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Are AGM charge currents incompatible with off the shelf charge controllers?

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  • Are AGM charge currents incompatible with off the shelf charge controllers?

    I was considering a Rolls 250Ah (20 hr rate) AGM battery bank. The Rolls suggested charge profile is attached. Note it starts with a constant current of 0.25*C20.

    If I had three strings in parallel to obtain 750Ah, the charge current would be 188A, if I am doing things correctly. This would require multiple charge controllers - probably 3.

    In the disadvantages/advantages of AGM batteries, I see that they can charge up to 5 times faster than FLA. The large current I am seeing would seem to be consistent with this.

    Am I on the right track? If so - large wiring, large currents, and 3 strings in parallel. Not to mention a need to double the size of my solar array. Time to switch to FLA?

    Don
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  • #2
    You are misinterpreting the chart and instructions. C/4 is the recommended rate, and C/3 is the max. You are correct AGM can be charged at a higher rate than FLA, but that does not mean you should. You can charge AGM as slow as you want, just DO NOT exceed the maximum rate which in your case is C/3.

    AGM are great batteries, but they should only be used for niche applications. The reason I say this is AGM's are twice the cost of FLA and only last half as long as FLA. That makes AGM roughly 400% higher in cost over its lifetime. So if you are going to spend that much you had better have a good reason to justify the extra expense. So what are those reasons:

    1. Mobile applications where spills cannot be tolerated.
    2. Unusual installation orientation like on their side or upside down where liquid electrolyte would spill out.
    3. Extreme cold and by extreme cold I mean below -40 F
    4. Extremely high charge or discharge rates are required like if you live in an area that receives lees than 3 sun hours in a day. With less than 3 Sun Hours means you would have to exceed C/8 charge rate for FLA.
    5. Confined locations where hydrogen gas cannot be tolerated.
    6. You are Lazy, incapable of routine maintenance, and rich who can afford frequent battery replacements.
    MSEE, PE

    Comment


    • #3
      Originally posted by Living Large View Post
      I was considering a Rolls 250Ah (20 hr rate) AGM battery bank. The Rolls suggested charge profile is attached. Note it starts with a constant current of 0.25*C20.

      If I had three strings in parallel to obtain 750Ah, the charge current would be 188A, if I am doing things correctly. This would require multiple charge controllers - probably 3.

      In the disadvantages/advantages of AGM batteries, I see that they can charge up to 5 times faster than FLA. The large current I am seeing would seem to be consistent with this.

      Am I on the right track? If so - large wiring, large currents, and 3 strings in parallel. Not to mention a need to double the size of my solar array. Time to switch to FLA?

      Don
      All you need is a set of Rolls 6CS25P batteries or two strings of Rolls S-550 if you like the FUNs of parrallel battery installaton brings.

      Comment


      • #4
        Originally posted by Sunking View Post
        You are misinterpreting the chart and instructions. C/4 is the recommended rate, and C/3 is the max. You are correct AGM can be charged at a higher rate than FLA, but that does not mean you should. You can charge AGM as slow as you want, just DO NOT exceed the maximum rate which in your case is C/3.
        The only thing I misinterpreted is you can charge these at a lower rate than "recommended". I admit I placed more trust in "recommended" than I should have. My question was, to charge such a bank using the recommended approach, would I be charging at 188A, and could that be done with 3 parallel charge controllers. It appears the answer is yes. I liked the fast charge time, but realized I would need much more PV (and charge capacity) to do it. If I can't take advantage of it, it reduces the reason to go this route. I did see a comment somewhere that AGM are good for smaller installations - my spitballing seems to support that. I don't want to have 3 parallel strings to get the Ah I need.

        AGM are great batteries, but they should only be used for niche applications. The reason I say this is AGM's are twice the cost of FLA and only last half as long as FLA. That makes AGM roughly 400% higher in cost over its lifetime. So if you are going to spend that much you had better have a good reason to justify the extra expense. So what are those reasons:

        1. Mobile applications where spills cannot be tolerated.
        2. Unusual installation orientation like on their side or upside down where liquid electrolyte would spill out.
        3. Extreme cold and by extreme cold I mean below -40 F
        4. Extremely high charge or discharge rates are required like if you live in an area that receives lees than 3 sun hours in a day. With less than 3 Sun Hours means you would have to exceed C/8 charge rate for FLA.
        5. Confined locations where hydrogen gas cannot be tolerated.
        6. You are Lazy, incapable of routine maintenance, and rich who can afford frequent battery replacements.
        Reasons 1, 4, 5 and 6 apply to me to some extent. This installation will be in the Adirondacks. I will be lucky to get 2 hours of insolation in the winter. I do have a generator, but I'd rather not run it excessively. I don't want acid on the floor of my basement, though I know I can build containment to some extent. I don't want acid vapors wafting around my basement, though I can do something to battle that as well. I am lazy and I have had the grid all my life - guilty. Thanks for the reply!

        Comment


        • #5
          Originally posted by Living Large View Post
          The only thing I misinterpreted is you can charge these at a lower rate than "recommended". I admit I placed more trust in "recommended" than I should have. My question was, to charge such a bank using the recommended approach, would I be charging at 188A, and could that be done with 3 parallel charge controllers. It appears the answer is yes.
          Well if the battery capacity is 750 AH then yes.

          Originally posted by Living Large View Post
          I liked the fast charge time, but realized I would need much more PV (and charge capacity) to do it. If I can't take advantage of it, it reduces the reason to go this route. I did see a comment somewhere that AGM are good for smaller installations - my spitballing seems to support that. I don't want to have 3 parallel strings to get the Ah I need.

          You don't have to use 3 parallel battery strings to get 750 AH batteries. AGM cells go up to 4000 AH.


          Originally posted by Living Large View Post
          Reasons 1, 4, 5 and 6 apply to me to some extent. This installation will be in the Adirondacks. I will be lucky to get 2 hours of insolation in the winter. I do have a generator, but I'd rather not run it excessively. I don't want acid on the floor of my basement, though I know I can build containment to some extent. I don't want acid vapors wafting around my basement, though I can do something to battle that as well. I am lazy and I have had the grid all my life - guilty. Thanks for the reply!
          OK then AGM is right for you.
          MSEE, PE

          Comment

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