2 volt batteries vs 6 volt

Tooltime
Tooltime Solar Expert Posts: 45 ✭✭
What I have read over and over is that 6 volt is much better then 12 volt, does that meen that 2 volt sd7 batteries would be a good choice.

Thank you '

Tom

Comments

  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: 2 volt batteries vs 6 volt
    Tooltime wrote: »
    What I have read over and over is that 6 volt is much better then 12 volt, does that meen that 2 volt sd7 batteries would be a good choice.

    Thank you '

    Tom
    Unfortunately I'm not familiar with "2 volt sd7 batteries".
    Tried Googling the phrase, but nothing came up. Do you have a better description? Make / manufacturer etc?
    What are your intended loads etc?
  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    Re: 2 volt batteries vs 6 volt
    Tooltime wrote: »
    What I have read over and over is that 6 volt is much better then 12 volt, does that meen that 2 volt sd7 batteries would be a good choice.

    All lead-acid batteries are made of 2V cells, so this is only a question of packaging.

    With 2V batteries you need more cables to connect, but 2V batteries are not as heavy as 6V batteries of the same capacity and model.
  • Tooltime
    Tooltime Solar Expert Posts: 45 ✭✭
    Re: 2 volt batteries vs 6 volt
    Unfortunately I'm not familiar with "2 volt sd7 batteries".
    Tried Googling the phrase, but nothing came up. Do you have a better description? Make / manufacturer etc?
    What are your intended loads etc?

    http://www.alcad.com/Products/Lead-Acid-batteries/SD-SDH-Series-Lead-selenium
  • Tooltime
    Tooltime Solar Expert Posts: 45 ✭✭
    Re: 2 volt batteries vs 6 volt

    The part that I just dont understand is this. For comparative purposes, this example uses eight "golf cart" batteries of 220 Amp hours and 6 Volts, or 1320 Watt hours (DC) each. Times eight that is a battery bank capacity of 10,560 Watt hours, up to 50% of which would be "usable" for 5,280wh.

    The same math is 120ah 2 volt is 240 watt hours, times 12 batteries for my 24 volts is only 2880 watt hours. Not even close to the 6 volt golf cart batteries
  • Tooltime
    Tooltime Solar Expert Posts: 45 ✭✭
    Re: 2 volt batteries vs 6 volt

    What are your intended loads etc?

    I am still working on the loads of the household. This will be a complete household completely off the grid. Just bought a kill a watt meter last night.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 2 volt batteries vs 6 volt

    More or less, a 100 lb lead acid battery (or 50 kG) will store about the same amount of power whether it is 2/4/6/12 volt... For example

    6 volt * 220 AH = 1,320 Watt*Hours of storage for "6 volt golf cart battery"
    12 volt * 110 AH = 1,320 Watt*Hours of storage for a "12 volt GC sized battery"
    4 volt * 330 AH = 1,320 WH for a 4 volt GC sized battery
    2 volt * 660 AH = 1,320 WH for a 2 volt GC sized battery

    Here is some real life similar sized batteries from Trojan at NAW&S (our host's store):



    Trojan RE Series Deep Cycle Batteries for Solar & Renewable Energy Systems


    Part Number
    Volts
    AH @ 20 HR Rate
    AH @ 100 HR Rate
    Length
    Width
    Height
    Weight (lb)


    T105-RE
    6
    225
    250
    10.375"
    7.125"
    11.75"
    67


    L16RE-A
    6
    325
    360
    11.575"
    7.125"
    17.7"
    115


    L16RE-B
    6
    375
    410
    11.575"
    7.125"
    17.7"
    118


    L16RE-2V
    2
    1110
    1235
    11.575"
    7.125"
    17.7"
    119



    Notice that weight (amount of lead per battery) is a big variable here... If, we instead take the V*AH and divide by the weight we see:
    • 6v*225AH*1/67 lbs = 20 WH / lb
    • 6v*325AH*1/115 lbs = 17 WH / lb
    • 6v*375AH*1/118 lbs = 19 WH / lb
    • 2v*1110AH*1/119 lbs = 18.7 WH / lb

    All the batteries have about the same amount of energy storage per lb of battery weight.

    The difference here is that we can put 2*6 volt * 375 AH in series 4,250 WH of storage (2 batteries * 118 lbs) or we can put 6*2 volt batteries at 1,110 AH for 13,320 WH of storage (6 batteries * 119 lbs each) in our system and still only have 6 cells to monitor (specific gravity, fill with distilled water).

    I would still only have 120 lb (55 kG) cells to wrestle around, and I get ~3x the storage.

    My other option would have been to get 6*6 volt cells (same weight) but then I would have 3 parallel strings of batteries (each string with 2 batteries in series). Now I have 18 cells to monitor, I have 3x as many parallel electrical connections to monitor. If I have any one cell fail (open or shorted), they will be masked by the other cells around it (and a shorted cell could take down a whole bank, or an open cell could damage a series string of batteries, etc.). Also, when you parallel battery banks, you should have a fuse/breaker per battery string (more wiring, more hardware, more costs) to prevent one shorted battery/wiring harness from being fed too much current from the rest of the strings.

    Also, when paralleling batteries (or any voltage sources), it can be difficult to make sure that the current "shares" roughly equally between all the strings of batteries. A dirty connection, slightly corroded cable, one battery hotter (or colder) than the rest, and the sharing of current goes out the window.

    None of the above are insurmountable issues... They just take a bit of extra care and money to address. My personal recommendation is to aim for one series string, and use 2-3 parallel strings maximum for a system (unless there are other issues--such as finding batteries with the larger AH capacity). Also, besides the usual tools of a hydrometer (specific gravity measurements) and a good quality volt meter (measure cell/battery voltages, voltage drops across wiring harnesses under load/charging), we now can get pretty reasonably priced DC Current Clamp Meters (like this $60 unit from a US store that does AC/DC and DMM--digital multi-meter--type functions). You can use a DC current clamp to just take a look at the current flow among the parallel battery strings once a week to once a month (under heavy load, and heavy charging) to make sure all is OK.

    So, like most engineering problems--There is no one right or wrong answer. There is a big gray/grey area that we can usually help people understand and meet their needs. For example, there are lots of people with three or more parallel strings of batteries that are very happy with them--But they tend to be maticulus and monitor their batteries pretty closely (and some folks even move cells around once a year to keep everything "balanced" on average).

    So, rather than trying to answer your question in the abstract--What are your needs and what is it you are looking for? For example, some folks in remote areas like two parallel strings of batteries with an A/B/Both switch between them. This allows them to switch out a failed string and still limp along until they can arrange to repair/replace the failing items.

    Other folks have found that if they have a 48 volt battery bank and one cells fails--They can take the one cell out of service and still have everything work pretty normally (their batteries were 2 volt cells--so it was "easy" to bypass).

    On the other hand, failure analysis tells us that having 2x as much of anything, pretty much doubles the chances that something will go wrong... Murphy's Law means there are no winners in his game.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Tooltime
    Tooltime Solar Expert Posts: 45 ✭✭
    Re: 2 volt batteries vs 6 volt

    BB, thanks for all that. Looks like I made up mind. The problem is the batteries I was looking at are just to small. This learning curve sucks.

    Tom
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 2 volt batteries vs 6 volt

    Hi Tom,

    I forgot to put in the Sears link to a $60 DC Current Clamp meter. In my previous post (it has been updated too).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: 2 volt batteries vs 6 volt

    As BB Bill alluded to, WHEN one cell fails, that could translate into replacing ALL of the other cells in the battery. If it is a 2V FLA batt, there are NO other cells, which means less cost and waste. Furthermore individual 2V cells allow monitoring the voltage of every cell in the string, and even charging and EQing an individual cell (with the proper equipment).

    Some batteries of more than 2V do have removeable 2V cells, allowing easy replacement, and remedial charging.

    In my opinion, run the fewest number of strings possible -- one string is ideal. AND run the lowest possible voltage per battery -- 2V for LA batts. This means finding batteries with sufficient Capacity with a single string (if at all possible). Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: 2 volt batteries vs 6 volt

    Then you have this one " Trojan L16RE-2V 1,110 Amp-Hour 2 Volt Deep Cycle Battery " that is 3 individual cells ( 370 amp ) paralleled in one case. To me it's no better than a 6 V. Seems like one bad out 3 cells would drag the others down.
  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    Re: 2 volt batteries vs 6 volt

    Great point BC, these batts lose most of the advantages of running "2V" batts, still same # of cells to check, and the risk of one of the three dragging the others down. The only benefit is that the mfg handles the interconnects. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: 2 volt batteries vs 6 volt

    Yea--Trojan sort of blew it on their two volt design. Those (appear to be) simply their standard 6 volt battery with the bus bars rearranged to put the three cells in parallel instead of series. Does not really meet the requirements for what I would suggest as a true "2 volt" cell.

    However, in a large AH battery bank, it does at least allow you to have a large string with "replaceable" 2 volt cells. I have not read anybody here yet who has used these cells and offered a review. Do they have any issues with "balancing" cells SG and sharing of current--Kind of difficult to tell since the cells are paralleled internal to the battery.

    I was trying to give the 50,000 foot view without adding a bunch of confusing caveats.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • DANMILLER
    DANMILLER Registered Users Posts: 1
    Re: 2 volt batteries vs 6 volt

    These 2v batterys are made by ALCAD and you can go to there web site and check them out. New cost is $500 ea so you don't see many people using them. Mostly power companys or remote oil locations.
    DAN
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Re: 2 volt batteries vs 6 volt
    DANMILLER wrote: »
    These 2v batterys are made by ALCAD and you can go to there web site and check them out. New cost is $500 ea so you don't see many people using them. Mostly power companys or remote oil locations.
    DAN

    They're made by FIAMM as you can see on the label in the photos, and only rated to 1000 cycles no depth of discharge mentioned. For that kind of price, better to go for the OPzS style sells (FIAMM also makes those): http://www.fiamm.com/Industrial-Batteries-LM-OPzS-Zone-19-typeLBU4L13-Prod-3333-559