Would you buy another battery?

litefoot
litefoot Registered Users Posts: 3
in Off Grid Solar & Battery Systems #1
First post here. Hello everyone. I'm putting in a 48V system consisting of a Magnum power center with a MS4448PAE (4400W) inverter and I have 2kW in panels. I inherited a set of 24 C&D KCT-450 batteries (2V and 450 aH). They were recently load-tested at over 100% capacity. As I was racking the batteries, I dropped one jar and made a mess. The cost of replacement (including freight) is over $600. My budget is running out and I'm pondering just running with 23 cells. I'd have to adjust my charge voltages down but are there any problems you see in running it this way?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #2
    Re: Would you buy another battery?

    With a "48 volt" nominal inverter, you might be able to get away with running a 44 volt nominal array...

    Usually the inverter cutoff voltage will be around 42 volts and the minimum recommended operating bank voltage would be ~46 volts (1.917 volts per cell) under moderate loads... So, this this means the minimum battery bank operating voltage would be around 44.08 volts instead... That still gives you a couple of volts for wiring drop and bank voltage sag during start up load surges.

    If your operating current/surge current is not too great, and the battery bank voltage does not get too hot (bank voltage falls as temperature increases), it would appear that you could successfully operate with just 23 cells (assuming you program the correct absorb, float, and equalize voltages in your various charge controllers). At this point, you would just be operating close to the minimum inverter input voltage and a slight reduction in supported maximum cost effective array size (power=v*I=~57volts*controller max current vs ~59volts*controller max current).

    And if you have problems operating the bank over the next couple months (hot battery, lots of surge current, inverter cuts out with low battery voltage), you could revisit adding the missing cell.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • H2SO4_guy
    H2SO4_guy Solar Expert Posts: 213 ✭✭✭
    #3
    Re: Would you buy another battery?

    Those appear to be very nice batteries and having talked to C & D lately, they discourage using them for alt energy as they are not designed to be used that way. I am anyway and am willing to take the risk of a shorter life. If you find a deal on any more of them share with the class please as batteries are always a concern with off-gridders. Flooded are even more forgiving than the sealed AGM type. I would make it a point to get another as soon as funds became available and you will probably have many years of life out of them. How old are they? You may find that you are a bit light on panel wattage as well, but more can be added later. Suggest reading some of the FAQ's on batteries so you don't 'Murder' them as so many do.

    Good luck and keep us posted.
    12K asst panels charging through Midnite Classic 150's, powering Exeltechs and Outback VFX-3648 inverter at 12 and 48 volts.  2080 AH @ 48 VDC of Panasonic Stationary batteries (2 strings of 1040 AH each) purchased for slightly over scrap, installed August 2013.  Outback PSX-240X for 220 volt duties.  No genny usage since 2014. 
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  • litefoot
    litefoot Registered Users Posts: 3
    #4
    Re: Would you buy another battery?

    A hearty and belated thanks to those who responded to my query. I did not purchase the 24th battery after finding that the Eq, Bulk and Float voltages on both my Outback charge controller and the Magnum Charger/Inverter are fully adjustable to compensate for the missing cell. So I have everything set up and on charge at this time. Today, however, I realized that the temperature compensation function is not so easily remedied (at least not on the Magnum) since the float voltage is adjusted at 5mV/Degree Centigrade/Cell. No sure how to change that cell multiplier or just how to mitigate the possibility of TC overcharging.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
    Re: Would you buy another battery?

    i think it will still be close enough to work.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #6
    Re: Would you buy another battery?

    I'm with neil, too small a difference to matter. 0.005V / cell for 24 cells would be 0.0052V / cell for 23 cells.
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  • litefoot
    litefoot Registered Users Posts: 3
    #7
    Re: Would you buy another battery?
    BB. wrote: »
    Usually the inverter cutoff voltage will be around 42 volts and the minimum recommended operating bank voltage would be ~46 volts (1.917 volts per cell) under moderate loads... So, this this means the minimum battery bank operating voltage would be around 44.08 volts instead... That still gives you a couple of volts for wiring drop and bank voltage sag during start up load surges.

    If your operating current/surge current is not too great, and the battery bank voltage does not get too hot (bank voltage falls as temperature increases), it would appear that you could successfully operate with just 23 cells (assuming you program the correct absorb, float, and equalize voltages in your various charge controllers). At this point, you would just be operating close to the minimum inverter input voltage and a slight reduction in supported maximum cost effective array size (power=v*I=~57volts*controller max current vs ~59volts*controller max current).

    And if you have problems operating the bank over the next couple months (hot battery, lots of surge current, inverter cuts out with low battery voltage), you could revisit adding the missing cell.

    -Bill

    Bill, is the 1.917vpc figure a representation of % SOC? 50% or 75% maybe?
    H2SO4_guy wrote: »
    Those appear to be very nice batteries and having talked to C & D lately, they discourage using them for alt energy as they are not designed to be used that way. I am anyway and am willing to take the risk of a shorter life. If you find a deal on any more of them share with the class please as batteries are always a concern with off-gridders. Flooded are even more forgiving than the sealed AGM type. I would make it a point to get another as soon as funds became available and you will probably have many years of life out of them. How old are they? You may find that you are a bit light on panel wattage as well, but more can be added later. Suggest reading some of the FAQ's on batteries so you don't 'Murder' them as so many do.

    Good luck and keep us posted.

    Yeah, the tech support people at C&D told me the same thing. However, in my case, my intended application (emergency backup for the home) is very similar to a telecom application. So I think they'll be perfect for now. Heck, I guess free batteries are about as perfect as it gets. LOL!
    stephendv wrote: »
    I'm with neil, too small a difference to matter. 0.005V / cell for 24 cells would be 0.0052V / cell for 23 cells.

    Thank you. When you look at it that way, then I see what you mean. I keep worrying about exceeding the maximum float voltage (2.29 vpc) for these batteries, but I have to keep reminding myself that the 2.29 figure is at nominal temperature of 25° C.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #8
    Re: Would you buy another battery?
    litefoot wrote: »
    Bill, is the 1.917vpc figure a representation of % SOC? 50% or 75% maybe?

    Just one more of the rough rules of thumb that take a whole bunch of things into account (or flat out ignore those "things" :roll:).

    More or less, ~11.5 volts would be a battery at ~50% state of charge with a good sized load (near 20% SOC without load--already damaging to a lead acid battery bank)... People keep asking for a "minimum" voltage that they should never let the battery bank go below--And it is really not that simple. Battery voltage is a combination of current, specific gravity/state of charge, temperature, age, etc. And it is not really possible to give a "magic number" that will keep the battery healthy.

    More or less, if you discharge to 11.5 volts (12 volt bank), your battery will probably be OK... and if you discharge to 10.5 volts, the battery bank will probably be permanently damaged.

    Poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings).
    leaf wrote: »
    Am trying to upload the charts I am using...

    Attachment not found. Attachment not found.

    attachment.php?attachmentid=3655

    attachment.php?attachmentid=3654

    I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.

    Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.

    I believe those charts are from Home Power #36, August- September 1993. Lead-Acid Battery State of Charge vs. Voltage ©1993 Richard Perez. Here is a link: http://www.scubaengineer.com/documen...ing_graphs.pdf (from vtMaps)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • H2SO4_guy
    H2SO4_guy Solar Expert Posts: 213 ✭✭✭
    #9
    Re: Would you buy another battery?

    I just purchased 24 of the C & D model KCT-450 today for $47 each and the resting voltage was 2.16 or so on most of them. I would be glad to keep an eye out for you if you want to pick up a single of one of these if more become available. I wanted to pick up at least 6 more for a 12 volt string so I will have spares for when the cells start to die in the 48 volt string. I can sell these back for $29 each, so the risk in cash is only $18 per battery. I can afford to take some chances with that little of money involved, but it can vary depending on the price of lead.

    What voltage are you going to be charging them at? I saw the float published for low float and hi float, but not for absorb or bulk charging.

    Thanks,


    Skip
    12K asst panels charging through Midnite Classic 150's, powering Exeltechs and Outback VFX-3648 inverter at 12 and 48 volts.  2080 AH @ 48 VDC of Panasonic Stationary batteries (2 strings of 1040 AH each) purchased for slightly over scrap, installed August 2013.  Outback PSX-240X for 220 volt duties.  No genny usage since 2014. 
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