Adding a battery

simagic
simagic Solar Expert Posts: 100 ✭✭✭✭
in Off Grid Solar & Battery Systems #1
I have (6) 100W panels running to my MPPT controller(s) feeding my 224AH (24 hour rate/they give 24hr rates, not 20) batteries. They are (2) 6v Concord AGM in series equaling 12v). I believe my charge rate is somewhere around 14%. I probably could add a little more battery. I'm thinking of adding another 84AH's to give me 308AH. Could i get a confirmation of what my charge rate would be if I increase to 308AH??( I think 10%?) Sooooooo, my current batteries are about 2-3 years old. Typically I never allow them to fall below a 85% SOC / 15% discharge, as i want them to be always "pretty much full" in the event of a power outage. When the sun is out, i use my system for everyday use. I pretty much use what i produce. When the sun is not out I don't use them. Sooooooo (again), I've read "not" to add new batteries to old, but since my current batteries never get discharged very far, might this allow for an exception. Can I add a "new" 84Ah battery. And how exactly do i do that?. The(+) on the new battery goes to where?? /// There are (2) 6 volt batteries therefore (2) plus posts. and same with the neg posts. Which + and which - posts do i attach the new battery......... thanks
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Adding a battery

    Whoops! Brakes on!

    You have 224 Amp hours @ 12 Volts.
    You cannot parallel 84 Amp hours @ 12 Volts to that.

    Right now your 600 Watts of panel can probably deliver about 38 Amps. That's a 17% peak rate.
    So you think bringing the capacity up to 308 Amp hours will reduce this rate to 12%. Logical, but with a fatal flaw: the resistance of the two battery strings would be vastly different. The smaller ones will receive a greater percentage of the current than the larger as a result. In fact they would be hit with about 20% rate while the larger would take 8%. In no time at all they will be unbalanced and likely the smaller ones would cook.

    You can add the same capacity/Voltage/type of batteries to a system, but not different ones. This is also why we warn against mixing old with new: the old ones will have decreased in capacity, possibly to the point where you'd have this severe imbalance.

    But if you did add two more of the same type batteries your peak charge rate would be pulled down to 8%, which is still viable for your system.
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  • simagic
    simagic Solar Expert Posts: 100 ✭✭✭✭
    #3
    Re: Adding a battery

    You say ""Right now your 600 Watts of panel can probably deliver about 38 Amps"". Actually with the MPPT ( when ALL 6 are fully cranking, I've see, but not always, 42amps). Most of the day I don't get that because of shade on one, but have indeed seen 42a on my monitor when theyre all in the sun.

    See, that's why ya gotta go to the "knowing people" before making a move. Would have NEVER thought it would be a problem. So you say "But if you did add two more of the same type (so you mean same AH ?????) batteries your peak charge rate would be pulled down to 8%, which is still viable for your system.".. Do you mean (2) more 6volt 224AH like i have now. I ass-u-me you mean that because you first said 17% charge rate and then said it would be 8% (approx 1/2). For some reason ( and i'm not an expert at the math part), but I thought my charge rate was 14-15% (not 17)
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  • simagic
    simagic Solar Expert Posts: 100 ✭✭✭✭
    #4
    Re: Adding a battery

    You also say """ In fact they would be hit with about 20% rate while the larger would take 8%. In no time at all they will be unbalanced and likely the smaller ones would cook."". So 2 questions to that. (#1), doesn't my CC stop any "cooking" .and (#2), if I do have a 17% charge rate (or more), am I (as you say) "cooking" my batteries that I have now.???? And back to #1, doesn't my CC stop the "cooking" for my existing batteries ( if you tell me 17+% charge rate is too much)???
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Adding a battery

    The 38 Amps is typically what you'd expect as peak charging current based on the standard formula for MPPT controllers: 600 * 0.77 / 12 = 38.5 Amps. Things can affect current output from an MPPT, including cold temps raising Voltage which it turns into more current, or higher elevation or light reflection increasing insolation.

    Yes: two more of the batteries like you've got giving you 448 Amp hours @ 12 Volts. That would be (38 Amps * 100 / 448 Amp hours) 8% rate, which is still above the 5% minimum and may work for you.
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  • simagic
    simagic Solar Expert Posts: 100 ✭✭✭✭
    #6
    Re: Adding a battery

    You also say """ In fact they would be hit with about 20% rate while the larger would take 8%. In no time at all they will be unbalanced and likely the smaller ones would cook."". So 2 questions to that. (#1), doesn't my CC stop any "cooking" .and (#2), if I do have a 17% charge rate (or more), am I (as you say) "cooking" my batteries that I have now.???? And back to #1, doesn't my CC stop the "cooking" for my existing batteries ( if you tell me 17+% charge rate is too much)???
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #7
    Re: Adding a battery

    No, the charge controller can only control the total current not how it gets divided between two unequal battery strings.

    With the one string of batteries you have three things going for you:
    1). the controller can limit the current so that not too much goes to it (although it appears you haven't programmed it to do so or the controller doesn't have that function).
    2). the batteries are to some extent self-limiting; their resistance will in part determine how much current flows through them.
    3). 17% is above the practical maximum but not the absolute maximum which will vary depending on the particular batteries. In combination with #2 this rate is not seen all the time, only periodically. If it were steady current it could be a problem.

    Which brings us back to the mismatched strings: with one always having lower resistance it will always carry more current. The exact balance may not be 60/40 or 75/25 but it will never be 50/50 which is the only one that works. The greater the difference in capacity between two paralleled battery strings the greater the problem will be and the greater the possibility of cooking the smaller.

    BTW, this is related to the issue of keeping wires on parallel battery strings equal in size and length and the issue of multiple parallel battery strings in general: all are a matter of current flow through different amounts of resistance and how that current is divided up between the paths available.
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  • simagic
    simagic Solar Expert Posts: 100 ✭✭✭✭
    #8
    Re: Adding a battery

    You said "... the controller can limit the current so that not too much goes to it (although it appears you haven't programmed it to do so or the controller doesn't have that function).. I programmed my Blue Sky 2512X controller for bulk@ 14.4 and float at 13.3. Does that have anything to do with my charge rate of ( lets say 17%)?? or are we apples to oranges here??
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Adding a battery

    Apples and oranges. Voltage is not current. Your Blue Sky controller does not have the ability to be programmed for current limit.

    A "high end" controller like the Outback or MidNite units can have their output current set to be no more than 'X' Amps rather than their maximum 60/80/90 Amps they are rated for. This is useful if you are in an area with many poor insolation days so you maximize the array Watts and let the controller keep the current from exceeding the desired level on sunny days.
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  • simagic
    simagic Solar Expert Posts: 100 ✭✭✭✭
    #10
    Re: Adding a battery

    Thanks for all that help. When the life of my 224AH batteries are exhausted, I'll buy 300-350 AH batteries. Thanks
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