Can't reach float; dead cell



  • EstragonEstragon Registered Users Posts: 4,495 ✭✭✭✭✭
    The absorb setpoint for both the charge controller and inverter/charger should adjust for battery temperature.  The setpoint will be adjusted lower for warm/hot batteries, or higher for cool/cold batteries.  The battery temp is (should be) determined by a remote temp sensor.  Being different brands, the Outback and Magnum would each need their own sensor, as they usually wouldn't be networked and able to share a sensor reading.  Do you have RTS's on both?  If so, are they mounted together on the same place/ same battery?  

    If the Magnum thinks the batts are warm, it would and should adjust the voltage down.  Adjusting down a couple of degrees sounds reasonable if the batts are warm.  Once the voltage reaches 57.x or whatever the adjusted absorb voltage is, the Magnum will hold it there and current will taper off as the bank gets full.  Do you have a laser thermometer you can use to check the temperature of the bank in various places on each battery?

    A spread of 0.8v between high and low batteries seems on the wide side to me, suggesting the low one(s) being chronically undercharged, and the high one(s) overcharged.
    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
  • BB.BB. Super Moderators, Administrators Posts: 32,015 admin
    I agree with Estragon, that wide of voltage is too much... You should be down in the 0.1 volt (or a few 1/10'ths of a volt) difference between "matched" 6 volt batteries in a string.

    If the batteries are in Absorb phase, they should be something like 2.45 volts per cell (3x that for 6 volts) or 7.35 volts per 6 volt charging nominal absorb. Or 58.8 volts charging for the whole bank.

    The batteries that are low--Even after you manually charged/checked them one battery at a time--Are probably toast.

    The batteries that are high voltage--Until you start to cycle them and see how much AH / Storage they have left (i.e., sulfation or other failure types) will tell you if they are still good or not (monitoring during discharge... If you have one or more batteries "collapse" during discharge--Then they are (probably) not good either.

    The Rolls battery manual has the basic requirements for voltages and SG levels during charging/discharging/operation:

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • terrynewterrynew Solar Expert Posts: 48 ✭✭
    Good news, I've got some answers. First, the inverter is kicking into Absorb and EQ charging at lower voltages then my settings to compensate for the high temperatures sensed by the remote temperature sensor. You folks probably realized that but I didn't.

    Second, the Rolls Surrette battery manual on their website says that bulk charging should be at 10-20% of C20 (400 Ah), so 40-80 A; therefore the inverter/charger is working fine when bulking at 60+ Amps.

    The manual also confirms that during Absorb charging, that current should drop down as the batteries reach their full charge. I did a second bulk-to-absorb grid charging yesterday, after spending 7.5 hrs grid absorbing the day before, and the second time I did see this decrease in current. My puzzle is why this decrease only went from 60A to 30A, even with a 7.5 hr Absorb charging time. That 30 A still seems high.

    I talked to Magnum's tech support, and they suggested reducing the inverter/charger's Max Charge Rage (ARC Settings>3C). It's set to 100%, and reducing it to 75% would reduce the maximum current sent to the batteries (so Bulk would be 45 A instead of 60, and Absorb would drop from 45 to something lower than 30 A, likely). I'm going to try this, to see if I can charge the batteries without a lot of boiling/gassing action. If successful, I'll do an EQ with that setting too. And report back.

    Wishing you all a prosperous 2021... Terry
  • BB.BB. Super Moderators, Administrators Posts: 32,015 admin
    Once the battery bank is fully charged--You should see 1% or less rate of charge / float charge. Or 4 amps or less.

    Personally, if you see more than 2% rate of charge after the bank is full... I would suspect that you have bad cell(s) or battery(ies). 2% or greater rate of charge is damaging to the batteries (this is basically a 2.5% to 5% EQ rate of charge)... You risk the bank overheating/"boiling" (gassing) dry--exposing plates, and even the risk of fire or Hydrogen gas explosion (admitially a rare failure--But it is a risk).

    Check the voltages on each battery/log it. And the SG for each cell (temperature corrected/logged). Any low voltage/low SG batteries at this point probably need to be replaced (do you have any "good batteries" from your old string left?).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Solar Expert Posts: 9,362 ✭✭✭✭✭
    With solar days, you will have difficulty getting a full charge at full amps.  Reducing amps will require more hours of charge, which you would need a generator for.  Partially charged batteries are dieing batteries.  
    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

    gen: ,

  • terrynewterrynew Solar Expert Posts: 48 ✭✭
    Sorry, Estragon & Bill, I posted yesterday before I saw your Dec 28 posts (didn't look at the new page). Thank you both for that valuable info. I do have separate temperature sensors for the Magnum and the Outback; they're one battery apart but I can move one over. No laser thermometer, though.

    Curious, the Rolls Surrette manual says "Temperature sensor should not be mounted to the battery terminals", yet the inverter's RTS has a terminal mount. Should I glue it to the side of a middle battery? What kind of glue does one use?

    Ahh, the manual also has a state-of-charge to SG table; that answers an earlier question of mine, asking how I'd know when I dropped to a 50-75% SOC. Good.

    My voltage spread is a concern. Here are the 8 voltages measured 2 days ago during high-current charging:
    7.19, 7.06, 6.84, 7.05, 6.99, 7.64, 7.12, and 7.12.
    If I take out the 7.64, the average is 7.05. That places my lowest, 6.84, just 0.21 Volts away. So if I were to replace the 7.64 V battery with one of my spares that had a 7.05 V charging voltage, the total would be 56.42 V, as opposed to Bill's ideal of 58.8 V. Is this something I should do? 

    Bill, you said the low V batteries are probably toast. Given the ideal of 7.35 V, that might mean those two of mine below 7 V. I can try replacing them with two from my 'spares' (recall I have 6 batteries with no dead cells from my second string). But those spares all have lower SGs (as I chose the highest SGs for my new one-string setup); doesn't that also mean lower voltages?

    As I'm writing this, I'm bulk charging off the grid again, after reducing the inverter's 03C Max Charge Rate from 100% to 70% and after two days of loads reducing the SOC to around 75%. Two hours in, at 57 V, the current is 44 A instead of 60+ A. What's interesting is that my 8 battery voltages are now all lower:
    7.00, 6.98, 6.82, 6.98, 7.41, 6.96, and 6.95.
    I'm guessing this is because I'm measuring while Bulk charging at 57 V, rather than while Absorb charging at a higher voltage. When is the best time to measure charging voltages to compare against the 7.35 V ideal?

    I just noticed another tuning issue: the Rolls Surrette manual recommends Absorb charging at 60 V and Floating at 54 V; this has changed since my 2010 setup. My inverter/charger was set at Absorb = 59.2, and Float = 52 V. I'll up them.

    Another side issue: could any of the low voltages be due to poor battery terminal connections? I always use petroleum jelly but have never given the terminals a cleaning with baking soda. I regularly check for tight connections.

    Phew, my head hurts. Time for breakfast. Thanks again, guys... Terry

  • VicVic Solar Expert Posts: 3,112 ✭✭✭✭
    Hi Terry,

    On the Temp Sensor (RTS, or BTS),   place  it midway down the side of one battery in the center of the pack.   Clean that area,   and use Painter's or Stucco tape to hold it in place.   You could,  then hog out an area of styrofoam (white stuff),   the size of the sensor an tape or strap that foam over the temp sensor.  This foam should be about the width of the battery.  The idea is to remove the effects of room ambient temperatures from  that of the battery.

    Your battery voltage measurements show a very wide range,   indicating a very wide variation in SOC/battery condition.   A very long Corrective EQ (after a full  Absorb stage,  as noted before).   This could take many hours.   As BB Bill noted,   some of the batteries may well be toast.

    Surrette has increased the recommended  charge voltages,  some years ago.  These will be best for your batteries,   since it appears that they have not been getting a full charge in the past.

    If you are fairly quick in measuring and recording battery voltages,   then you can do this in Bulk,   or Absorb.   The voltage will be climbing,  during Bulk,   and the voltage will be fairly constant in Absorb,   but the current diminishing.  Would suggest doing these measurements near the end of Bulk,  or,   near the beginning of Absorb.   If you do this fairly rapidly,   it will make little difference.  Doing it when the charge current is high,  should be the most meaningful.

    You can measure the voltage drop between the cable lug,  and the battery terminal on the battery interconnects,  with your DMM,   set to the 0.2 (200 mV) range to see what is the voltage drop.   This will not show the voltage drop between the cable and the cable lug,  of course.

    You can feel the heating at these cable lugs,  with the back of one finger,   or,  use an IR no contact thermometer,  as well to infer lug/termination voltage drop.

    SG measurements are the standard for SOC of FLA  batteries.  Make and record the SGs when you have completed a full (or overly-long) Absorb.  Temperature compensate these readings,  for the best infol

    And so on,   many details,  much advice. Good Luck,   HNY,  Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • BB.BB. Super Moderators, Administrators Posts: 32,015 admin
    Hi Terry,

    If you glue the sensor to the side of the battery--I would try some sort of silicon adhesive and a styrofoam "cap" over the sensor... There are better epoxy adhesives, but you run the risk of damage to the battery case and/or sensor when you need to move/service/change the batteries. But I do not have any direct experience--So others with their suggestions are probably better than mine.

    Where to mount--Your choice. Mounting to the battery post near acid misty/fumes would be a concern... As is any small wiring near all that heavy gauge wiring (during installation/cleaning/etc.)...

    Regarding battery voltages... Are those voltages during charging, or after a few hours of rest?

    "High voltage" batteries during charging can reduce charging current (because of sulfation or other internal battery problems). At this point, your low bank charging voltage and high charging current is probably due to the "low voltage" batteries in the string.

    Have you measured the Specific Gravity of all the cells (logged to note book)? Do you see the SG rising on the cells, or is it staying constant between checks (every 1 hour)? If the SG is no longer rising, then that is the "new" full charged SG reading.

    I would not try to raise the charging voltage yet--You have lots of current flowing through the battery string... And you should see the current drop as the cells/batteries become fully charged... And the charging current should fall to less than 1% of bank AH capacity. The fact you are not seeing bank charging current fall--More than likely one or more cells/batteries is failing.  There is no reason to "up" the charging voltage setpoint (or even EQ'ing the bank) until you figure out why the charging current is not falling.

    This should be a simple math issue... If you have a 400 AH bank at 50% state of charge, then you need to replace 200 AH... If you are charging at 40 Amps, then 200AH/40A=5hours of 40 amp rate of charge should get you neart 100% State of Charge (some losses)...

    If you have put 40 amps for >10 hours into that string--Then you should be seeing the batteries really "boiling" (gassing) and getting very warm/hot... Putting more hours of current into the bank is not helping at this point--It is just converting water into hydrogen and oxygen (and generating heat).

    I suggest you stop at this point... The string is probably not salvageable -- Other than if you replace the "low voltage" batteries with some of your spares (spares already fully charged) and try again.

    There is corrective EQ that you can do... But until your batteries "stop accepting" >1% current at normal charging setpoints, you cannot do a corrective EQ (with low + normal batteries in a string, the "normal" batteries are getting the EQ already, and do not need more).

    Bad battery connections (corroded/loose/etc.) can cause problems... The bad connections can overheat. And they will limit charging/discharging current--Neither of which seem to be a problem at this point.

    Look at the current (40 amps) and time (hours) that you are putting into the bank... Lead Acid batteries are very near 100% efficient at storing/releasing current (i.e.., 400 AH / 40 Amp current = 10 hours from empty to full). Anything more than 125% AH into the battery bank is wasted. And will usually generate excessive heat and gassing (and water loss).

    If the battery string is in good shape, then at 57 volts charging, the string should drop down to 4 amps or less charging current over time. If you EQ the bank at 60+ Volts--Then you will see around 2.5% to 5% EQ current being "forced" into the bank (bringing up low SG cells, gassing already 100% charged cell).

    There is no reason (that I can see) to "beat up" the present string of batteries... Replace the "low voltage" batteries with your possible "good batteries" and try charging at 57+ volts again and see what happens--If you are still seeing "hours" of 40+ amp current, battery voltages+SG failing to rise, and lots of bubbling/gassing/heat--This is the end of your present bank...

    I hate to mention it--There is one possible issue that we have not addressed. Your DMM... Have seen DMMs with low battery cause them to read really strange voltages. Assuming your DMM and your Charge Controller(s) are reading similar bank voltage--Then probably not an issue... But if you have a backup meter you can borrow--It would not hurt to check (you can even check your car battery to make sure the meter is "sane"--Typical 12.7 volts resting and 14.0 to 14.4 volts charging/engine running). A "bad" meter or meter with a weak battery can really have you chasing your tail. I don't think you have a bad meter/meter battery--But just something to watch out for.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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