2V vs. 6V batteries

AGreyManAGreyMan Registered Users Posts: 3
Hello Folks:

I am wondering about the advantages/disadvantages of 2V batteries vs 6V batteries. Specifically, I am looking at the Trojan L16 series.

I have 24V panels purchased and now need a battery bank. Presuming the cost of the batteries is fairly similar, would you go with 12 2V batteries or 12 6V batteries?

Thanks for any insight you may be willing to share.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,366 admin
    Re: 2V vs. 6V batteries

    Personally, I like to recommend 1-3 parallel strings of batteries (1 being better, 3 being not as good in terms of maintenance and failures/life).

    So, that means you want a cell large enough AH capacity so that 1 to 3 of them in parallel will meet your power needs (i.e., XX volt battery bank at YYY amphours).

    The next issue is the physical weight of the cells... Most people find that ~150 to 300 lb cells/batteries are about as large as they can move around with one or two people and a hand truck (unload from pickup, move down stairs to basement, etc.).

    If you have a forklift or overhead crane to unload/move to point of use, you can get 12 or 24 volt fork lift batteries that weigh 1,000 to 2,000+ lbs. And you just use a pallet jack to slide them on a smooth concrete floor into place.

    Then, there is the issue with cell design itself. Most cells have one cap per cell. If you have a 24 volt battery bank with 12 cells--That is 12 caps to check for water/specific gravity.

    If you choose an L16 battery in 6 volts, that is 3 series cells (for 12 volts), and 4 batteries (times 6 volts) for a 24 volt string.

    If you choose an L16 battery in 2 volts--For some vendors that is one cap and one cell. For Trojan (as I remember, you need to confirm), they actually are three cells in parallel--So you have three caps to check water/s.g. reading on. That is probably less than ideal for many people...

    You have a single string L16 2 volt battery in Trojan--You now have 3x12=36 cells to check.

    If you have the same L16 2 volt battery from a different vendor, you should only have 1x12=12 cells to check (per string).

    If you have 2-3 strings, you have 2-3 times more cells to check (3 parallels strings of Trojans, that would 108 cells for your 24 volt battery bank--That is a lot).

    So--What size (voltage/AH) battery bank are you looking at? Do you have a maximum cell/battery weight limit?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • AGreyManAGreyMan Registered Users Posts: 3
    Re: 2V vs. 6V batteries

    Wow! Thanks for the rapid, kind and informative response!

    I am still in the beginning/planning stages, but using the online calculators, advice, and etc., I am thinking 1200-1800 amp hours at 24V.

    As far as I know, I don't have a weight limit. It will be relatively simple to get the batteries into position, if I can get them up my narrow and convoluted driveway! Tractor-trailers are out. The flooring under them is concrete.

    Thanks.
  • BB.BB. Super Moderators, Administrators Posts: 29,366 admin
    Re: 2V vs. 6V batteries

    You are very welcome AGM,

    For a "cost effective" off grid system--You really need to "nail" your loads (by month/season if it matters). In general with off grid solar, you can go 50% smaller to ~100% larger -- If you need to move your system beyond these limits, it gets hard and expensive (sometimes, it is a complete replacement of most of the major components).

    In my humble opinion, a 1,200 to 1,800 AH @ 24 volt battery bank is pretty large. For solar we look at ~5% to 13% rate of charge... For a genset, around 10%-20% rate of charge is pretty common.

    10% of a 1,800 AH battery bank is 180 amps of charging current. 20% will be ~360 amps. Those are pretty huge numbers.

    You might want to take a look at a 48 volt battery bank--It will cut your current in 1/2 and instead of ~2x 80-90 amp solar MPPT charge controllers, you may be able to get away with 1-2 charge controllers (controllers are usually current limited... 80 amps at 12 volt/24 volts/48 volts is all similar--But the arrays are 2-4x more power for the same controller when operating at higher battery bank voltage).

    48 volt systems have their own issues (more battery cells in series, higher voltage need higher rated voltage breakers, and the AC inverters tend to be larger rated capacity at higher battery voltages).

    But for such a large battery system--You will need a large solar array and backup genset/battery charger... For example 5% to 13% rate of charge--Use 10% as "healthy nominal:

    1,800 AH * 29 volts charging * 1/.77 panel+controller derating * 0.10 rate of charge = 6,779 Watt solar array "Nominal"

    To charge using higher end inverter/chargers:

    1,800 AH * 29 volts charging * 1/0.90 charger eff * 1/0.95 Power Factor * 1/0.80 genset derating * 0.1 rate of charge = 7,632 Watt/VA rated genset minimum

    Note that the above (size of solar array, size of genset, power rating of inverter, storage capacity of battery bank) does not really matter at this point... a 24 volt @ 1,800 AH vs 48 volts @ 900 AH is exactly the same amount of stored power, the same weight and cost of batteries, etc... You just can use smaller diameter copper cables (lower current).

    Such a battery bank, assuming 2 days of stored energy and 50% maximum discharge (1/4 of battery for your daily loads):

    1,800 AH * 24 volts * 0.85 inverter eff * 1/2 days of storage * 0.50 maximum discharge = 9,180 WH = ~9.2 kWH per day (or 276 kWH per month).

    That is a pretty big system (about the maximum for an off grid home with "near normal" appliances usage). A very efficient off grid home (using wood/propane/etc. for heating/cooking/hot water) could get as low as ~3.3 kWH per day (100 kWH per month). Energy usage is a highly personal set of choices. In general, "extreme" conservation is usually a better "investment" vs just building a larger off grid power system.

    You need to do your paper designs first (after confirming/estimating power needs) before you lay out any money. It is easy to start buying hardware a get yourself in a box where you cannot use some of it without other issues.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: 2V vs. 6V batteries

    For my replacement bank installed this past Summer, I went with Surrette 2 volt L-16 for my 12 volt system.
    3 reasons;
    1) After tending a 3 parallel string bank for 11 years and struggling with keeping them balanced, I wanted a single string.
    2) It seems Surrette is one of the few, or perhaps only manufacturer to build a 2 volt L16 with one single large 2 volt cell with one fill cap, instead of three smaller parallel cells, each with it's own fill cap, all within the L16 container. Thus I only have 6 cells to check electrolyte levels and SG instead of 18. After 11 years of checking 18 cells, it's a pleasure to now only have 6.
    3) Surrette, although more expensive, has their factory rather locally for me, so could pick them up at the factory and avoid shipping costs.
    Am I sorry? Not yet. So far everything is as good as expected. Ask me in 10 years. :D
  • AGreyManAGreyMan Registered Users Posts: 3
    Re: 2V vs. 6V batteries

    Thanks again for the continued information. I am going to add my questions/comments/ponderings in red. Please forgive me if I ask something that has been answered a million other times. I am pretty new to the RE stuff.


    BB. wrote: »
    You are very welcome AGM,

    For a "cost effective" off grid system--You really need to "nail" your loads (by month/season if it matters).

    I am guessing you mean that I should really get hard numbers as to my energy usage frm different appliances, etc. That is a something I really need to do, but have been remiss in doing. Thanks for the reminder.

    In general with off grid solar, you can go 50% smaller to ~100% larger -- If you need to move your system beyond these limits, it gets hard and expensive (sometimes, it is a complete replacement of most of the major components).

    So, for any given sytem you can size it up (by ~100%) or down (by 50%) by substituting a component, or adding a few panels? Again, sorry to be dense, but I am still learning!

    In my humble opinion, a 1,200 to 1,800 AH @ 24 volt battery bank is pretty large. For solar we look at ~5% to 13% rate of charge... For a genset, around 10%-20% rate of charge is pretty common.

    I am afraid I got caught in your last warning at the end of the post of buying hardware before you have a plan: Based on only a rough estimate of my needs/wants I have purchased 10 270W 24V panels.

    10% of a 1,800 AH battery bank is 180 amps of charging current. 20% will be ~360 amps. Those are pretty huge numbers.

    You might want to take a look at a 48 volt battery bank--It will cut your current in 1/2 and instead of ~2x 80-90 amp solar MPPT charge controllers, you may be able to get away with 1-2 charge controllers (controllers are usually current limited... 80 amps at 12 volt/24 volts/48 volts is all similar--But the arrays are 2-4x more power for the same controller when operating at higher battery bank voltage).

    48 volt systems have their own issues (more battery cells in series, higher voltage need higher rated voltage breakers, and the AC inverters tend to be larger rated capacity at higher battery voltages).

    But for such a large battery system--You will need a large solar array and backup genset/battery charger... For example 5% to 13% rate of charge--Use 10% as "healthy nominal:

    1,800 AH * 29 volts charging * 1/.77 panel+controller derating * 0.10 rate of charge = 6,779 Watt solar array "Nominal"

    To charge using higher end inverter/chargers:

    1,800 AH * 29 volts charging * 1/0.90 charger eff * 1/0.95 Power Factor * 1/0.80 genset derating * 0.1 rate of charge = 7,632 Watt/VA rated genset minimum

    Note that the above (size of solar array, size of genset, power rating of inverter, storage capacity of battery bank) does not really matter at this point... a 24 volt @ 1,800 AH vs 48 volts @ 900 AH is exactly the same amount of stored power, the same weight and cost of batteries, etc... You just can use smaller diameter copper cables (lower current).

    Such a battery bank, assuming 2 days of stored energy and 50% maximum discharge (1/4 of battery for your daily loads):

    1,800 AH * 24 volts * 0.85 inverter eff * 1/2 days of storage * 0.50 maximum discharge = 9,180 WH = ~9.2 kWH per day (or 276 kWH per month).

    That is a pretty big system (about the maximum for an off grid home with "near normal" appliances usage). A very efficient off grid home (using wood/propane/etc. for heating/cooking/hot water) could get as low as ~3.3 kWH per day (100 kWH per month). Energy usage is a highly personal set of choices. In general, "extreme" conservation is usually a better "investment" vs just building a larger off grid power system.

    OK, I am a little slow with the mathematics, but I think I get the general idea if the above. I am continuing to try to wrap my head around it. Our home will definitely be made more efficient: I have heard that for every $1 spent in efficiency, it's like spending $4 on more panels, larger inverters, bigger battery bank, etc. Though we have been making some progress in the efficiency realm (CF bulbs in all fixtures, etc. ) we will have to step up our game. Our plan was to purchase one of the Outback pre-configured power systems, with grid-tie capability. That way any shortfall in our RE system could be picked up by the grid, and as we replaced energy inefficient appliances, we would "wean" ourselves from the grid.

    You need to do your paper designs first (after confirming/estimating power needs) before you lay out any money. It is easy to start buying hardware a get yourself in a box where you cannot use some of it without other issues.

    Very definitely...I can see getting into trouble by not pre-planning. Again, excellent advice that I intend to follow. I think the first step is hanging out here and asking questions, so I know for what to plan!

    -Bill


    Thanks for taking the time to help me understand this stuff!

    -Matt
  • BB.BB. Super Moderators, Administrators Posts: 29,366 admin
    Re: 2V vs. 6V batteries

    Matt,

    AGreyMan wrote: »
    I am guessing you mean that I should really get hard numbers as to my energy usage frm different appliances, etc. That is a something I really need to do, but have been remiss in doing. Thanks for the reminder.

    Since you have Grid Power (I am guessing)--You should just monitor your home electric bill, and you can get a whole house meter (something like this or others).

    You will have other electrical issues to resolve too--Figuring out how to interface with your main service panel, create a sub-panel (after the AC off grid/hybrid inverter), and some/all of the circuits that you want to run off grid there is probably not going to be cheap or easy (difference between running a few circuit off grid in an emergency vs moving whole home off grid).
    So, for any given sytem you can size it up (by ~100%) or down (by 50%) by substituting a component, or adding a few panels? Again, sorry to be dense, but I am still learning!

    Pretty much... The hardware between a smaller system and a larger system is not really something that can be "migrated" back and forth (you start with a 12 volt system and a 300-1,200 watt AC inverter, and want a 48 volt system with 4kW AC inverter--Pretty much a total clean slate re-design). You can sometimes salvage some components (solar panels)--But even then, it is not always easy (older solar panels may not "match" newer solar panels purchased 2+ years later).

    Frequently, people will keep the older system for backup/guest cabin, or sell to neighbor who wants to get started. Batteries are only good for ~3-8 years or so. Charge controllers and AC inverters are usually pretty much near end of life after 10+ years (parts age, hard to get replacement electronics, etc.).
    I am afraid I got caught in your last warning at the end of the post of buying hardware before you have a plan: Based on only a rough estimate of my needs/wants I have purchased 10 270W 24V panels.

    It happens--The panels are probably fine and some of the larger panels are coming with similar Vmp ratings--Makes it easier to parallel older and newer panels in a single array...

    But details matter. For example the "24 volt panels"... That term is loose enough that it could mean Voc=24 volts, Vmp=24-30 volts, or Vmp~35-39 volts or so... Of all those combinations, "we" would call the Vmp~35 volt panels true "24 volt" panels that are capable of charging a 24 volt battery bank with a PWM charge controller. Because Voc and Vmp change with temperature--You have to be careful that your panels and charge controllers will work together with your chosen battery bank (12/24/48 volt).

    And then you get into issues like--Say these are Vmp~30 volt panels (very common GT panel voltage). One panel is not enough to charge a 24 volt battery bank, and two panels in series (minimum) with an MPPT charge controller would be needed instead (efficiency and cost issues).

    If you decide to go with a 48 volt battery bank, you need a Vmp-array of ~72 volts minimum... That means you need three panels in series with an MPPT controller. And for standard MPPT controllers, 4 panels in series may be too high of array voltage... So you need your strings in groups of 3 series strings... I.e., 3, 6, 9, 12, etc... And you have 10 panels. So to have a "typical" 48 volt capable charging system, you would need either to not install one panel, or get two more "identical panels".

    And for the size battery bank you talked about, I would be highly suggesting a 48 volt battery bank (first guess).

    Note--It is not that your entire panel purchase is "wrong"--It just may work out to not be "optimum" use of your money--And off grid solar will be a series of trade-offs to try and get near optimum use of money to meet your needs. There will be lots of "shades of gray" types of decisions ahead.
    OK, I am a little slow with the mathematics, but I think I get the general idea if the above. I am continuing to try to wrap my head around it. Our home will definitely be made more efficient: I have heard that for every $1 spent in efficiency, it's like spending $4 on more panels, larger inverters, bigger battery bank, etc. Though we have been making some progress in the efficiency realm (CF bulbs in all fixtures, etc. ) we will have to step up our game. Our plan was to purchase one of the Outback pre-configured power systems, with grid-tie capability. That way any shortfall in our RE system could be picked up by the grid, and as we replaced energy inefficient appliances, we would "wean" ourselves from the grid.

    The math is not that bad--I write the equations in sort of a English grammar type of way... If you use a simple calculator (don't use parens), the math will work out.

    There are lots of SWAGs (scientific wild a$$ guesses) fudge factors--But they are usually "good enough" to design your system (and we try to be a bit on the conservative side--Want your system to not only work tomorrow, but 5 years from now too as components and batteries age, panels get dusty, etc.).
    Very definitely...I can see getting into trouble by not pre-planning. Again, excellent advice that I intend to follow. I think the first step is hanging out here and asking questions, so I know for what to plan!

    You really need to decide what your end game is... Is it to save money, go green, a few weeks of emergency power, or months to years of emergency power, or just going off grid.

    In general, if you want to save money/go green--Your best bet is to go with Grid Tied Solar (panels+GT Inverter, no battery bank). In many places with expensive power (usually both east and west coast states), you can save significant money--especially if you use a lot of power (summer A/C, pumping, electric heat, etc.).

    If you want a couple of weeks of emergency power... Get a genset and wire that up to the house (you have fuel choices to make). Usually much cheaper than off grid solar. Some folks will do a hybrid system... A small battery bank + some solar for "off peak power" (lighting, computer, radio, TV, refrigerator) and use the genset for larger loads (shop power, water pumping, washing clothes, cooking, etc.).

    If you want months to years of emergency power or to go off grid... This is going to be expensive. And remember that batteries will last around 3-10 years, and electronics (chargers, inverters) around 10+ years--You are your own power company and are now 100% responsible for maintenance and cash flow. Don't forget that genset and fuel storage/costs for those days/couple weeks at a time of bad weather--Solar needs sun, and for most people, Wind is not a viable alternative. And few people have a stream they can tap for hydro.

    If you are going to "grow" into a system, my humble list:
    1. monitor/measure your loads
    2. figure your costs (both at $0.15 per kWH for grid power and $1-$2+ for off grid power)
    3. Conservation (lots of it). More insulation, double pane windows, Energy Star appliances, new heat pumps (heating, cooling, even hot water is possible from electricity/solar power), turn stuff off (DVRs, desktop computers, satilite/digital receivers, etc. can consume lots of power on "standby"), etc...
    4. More conservation.
    5. Measure/Evaluate your new power needs. If Grid Tied (saving money)--Now you have an efficient home and your GT system needs will be smaller (people that have never conserved before can, many times, save 50% or more off their power bill).
    6. If going off grid and/or emergency power (this list may take a few years to complete):
      • Paper design
      • Install generator and fuel supply
      • Install Battery bank + AC inverter + AC Battery charger (from grid/backup genset)
      • Install Solar Panels + Charge Controller
    You can flip the above and install battery bank+Inverter+AC charger and make a "home UPS system" (assuming outages are a couple days or less. Or do the genset first if you are protecting against several week+ outages.

    And you can install a small battery bank and AC inverter to just provide lightning/radio/computer/cell phone service and a Honda eu2000i genset (1,600 watt) or Similar Yamaha. Fuel choices can be from natural gas, or propane, or gasoline (diesel for larger gensets--But that may not fly in a typical suburban neighborhood (smoke, noise, smell, and in some states, smog laws).

    There is no "right plan"--This is all based on your needs and abilities (knowledge, bank account, etc.). Doing the paper design first, listing its abilities (at various stages), and costing it out (roughly)--All will further clarify your actual capabilities & needs.

    As always, more questions than answers at this point in the process. I highly suggest you you work with the numbers first--And not pay too much attention to hardware. There are lots of options and configurations and it is easy to get lost in the details. Just get a rough sizing of power needs and some generic costs first.

    At that point, when the "generic" design is all figured out, we can go into the details of selecting each major component.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: 2V vs. 6V batteries
    BB. wrote: »

    The hardware between a smaller system and a larger system is not really something that can be "migrated" back and forth (you start with a 12 volt system and a 300-1,200 watt AC inverter, and want a 48 volt system with 4kW AC inverter--Pretty much a total clean slate re-design).
    -Bill
    And unfortunately, that's where I am. Never in my wildest dreams did I ever suspect I'd see the day when I would be living independent of the grid when I first started out with my 12 volt system and thinking I might use it to run perhaps a couple of lights, a radio, and in extreme cases, a water pump for a few minutes to get by during a grid outage. As a result, I painted myself into the 12 volt corner and now, without huge expense, can't get out of the 12 volt corner. Fortunately I'm able, just able, to get along with 12 volts, but if only I had gone 24 volts from the start, I wouldn't be blocked in now. For my usage and loads, 24 volts would have been the answer. Hind sight is awesome isn't it. :cry:
    In your case, 48 volts may be best, I don't know, it will depend on your loads etc.
    Look into the future, of where you might go with your usage, and if at all possible, design for that from the start. And remember what others have so often told me: "Your loads almost always grow".
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