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Thread: 2v batteries or 6v - which is better?

  1. #1
    bernie1 Guest

    Default 2v batteries or 6v - which is better?

    Hi,
    This is my second solar project. I have moved and am starting all over again. I find my brain trying to refresh itself. I am starting to look for batteries and hope some experienced folks can help me make the right decisions.

    In my old system I had 16 Trojan L16 batteries (6 volt). I had two banks of 8 batteries, times 6 volts = 48 volt system. It did great, but I also had a hydro-electric system, so I know that helped me tremendously at night.

    My new place just has solar (for now) - if I wanted to increase my battery storage capacity would I be better off going with 2 volt batteries this time? I realize I could get 32 batteries and double my old system but that seems like a LOT of batteries.

    My gut instinct says YES go with 24 2 volt batteries, but my brain is not lighting up the reasons why. I was planning on using another 48 volt inverter to hook them up to - which would be the most intelligent?

    I'm thinking 24 two volt batteries in series = 48.

    -- I have not started collecting all the pieces yet. Any general words of advice? My old system we used a Xantrex SW5548 inverter. Outback MPPT charge controller. Are there any big advances I should know about in batteries or major components? Any super great system I should know about? New things I should consider?

    Very best regards and thanks for putting up with my ignorance!

  2. #2
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    Default Re: 2v batteries or 6v - which is better?

    My two cents... I do not like paralleling a large number of strings of batteries for several reasons:

    • lots of cells to check water levels (if not AGM/Sealed)
    • difficulties in sharing current equally between strings--have to pay attention to equal cable length/resistance and even bolt torque to ensure equal distribution wring resistance
    • difficulty in finding "bad" cells/batteries when everything is paralleled together
    • Each parallel string should have its own protective fuse/fuse holder to prevent short circuits from damaging wiring / starting a fire

    To that end, from my experiences in non-battery applications, I don't think I would put more than three strings together in parallel. Certainly people do put 4-8 or more strings together in parallel and are very happy with their bank--but I, personally, would avoid doing that if possible.

    So, that leaves "picking a battery/cell size"... In the old days, you purchased a bunch of 6 volt "golf cart batteries" and series / paralleled them to the bank capacity / voltage you needed.

    You can get much larger multi-cell batteries -- but those things literally can weigh a nearly a ton each.

    Crown Industrial Battery 12/125/13
    24 volt, 935 AH


    Some battery manufacturers are now making 4 volt and 2 volt cells in, roughly, the form factor of golf cart batteries today. Basically, you can now get a relatively high AH capacity 2 volt cell that one or two guys can horse into position without a crane or forklift access.

    Surrette 2KS-33PS Deep Cycle Battery
    1766 Amp-Hour, 2 volt

    Trojan L16RE-2V 1110 AH 2-Volt Deep Cycle Battery


    The other thing to thing about is the working voltage of your system... You have picked 48 volts--and that is usually a good idea... Instead of two banks of 24 volt strings, you can now have one bank of 48 with one string (or whatever battery/capacity works out for you).

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  3. #3
    bernie1 Guest

    Default Re: 2v batteries or 6v - which is better?

    Thanks for the reply! The internet "worked" for once. I will remember this when making my purchases. For confirmation here, it looks like you are telling me that

    (a) 2v batteries will be more maintenance in terms of more cells to top off when I'm watering + installing is more tricky, need to keep cables proper length and even need to somehow apply equal strength when I wire them up
    (b) the 24v battery from Crown looks like the winner, I'd just need to be able to buy 2 of them for my 48v system and have the ability to move them around + enough solar to keep them charged up.
    (c) you mention Trojan's 2v battery but not the 6v. this is because I can get so many more AH out of the 2v battery, right? I assume that's the point, I'm just trying to make sure my thinking cap still works.

    Sorry for my lack of confidence in my own intelligence. The investment is high dollar and I want to make sure I'm not making any mistakes in my selection.

    Still open to any advice/thoughts from anyone who can shed light on the best inverter and MPPT charge controllers on the market these days.... any additional battery thoughts? awesome message forum! Thanks so much.

    Bernie

    PS: moderator Bill, do you prefer I have separate thread for this inverter and MPPT charge controller questions? I'd like to be a well behaved newbie.

  4. #4
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    Default Re: 2v batteries or 6v - which is better?

    Quote Originally Posted by bernie1 View Post
    (a) 2v batteries will be more maintenance in terms of more cells to top off when I'm watering + installing is more tricky, need to keep cables proper length and even need to somehow apply equal strength when I wire them up
    Not exactly--I personally would rather have, for example 24x 2 volt 1,000 AH cells instead of 4 strings x 4 x 12 volt 225 AH batteries (24 cells/connections to keep up vs 96 cells to keep watered plus 4 sets of parallel connections).

    The issue about "balanced" cable length/sizes is that if one string had long/small awg cable and another string had short/large awg cable--the string with the short/heavy cables will do most of the charging/discharging because it has the lowest resistance between the battery and the load.

    Smart Gauge

    Here is a thread with a little bit more on battery wiring:

    Battery wiring configuration questions...

    (b) the 24v battery from Crown looks like the winner, I'd just need to be able to buy 2 of them for my 48v system and have the ability to move them around + enough solar to keep them charged up.
    Before you get caught up in Tony's (Icarus') line of "Ready, Fire, Aim"--you should understand your daily loads in AH or WH... We usually recommend a starting point of sizing your battery bank to handle 3 days without sun and 50% maximum discharge for long life. Or 6x your daily load.

    Then, you can work backwards and figure out how much solar power you need based on where you live.

    For the most part, you can treat everything in an off grid system pretty badly (over load, under load, leave sitting for months unconnected, etc.) -- except for one component--your Battery Bank.

    Treat them badly, and you will knock years of their lifetime.

    (c) you mention Trojan's 2v battery but not the 6v. this is because I can get so many more AH out of the 2v battery, right? I assume that's the point, I'm just trying to make sure my thinking cap still works.
    Yes--I am not recommending or slighting any particular battery configuration or vendor here--just showing an example.

    Pretty much, I would measure/estimate your daily load--Multiply it by 2x if you are planning on growing your loads--then multiply that by another 6x (3 days of no sun, 50% maximum discharge)...

    As you can see, we can quickly increase the size of your system if we are just guessing at your daily power requirements--And size of the battery bank (and loads) drives your min/max "rule of thumbs" solar array size.

    Too small of array, you undercharge your battery bank and/or have to use a generator more often to keep them up.

    Too large of solar array--you are pretty much just throwing money away because the battery bank will not be able to store all of the power generated.

    Sorry for my lack of confidence in my own intelligence. The investment is high dollar and I want to make sure I'm not making any mistakes in my selection.
    "Measure twice, cut once" is the old carpenter saying...

    Still open to any advice/thoughts from anyone who can shed light on the best inverter and MPPT charge controllers on the market these days.... any additional battery thoughts? awesome message forum! Thanks so much.

    Bernie

    PS: moderator Bill, do you prefer I have separate thread for this inverter and MPPT charge controller questions? I'd like to be a well behaved newbie.
    If you are going to talk about your entire system--frequently is is easier to keep everything in one thread--otherwise we (those of use with short attention spans) quickly mix up your system requirements with other system questions.

    MorningStar right now makes a very good range of MPPT (and PWM) type charge controllers. They also make a very nice 12 volt 300 watt TSW inverter.

    Schneider (Xantrex) makes a very nice 4-6kW Hybrid Inverter system (XW Inverter in 24 or 48 volt banks). Does both Grid Tied and Off-Grid with battery and generator backup.

    There are some other options out there too--A lot depends on the eventual sizing of your system.

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  5. #5
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    Default Re: 2v batteries or 6v - which is better?

    i pretty much agree with bb and i would add that if you can afford it that agm batteries will give you more efficiency and no gassing when properly charged in addition to far less maintenance. there are some drawbacks as many will argue, but i had to let you be aware of this option. you will see that there is a variety of them and this is only concorde's sunxtender line as other manufacturers do make them too and some may be cheaper.
    http://www.sunxtender.com/xtender_main.php
    shopping is always fun to do and you will have to make up your mind on fla or agm and from whom.
    could i ask how long those batteries lasted you?
    NIEL

  6. #6

    Default Re: 2v batteries or 6v - which is better?

    Interesting info here.

    If I may, I'd like to understanding something more about batteries.

    I was of the impression that essentially ALL batteries were some composite of 2V cells. Thus, a "12-Volt" battery really consisted of 6-2Volt Cells, etc.

    So, how does configuration matter as to number of cells to water/maintain?
    Or, are we counting openings only here? That is, one opening watering an array of 6-2volt cells at once.

    And, with sealed cell batteries, watering ceases to be an issue at all, right?

    I'm as big a newbie as anyone, so sorry if this sounds like it.

  7. #7
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    Default Re: 2v batteries or 6v - which is better?

    One analogy might be--you need ten gallons of milk.

    You can buy 2x 5 gallon containers or 40x 1 quart containers. You still have 10 gallons in the end.

    But, perhaps your refrigerator cannot hold 2x 5 gallon pails... So you get quarts.

    Perhaps you cannot lift 5 gallon pails (40+ lbs) to put the milk in the top of the soft serve machine... So you go with quarts.

    But, now you need to hook this to a dispenser--You can drop two hoses in two pails or 40 hoses in 40 quart containers. Big pain to keep 40 dispenser hoses clean and running.

    Nothing right or wrong with either solution--but you look at your needs and abilities.

    Technically, paralleling lots of batteries is sort of difficult to do (current sharing, wiring, lots of connectors), each string needs a fuse+holder (another $30-$50 of hardware), etc...

    Note that each device type has its own idiosyncrasies... Batteries being voltage devices do not parallel / share energy well. Solar panels being current source devices (more or less) actually share current very nicely.

    For a 48 volt bank, taking care of 24 cells once a month for water and specific gravity checking is doable... Get 4 strings, and you are now up to 96 cells to check.

    And for debugging, many times it is easier to find a bad cell in one series string (battery bank voltage drop 2-4 volts with a dead cell--just take a DMM and measure each battery/cell).

    if you have 4 strings in parallel--you can lose "one cell/battery" and the not really see a drop in voltage/performance (a 25% performance hit will not be obvious in an otherwise "good" bank).

    You can purchase a DC Current Clamp meter for $100 or so and check for unbalanced current output/acceptance for each string in the bank to look for problems. Are you going to do this once a week/month?

    Again, this is my personal bias--I would recommend aiming at one string of large capacity batteries--and no more than two or three parallel connections.

    And you have a choice of cells to meet your ability to transport and install... Big 24 volt (or even larger?) forklift/traction batteries (need fork/crane to move/position)... Or some nice 500-1000 AH 2 volt cells that can be moved by one or two people without a crane/lift.

    You are correct that AGM/Sealed batteries do not cells to check water in--but they still need fuse+holder, proper wiring to balance current loads, and still have the issue where one or more "failed" cells can take a string out of service (if cell is open) or even discharge the rest of the bank (if internal short).

    I have used current clamps, and even used a DVM set to millivolt scale and use identical cable sections in different strings as an impromptu current shunt (the cable in each string should have the same drop across the bank when under load/charge).

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  8. #8
    Join Date
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    Default Re: 2v batteries or 6v - which is better?

    you are also correct that each cell is 2v and it matters not if separate or in one battery or many batteries as each has its own opening. they do not share electrolyte so they will remain separate be it fla type or agm. agm you can't check as there is nothing to check anyway, but this means no watering. it will vary somewhat, but you could roughly see about a 10% boost in efficiency with an agm. yes, it could mean you may need less in pv to charge, but i would not bank on the extra efficiency to always be 10% more than the fla type.
    for the record, each battery can have x number of cells, but when it is only 1 cell it is not really a battery as it is only a cell. this gets abused and misused all of the time.
    NIEL

  9. #9

    Default Re: 2v batteries or 6v - which is better?

    Another advantage of 2V cells is that most inverter/chargers and solar charge controllers can be configured to work with a 46V nominal or 50V nominal battery bank. So if 1 cell fails, you could take it out for repair and reprogram your chargers to work with a temporary battery bank voltage of 46V without losing power to the whole system.

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