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Charging and discharging in cold temperatures

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  • Charging and discharging in cold temperatures

    Hi, I have a 900AH AGM battery bank. I have a midnight solar battery state meter hooked up and set for AGM batteries. I noticed that I couldnt get the weekly meter to go green and figured out that I needed set my voltage a bit higher. I dont know the math for temp to cell. Can anyone explain easily?

  • #2
    Higher Voltage ,

    You need to get the Volts per 10 Degrees C from the AGM Battery manufacturer
    A typical value may be 0.028 Volts / 10 Degrees C
    That is ... per cell

    And there may be a Low Cut-off Temperature, to limit the Max Voltage.

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    • #3
      Thanks

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      • #4
        As an example, Rolls-Surrette recommends a temperature compensation of -0.004 V per degree C per cell for its AGMs. For my 48V battery made up of 24 cells (eight S6-460 6V batteries made up of three cells each), that works out to 0.096 V/deg C.

        Right now on this winter morning in WA, the battery is 4 degrees C, so the increase in voltage set points is (25-4)*0.096 = +2.016 V.

        Unfortunately, my Outback Radian seems to have a fixed "one size fits all" temp comp coefficient of -0.005 V/deg/cell, which means I will have to change the charging set points somewhat depending on the average battery temperature. Come summer when the battery temperature is averaging 27 degrees C, the wintertime set points will be about half a volt too low. Probably not a big deal, and the numbers would still be well within Rolls's range of acceptable charging voltage values.

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        • #5
          Im in CAN. We are in the minuses now. As low as -12 C.I am also 48v. I still use a 24 cell set up for voltage but I am running 48 12v ups batteries. C&D Tech.UPS12-300MR. Not my preference but I paid almost nothing so I cant complain. I dont have a BTS for my controller, which I should have. I guess for me it is a money thing. So for me at -5 we are : 30x.096=2.88+56.8=59.68V. Is this correct? My inverter does not like the higher voltages . Thanks for the math lesson.Ill see if I can find the temp.comp. for my batteries.

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          • #6
            If I were to get a BTS ,where would be the best place to install?

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            • #7
              Originally posted by Higher Voltage View Post
              Im in CAN. We are in the minuses now. As low as -12 C.I am also 48v. I still use a 24 cell set up for voltage but I am running 48 12v ups batteries. C&D Tech.UPS12-300MR. Not my preference but I paid almost nothing so I cant complain. I dont have a BTS for my controller, which I should have. I guess for me it is a money thing. So for me at -5 we are : 30x.096=2.88+56.8=59.68V. Is this correct? My inverter does not like the higher voltages . Thanks for the math lesson.Ill see if I can find the temp.comp. for my batteries.
              A quick look through your battery's manual didn't reveal anything about temperature compensation, but they recommend the same float voltage as for mine: between 2.25 and 2.30 V per cell. That works out to 54.6 V in the middle of that range, for float service. The "freshening charge" they recommend (clearly only intending these for backup usage) works out to 57.6 V for your battery bank.

              Assuming your batteries (also AGM) would also use the -0.004 V/degree/cell temperature compensation, yes, your adjustment would be the same. Say your average battery temp is -6 C, or 31 C below the standard room temperature for which battery charging parameters are published. You would add +3 V to these numbers: "Freshening charge" at 60.6 V and float at 57.6 V. Or, using the 56.8 V figure you cite, 59.8 V instead.

              The temp sensor should be taped securely to the side of a battery in the middle of your battery bank, about halfway up. Don't use double-stick foam or anything that puts thermal insulation between the battery and the sensor.

              Remember, these are UPS batteries and are best used for float service without much cycling.

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              • #8
                If you care about your batteries, you won't wing-it with manual temperature compensation adjustments, but use a controller with a temp-sense wire attached to the battery instead.

                The lead in batteries don't react quickly to ambient temperature changes, so it is best to automate this if your batteries are not in a temperature-controlled environment. Simple manual adjustments for temperature can lead to under / overcharge as the battery is slowly ramping up or down in temperature.

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