Battery question... One 12v or two 6v

lazykdanlazykdan Posts: 8Registered Users ✭✭

Is there a rule of thumb as to which battery scenario is 'better': one 12v or two 6v in series? What are the pros/cons of each?

My use case is a small system to power a pond and a fountain.  The solar panel will be on top of a nearby storage shed and all the electrical components will be inside the shed. 

Thanks in advance!
Dan

Comments

  • littleharbor2littleharbor2 Posts: 979Solar Expert ✭✭✭✭
    One typical 12volt battery will be about 100 - 120 amp hours One typical 6 volt GC2 golf cart battery will be 210-240 amp hours. If you need 100 - 120 amp hours then the 12 volt battery is fine. If you need more  you shouldn't parallel 2 12 volt batteries when you can series 2 6 volt batteries. Many 12 volt batteries like the "deep cycle RV/marine" batteries aren't true deep cycle batteries. GC2, golf cart batteries or something like a Trojan T-105 are deep cycle batteries and will give you better daily cycling performance. 

    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.

  • lazykdanlazykdan Posts: 8Registered Users ✭✭
    Thanks for the info. I do need about 100 ah and was initially considering one 12v but this has made me reconsider.  Having 200+ah would allow me to throw some lighting onto the system as well and would be a good safety for even more cloudy days...not that we have too many of those here in PHX.
  • EstragonEstragon Posts: 2,597Registered Users ✭✭✭✭
    Just to clarify, you don't generally want to draw down a deep cycle lead acid battery below 50% state of charge for decent battery lifetime, so a pair of golf carts would give you ~100ah of usable capacity.
    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
  • lazykdanlazykdan Posts: 8Registered Users ✭✭
    Understood...to clarify, if my math is correct, I actually only *need* 45ah which is how I got my 100ah sizing.  That said, 200 is still better than 100... 

    Thanks for the assistance!
  • tabbycattabbycat Posts: 42Solar Expert ✭✭
    When you double your battery capacity you need to double your absorb charging current. If you look at battery manufacturers websites they recommend a finish charge of three percent of battery capacity.  For a 100 AH bank that is three amps; for a 200 AH bank that is six amps. Is your panel capacity up to this requirement?
  • lazykdanlazykdan Posts: 8Registered Users ✭✭
    Well, I thought it was. I went with a 100W/12v panel pared with a single 12V/100AHr battery. By my math, that should be 8A.  But at the end of the day, the battery status isn't what I expect.

    For example, I'll start the day with 100% battery (according to the controller readout).  I'll turn on the pond pump (12V/4W) when I leave for work. The controller shows the panel at 20V+ and 0.3A going to the pump.  When I return 9 hours later, the battery is at about 85%.  

    I live in Phoenix where we're in our warmest, and driest, winter in memory.  We haven't had any clouds for weeks so it should be 9 hours of clear sun. I don't quite understand why the charge isn't keeping up with the load...seems like it should be more than enough.
  • mike95490mike95490 Posts: 7,786Solar Expert ✭✭✭✭
    lazykdan said:
    Well, I thought it was. I went with a 100W/12v panel pared with a single 12V/100AHr battery. By my math, that should be 8A.  But at the end of the day, the battery status isn't what I expect.
    100W normally is 80w actual output with precision aim every 20 minutes, for an hour at solar noon.
    This time of year, lower sun angle = lower harvest, and you only can get 5 hours in non-summer months, Sure the sun is up, but the soup of atmosphere cuts the power down.
    Charging 80W @ 15V = 5.3 amps for just 5 hours - see the problem ?
    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 ,

  • EstragonEstragon Posts: 2,597Registered Users ✭✭✭✭
    Even considering the low sun etc, something doesn't sound right to me. 15% DOD is 180wh - more than you'd expect with only a 4w load and controller self-consumption.

    It may help to confirm the morning 100% SOC as a starting point. With no loads or charging sources, it should be ~12.8v
    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
  • lazykdanlazykdan Posts: 8Registered Users ✭✭
    mike95490 said:
    lazykdan said:
    Well, I thought it was. I went with a 100W/12v panel pared with a single 12V/100AHr battery. By my math, that should be 8A.  But at the end of the day, the battery status isn't what I expect.
    100W normally is 80w actual output with precision aim every 20 minutes, for an hour at solar noon.
    This time of year, lower sun angle = lower harvest, and you only can get 5 hours in non-summer months, Sure the sun is up, but the soup of atmosphere cuts the power down.
    Charging 80W @ 15V = 5.3 amps for just 5 hours - see the problem ?
    Thanks for the reply! Unfortunately, since I'm new at all of this I don't. I understand the seasonal adjustment to output (and I have angled the panel based on an online solar angle calculator to get as much power as it can) but I guess I don't understand where the numbers are going wrong. Even at these seasonally reduced values/estimates, it seems 5.3A coming in should be more than enough to cover the 3A that the pump uses over the day.

    I will log values tonight after the pump is off and tomorrow before turning it on again. The only source I have, though, is reading from the controller LCD screen.  Hopefully that'll work.
  • EstragonEstragon Posts: 2,597Registered Users ✭✭✭✭
    Just to clarify a couple of sometimes confusing terms, the 5.3a is a rate, like MPH. The distance covered (like miles) is Amp-Hours, so your "distance" is 4w × 9 hrs = 36 watt-hours = [email protected]

    One other thought - is there any chance the panel gets shaded at all (even a bit by a nearby wire, leaf, bird bomb, or whatever) at any point during the day?
    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
  • lazykdanlazykdan Posts: 8Registered Users ✭✭
    edited February 11 #12
    is there any chance the panel gets shaded at all (even a bit by a nearby wire, leaf, bird bomb, or whatever) at any point during the day?
    There shouldn't be any shade on the panel during the day. It's above everything close and no trees nearby.

    The forecast for tomorrow is overcast in the AM so I'm not sure it'll be an optimal test day.  We'll see...
  • mcgivormcgivor Posts: 2,044Solar Expert ✭✭✭✭
    edited February 12 #13
    @lazykdan Using the display on the charge controller to determine state of charge is very misleading, usually the 100% represents ~14.4 volts, as soon as the panel is not exposed to sunlight the voltage will drop, hopefully stabilizing at 12.7-12.8 volts, without load, this may be represented as 85%, when in fact it is closer to 100%. If the only load is the 300mA pump operating only during the day, the 100W panel should be more than adequate, to support both load and what little charging would be required, using any other loads would change things. Having a multimeter to measure actual values would be extremely helpful, if possible can you post what controller and battery you have?

      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • littleharbor2littleharbor2 Posts: 979Solar Expert ✭✭✭✭
    Are you sure the pond pump isn't 12volts @4 AMPS? Your numbers sound very low for a water pumping device.

    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.

  • lazykdanlazykdan Posts: 8Registered Users ✭✭
    mcgivor said:
    @lazykdan Using the display on the charge controller to determine state of charge is very misleading, usually the 100% represents ~14.4 volts, as soon as the panel is not exposed to sunlight the voltage will drop, hopefully stabilizing at 12.7-12.8 volts, without load, this may be represented as 85%, when in fact it is closer to 100%. If the only load is the 300A pump operating only during the day, the 100W panel should be more than adequate, to support both load and what little charging would be required, using any other loads would change things. Having a multimeter to measure actual values would be extremely helpful, if possible can you post what controller and battery you have?


    Thanks for the response! The battery is a "Universal UB121000-45978 12v 100AH Deep Cycle AGM Battery". I have it hooked to a Renogy 100W panel and controller (https://www.amazon.com/gp/product/B01N5L2GM9).  

    Your scenario about how the numbers drop as soon as sunlight stops sounds very much like what happens. Yesterday, I was able to watch the setup all day. The controller showed 100% battery with the panel at 14.4v in the AM.  In the afternoon, still full and 14.4. As the sun started to set, the panel showed 13.4 while the battery showed 12.7V and 84%.  After dark, the panel dropped to near zero but the battery still showed 12.7v/84%.  

    I'm going to let the pump run all night tonight and see what the status is in the morning.  I haven't done that yet as I've been thinking there was a problem.  It seems I may have an understanding problem and not an equipment problem...  
  • lazykdanlazykdan Posts: 8Registered Users ✭✭
    Are you sure the pond pump isn't 12volts @4 AMPS? Your numbers sound very low for a water pumping device.
    Thanks for the response!  I also thought that but the numbers seem to jive. The pump in question is https://www.amazon.com/gp/product/B01IAFCRF2.  According to the pump specifications, this pump requires 4W.  This matches with the controller display which shows ~0.33A being sent to the pump.
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