My older batteries are behaving odd...to me

northernsunnorthernsun Registered Users Posts: 5 ✭✭
My off grid solar system dates from my 2007, I have been its sole operator. The system consists of an xantrex sw-4024, 4x crown cr-395 in series for 24v nominal. The current panel and charger configuration is 6x 305w panels, a midnight classic with a wizbang and endamp charging enabled. I've added a diversion load last year and I've noticed a funny phenomeneon. When the diversion load is active the battery voltage drops from 28.8 to 27.6 even though the net amps are still in the positive ( around 6-7 amps).I should add that when I check the SG of the batteries all cells are getting a good charge (SG 1.275-1.285). Is there a technical explination for whats going on here? My suspicion is that it has something to do with the age of the batteries although I'd love to hear a more detailed explanation.
6x Hanwha Solar G4 305watts
Midnight Solar Classic 150 w/ wizbang jr
4x 6v crowm 395 ah
Xantrex SW4024
location - Lanaudiere, Quebec, Canada

Comments

  • mcgivormcgivor Solar Expert Posts: 3,274 ✭✭✭✭✭
    What you are seeing is probably something normal but may be potentially damaging to the batteries if I'm understanding correctly what it is you are doing. By adding a load during charging it is possible that if the load is large enough it would drag the voltage down, the array may not be able to both support the charging and load together.

    Using the 28.8V figure would indicate the batteries are still charging, the diversion load could potentially prevent a complete charging cycle. This may, if done on a daily basis over an extended period cause undercharging related problems.

    To understand more clearly could you provide details related to how this diversion load is connected, how large and for how long, how often etcetera.

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery bank 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah FLA 24V nominal used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • EstragonEstragon Registered Users Posts: 4,451 ✭✭✭✭✭
    Does the voltage recover fairly quickly (a few seconds to minutes)?  If so, I think that would be pretty normal.  The controller may take a few seconds to sweep a new I/V curve, and a bit more to get the bank back to target voltage.  I often see this when powering a pump or whatever mid-day.

    If the voltage sag lasts more than a minute or so, pv may not be sufficient to power both the load and charging, as mcgivor suggests.
    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
  • northernsunnorthernsun Registered Users Posts: 5 ✭✭
    edited July 2019 #4
    As I've mentioned in my initial post, when the diversion load is active in good sun I'm seeing a net charge of 6 or 7 amps, roughly 35 amps as system load and 40 amps coming from the charger. I'm getting those numbers from both the Wizbang jr and my trimetric. The diversion load is triggered by the midnight classic (waste not Hi). I guess what I'm really asking is how its possible to have a positive charge to the battery and have voltage drop at the same time? I shoud also mention that I'm hitting float via endamps charge pretty much every day, so I'm not overly concerned, just curious.
    6x Hanwha Solar G4 305watts
    Midnight Solar Classic 150 w/ wizbang jr
    4x 6v crowm 395 ah
    Xantrex SW4024
    location - Lanaudiere, Quebec, Canada
  • EstragonEstragon Registered Users Posts: 4,451 ✭✭✭✭✭
    It's possible for voltage to drop even with positive amps to the battery.  Basically it takes 'X' amps to get to and maintain 'Y' volts.  To maintain float (~26-27v) on a full bank, for example, may take ~1% of AH capacity.  In absorb (~29v), it might take anywhere from ~2% to 15-20% depending on SOC etc.  If the 6a remaining after loads isn't enough to hold 28.8v, voltage will drop to whatever can be held with available current.


    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
  • PhotowhitPhotowhit Solar Expert Posts: 5,101 ✭✭✭✭
     I guess what I'm really asking is how its possible to have a positive charge to the battery and have voltage drop at the same time?
    I suspect you have your diversion load set to begin when you hit float and you are just seeing the natural switching of the charge controller dropping into float. At  27.6 volts and a SG of 12.75, you are fully charged.  I might even suggest lowering the float voltage to 26.6-27.2.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
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