Bat SG, and voltage drop

Chris11Chris11 Registered Users Posts: 64 ✭✭
edited February 12 in Solar Beginners Corner #1
A few questions if someone doesn't mind taking a stab at the answers.

10 100 watt panels, into 2 strings of 5 each.  Going into a combiner, then into an Outback 80.  Onto 4 Costco 6 volt DC batteries in series for 24 volts.  All wiring is very short.  The inverter is a 2500/5000 watt Chinese using 1/0 wires to it....and between bats.

The charging side has been "on" all winter without turning on the inverter.  I've just started to go to the property and have been able to get my Plug In Prius up to the box that contains everything.  A couple of days ago I checked the SG and it was at 1300 in all cells.  Same reading as the end of last year.  I  tried to lower the Float rate but even once I lowered it it still didn't seem to lower the SG.  Hydrometer is a decent one but I haven't checked it against another one.  I didn't compensate for the temperature.  Are my readings too high?  

The couple of days ago I mentioned I had plugged in my Prius.  It requires 110 to 120 volts @ 12 amps for 2 1/2 hours to fully charge my onboard battery.  First day I tried it I noticed that after an hour or so the voltage reading on the Outback read 23.4.  I had even shut it off once because the inverter started to buzz low voltage.  When I originally plugged it into the car the voltage is over 28...then it slowly goes down.  The sun was bright and it was around solar noon.  Angle of panels is very close to what it should be for this time of year.  
Today the inverter didn't sound the low voltage warning but the voltage out of the Outback again went slowly down to under 24.   I  know I'm getting some energy from the sun to power the inverter, plus drawing from the bats.  I would have thought the batteries could supply all necessary energy to fully charge my Prius bat even without any sun.   Is this drop in apparent voltage normal? 

Thanks,  Chris

Comments

  • mike95490mike95490 Solar Expert Posts: 7,407 ✭✭✭✭
    For 4 costco batteries charging a car, I would call it normal, even if the sun was shining.   I'd expect the 4 batteries to give about 1/4 of a charge into the car.

    If 4 costco batteries could power a car, for $400, why are $9K battery packs needed ?? Think about it for 90 seconds.


    A couple of days ago I checked the SG and it was at 1300 in all cells.  Same reading as the end of last year.  I  tried to lower the Float rate but even once I lowered it it still didn't seem to lower the SG.  Hydrometer is a decent one but I haven't checked it against another one.  I didn't compensate for the temperature.  Are my readings too high?
    Your readings are fine, if you temperature compensated the readings. You want the batteries to be fully charged, any less and they start to sulfate.
    If you are adding water 1x a month, that's good.  If you have to add water weekly, your Absorb time is too long.  Don't lower the Float voltage.

    120V x 12A = 1440watts x 2.5 hr = 3600 watt hours

    6V 180ah battery x 4 = 1080watt hour total capacity of battery pack.   You need 6x the batteries to reliably fully charge the car.  And don't forget about 3 conversion losses from 24vdc to 120VAC to car battery



    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 ,

  • mcgivormcgivor Registered Users Posts: 1,404 ✭✭✭✭
    Another way to look at why there is volt drop. With 1000W of panels there would be around 800W average output, divided by voltage, using nominal 24V for simplicity, would produce ~27.5A.
    The charge current required for the car would be 12A at 120V or 60A at 24V, for a net deficit of 32.5A which must come from the batteries, hense the drop in voltage. Note the actual voltages and current will be constantly changing as the battery voltage drops, hense the use of nominal voltage.
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • PhotowhitPhotowhit Solar Expert Posts: 4,335 ✭✭✭✭
    Chris11 said:
    When I originally plugged it into the car the voltage is over 28...then it slowly goes down.  The sun was bright and it was around solar noon.  Angle of panels is very close to what it should be for this time of year. 
    A few things on the learning curve, I would suspect if the batteries were at 28+ volts, that you had likely just reached float or were not yet in float. Flooded lead acid batteries are typically 'floated' between 26.2-27.4.

    The 28+ volts is a charging voltage and would indicate that the charge controller was holding the voltage at the higher level to facilitate the flow of current from the solar array to the battery bank. If removed from the charger, the battery voltage would change over an hour or 2, to roughly 25.4 volts, the resting voltage of a fully charged 24 volt, battery.

    Similarly the voltage dropping to would indicate that the solar panels are not strong enough to carry the load on their own and you are drawing from the battery bank. Which you indicated by the 12 amps at 120 volts required. 1,440 watts x 1.1 for inverter inefficiency is roughly 1600 watts, and your array of 1000 watts should produce about 750 watts in normal conditions... 
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, ForkLift battery. Off grid for @13 of last 14 years. 1000 watts being added to current CC, @2700 watts to be added with an additional CC.
  • Chris11Chris11 Registered Users Posts: 64 ✭✭
    Thanks guys.

    Mike....You've made a good point about using ordinary fla versus the lithium ion bats.  When I bought the bats from Costco I asked the capacity and the guy said they were 208 AH.  208 ah x 24 volts is 4992 watt hours.  That's why I thought that just the bats alone could come close to fully charging the Prius.

    McGivor....great explanation.  I figured the inverter was using both sun energy and bat energy.  I'm going to be adding another 2 panels soon...that'll help with less draw from the bats. (And hopefully the voltage drop).

    Photowhit....I think you're correct.  It was in float when I started so 27 volts or so would be right since I have float at 27.4 volts.

    I'm attaching a pic.  As you can see it's already put out 1.4 kwh so this pic was just close to the end of the charge.  The reason I took it was for another question which I forgot to ask.  So here goes....

    It started in float mode.  And here we are a couple of hours charging later and it still says float.  Why?  Shouldn't it switch back to bulk?  Or is this just how the display shows it?  

    Thanks again guys....I really appreciate your time and knowledge.

    Chris

      
  • mcgivormcgivor Registered Users Posts: 1,404 ✭✭✭✭
    edited February 12 #6
    The power was being drawn from the panels and the battery, but because the load exceed the solar, the suppliment was made up by the battery, therefore no charging.

    If the charging commenced while in float and the battery voltage dropped to 23.4V but remained in float perhaps the re bulk setting is too low, if available ( not familiar with Outback controller ) around 25v is a reasonable for re bulk. 

    If this will be a regular thing I would suggest getting more than a couple of 100W panels.

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

  • EstragonEstragon Registered Users Posts: 1,810 ✭✭✭✭
    I don't know about OB charge controllers, but IIRC my Outback inverters had pretty low defaults for low battery cut off and rebulk - more to protect the inverter from high current than to protect the battery I suppose.

    There may also be a setting for rebulk time (voltage has to drop to Vrebulk for X seconds or whatever), which is adjusted to prevent rebulking after pump starts etc. It would be worth checking if there is such a setting, and if so, it's set to a reasonable value (eg 30-60secs, depending on what you have for short, high current loads). There may be a similar setting for low battery cut off.
    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
  • Chris11Chris11 Registered Users Posts: 64 ✭✭
    McGivor....I seem to remember, but I'm not certain, that I increased the rebulk rate originally.  I'll check it next time i'm at the property.
    I agree that two panels aren't enough...but the two spares are all I have at the moment.  Perhaps more in the future.

    Estragon.... I looked at the Outback manual online...and the default for rebulk is 6 volts, which even the manual says that low setting is basically useless....lol.    As I told McGivor, I "think" I raised it. And the low voltage has to be low, as you suggested, for 90 seconds before rebulk takes affect.  I'll check that also.

    Thanks very much for replying,

    Chris 
Sign In or Register to comment.