0.52 off grid AC system eff * 5 hours of sun per day (sunny summers) = 3,198 Watt*Hours per day

    That is enough power (and you have a large enough battery bank) that it should run your refrigerator+other loads quite nicely for non-winter occupation... Of course, you have to have the sun and no sunny area.

    If I found the correct inverter, this is an MSW (modified square/sine wave) inverter.

    http://powerbright.com/manuals/PW3500-12_MANUAL_english_french_02.pdf

    If it goes bad, or you think about going 24 volt volt battery bus--I would be suggesting a 1,500-2,000 Watt PSW (pure sine wave) inverter--MSW inverters are hard on induction motors (refrigeration compressors) and some electrics (computer AC power adapters, and even some chargers for tool batteries) can cause early life failure too.

    A 3,500 Watt 12 volt inverter (and remember it supports a 7,000 Watt surge) would draw:

    • 3,500 Watts * 1/0.85 AC inverter eff * 1/10.5 volts battery cutoff = 392 Amps at rated load (and 2x that at 7 kWatt)

    That inverter has some pretty heavy/short cabling and heavy currents for this size battery bank (AH and voltage).

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭

    +1 on fewer bigger batteries. I'd likely go with 6v L16 size (~350-400ah). 3 strings would be in the ballbark capacity you want. Among other things, bigger cells means fewer cells for given capacity. L16s would be 1/2 as many cells to check/water.

    Also, lots of 12v aren't "true" deep cell, they are often hybrid/marine types not especially well suited for our application.

    Off-grid.  
    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
  • mike95490mike95490 Solar Expert Posts: 9,022 ✭✭✭✭✭

    Looking at the battery wiring, you should really read up on best practices here

    http://www.smartgauge.co.uk/batt_con.html

    because your massive parallel bank, will destroy cells before their normal end of life.

    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || 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 ,

  • littleharbor2littleharbor2 Solar Expert Posts: 1,557 ✭✭✭✭

    You could put together a bank of Fullriver 1150 amp hour 2 volt batteries. 6 batteries, 5 simple interconnect cables and a SINGLE string. It doesn't get any simpler than that.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • jsboettcherjsboettcher Registered Users Posts: 3

    littleharbour2


    How much??

  • BB.BB. Super Moderators, Administrators Posts: 31,063 admin

    From our host (these are List prices--Create an account and some items are less):

    $657 per 2 volt cell, 6x cells = $3,942 for a set (AGM batteries are much more expensive than normal FLA)

    Or you could go with:

    $385 per cell * 6 = $2,310 for the set (FLA batteries)

    There are lots of 2 volt large AH cells:

    Fullriver is an AGM battery made in China--The present "trade disputes" between China and USA can dramatically affect the import duties.

    Vs your 6 volt @ 200 AH batteries (2 batteries in series * 6 parallel strings for 12 volts @ 1,200 AH) would be:

    $80 per batt * 12 batteries = $960 for the set

    There are reasons for choosing high AH single 2 volt cells, and AGM cells, but low price is not one of them.

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • littleharbor2littleharbor2 Solar Expert Posts: 1,557 ✭✭✭✭
    edited March 2019 #10

    "Low price isn't one of them" Exactly.

    One thing about the Trojan 2 volt batteries shown is they are actually 3, 2 volt cells in parallel. This kind of bothers me in that if buying 2 volt, large capacity batteries to avoid parallel connections, why would you start out with this format. As shown in the 2 volt link bill has given there are single cell 2 volt batteries. They seem to be larger capacity, more suited for industrial applications maybe. The beauty of these is the fact that, in a 12 volt bank for example , that you have 6 cells to check, not 18.

    OK rant over. I personally haven't had the need for large capacity 2 volt cells. A friend has a 24 volt bank of the Fullriver 2-1150's and the simplicity of his large battery bank is quite admirable. I know AGM batteries are costly but the low maintenance and ability to sit for longer time with the system shut down make them my choice. Typically in this part of Baja, most folks leave for the summer, due to the heat and humidity. People with flooded batteries have to leave their systems up and running and have someone check the electrolyte levels in their batteries. I've yet to be one of the brave souls to volunteer to go check on a half dozen or so , other people's systems in the August inferno.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

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