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  • Question about battery charging.

    Hy. I have a 100W solar panel rated at 5A, i did a simple charger using a simple LM388 ( 5A linear regulator for testing, i know a mppt would be better but the cost of it dosent realy make sense for me for that price i buy one more 100W panel and with loses and everything still i will get more that the mppt can get out from the singel 100w panel).

    Everything was fine so i got a 12V lead-acid battery , put it to charge, it was drawing 4.2amps yesterday tuday it draws only 0.8A so it prity much charged so i linked the inverter to the battery ( to drive a 50w water pump) the inverter needs about 3.8amp to drive the pump so i was hoping that the inverter will pull 3.6 +/- amps from the solar panel and if it requires more he can get it from the battery but it dose the reverse. When i start the inverter the panel give 1.2amps instead of 0.8 but thats all so the inverter pulls the 3.6amps from the battery wasting the power of the panel....

    Is this normal because i used a linear regulator ? would a buck-converter (PWM step-down) solve this problem ?

    THX for any help.

  • #2
    What you need to figure out is what the short circuit current from the panel is with the light that is hitting it.

    If you have a 100W 5A panel, that will correspond to a maximum power voltage of 20V IF you are getting full sun to the panel on a cool day.

    You may also be having a problem with the regulator, but not because it is linear.
    Its output is in parallel with the battery, so the voltage drop in the wiring from regulator to battery may be a factor.
    You may just need to raise the set voltage of the regulator. What voltage are you using right now? Depending on the battery you may want to go as high as 14V.
    Last edited by inetdog; 08-16-2015, 01:37 PM. Reason: Added voltage info
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      Originally posted by inetdog View Post
      What you need to figure out is what the short circuit current from the panel is with the light that is hitting it.

      If you have a 100W 5A panel, that will correspond to a maximum power voltage of 20V IF you are getting full sun to the panel on a cool day.

      You may also be having a problem with the regulator, but not because it is linear.
      Its output is in parallel with the battery, so the voltage drop in the wiring from regulator to battery may be a factor.
      The panel is rated at 5.5A and 22.3V (open circuit) the max i measured in short-circuit was 5A. Well the voltage drops about to the battery voltage +0.2V ( if the battery is at 12.4 and i connect the regulator to it i measured somewhere around 12.6V first i was thinking that my regulator is messing around when i connect to the battery and drops the voltage so i tryed to connect the solar panel to the battery directly for a very short time just enought to check out the voltage ( 3-4 sec ) but the result was the same so i think thats what the panel is capable to put in the battery around 3.8 amps in a very sunny day i measured 4.2 amps. But still i dont get it why he can provide 3.8Amp to the battery and when the battery is almost charged and only draws 1A the inverter will eat my battery instead to get 3.8A(panel) - 1A( battery drawing without invertor) = 2.8 Amps that the panel can provide then take another 1.2amps from the battery ( the inverter needs 4A to run the AC pump). The inverter draw it's power from the battery and it looks like tha battery is drawing the same 1A+200mA from the panel instead to get the full 3.8Amp and feed it to the inverter.

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      • #4
        So i think that until the battery voltage drops ( so the battery will discharge to maybe 70%) and it will pull again the 3.8 - 4 amp charging current it's not possible to draw more current from the panel than the battery wants to pull. So if i charge the battery to the level where he only draws 1A or less until it discharge to a level the power from the panel will be wasted bk the inverter will draw from the battery without the panel to charge it back at the same rate the inverter pulls.

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        • #5
          your only going to get the panel to supply the power when the battery is fully charge floating (in the 13.7 volt area or higher). On my system 12 volt 102 ah battery connected to a 240 watt panel. if the battery is reading 14.3 volts (which is where i have it floating) it might be providing the battery only 1 amp or less, as soon as I connect a load a 7 amp fan, the controller/pnl amperage goes up to about 8 amps. The battery is still running the fan but the panel is replacing the exact amount of amps I'm using.

          if its morning (about 12.8 volts on battery) and the panel is only providing 3 amps, if I connect the same 7 amp fan, the battery will have to provide most of the power. the panel will only provide 3 amps which is the maximum it can produce with the limited sun available.

          If your battery is only reading 12.6 volts its not fully charge, its about 70/80 percent. Your 100 watt pnl/regulator is not charging it. When you are charging you need to see 14.4 volts and it needs to stay at 14.4 volts for at least 2 or 3 hours. What you need to see is 12.6 rise up slowly until it reaches 14.4 (it will take hours at 5 amps). Once it gets to 14.4 volts the amps will slowly drop as the battery gets full.

          You are correct mppt will not be an advantage on your 22 volt panel. You need at least 30 volt panels to give you an advantage.

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          • #6
            [QUOTE=bndi94;169364]Hy. I have a 100W solar panel rated at 5A, i did a simple charger using a simple LM388 ( 5A linear regulator for testing, i know a mppt would be better but the cost of it dosent realy make sense for me for that price i buy one more 100W panel and with loses and everything still i will get more that the mppt can get out from the singel 100w panel).

            A simple *quality* pwm would be a better choice, like a Steca or say Morningstar SS-10-12v.

            Everything was fine so i got a 12V lead-acid battery ,....
            New - used? A battery that is sulfated from abuse or neglect will charge allright - usually faster than expected, but not really support your loads. What brand/make/model and ah capacity is it?

            The linear regulator can work, but a pwm waveform is actually nicer to the battery during absorb, especially if it has already been abused.

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            • #7
              Originally posted by bndi94 View Post
              So i think that until the battery voltage drops ( so the battery will discharge to maybe 70%) and it will pull again the 3.8 - 4 amp charging current it's not possible to draw more current from the panel than the battery wants to pull. So if i charge the battery to the level where he only draws 1A or less until it discharge to a level the power from the panel will be wasted bk the inverter will draw from the battery without the panel to charge it back at the same rate the inverter pulls.
              The problem is, I think, that your regulator is set to too low a voltage. The .2V voltage drop in the wiring and in the regulator itself does not allow the regulator to supply more current without a significant drop in battery voltage. That drop in battery voltage can only occur if the battery supplies more current.
              SunnyBoy 3000 US, 18 BP Solar 175B panels.

              Comment


              • #8
                Originally posted by bndi94 View Post
                The panel is rated at 5.5A and 22.3V (open circuit) the max i measured in short-circuit was 5A. Well the voltage drops about to the battery voltage +0.2V ( if the battery is at 12.4 and i connect the regulator to it i measured somewhere around 12.6V first i was thinking that my regulator is messing around when i connect to the battery and drops the voltage so i tryed to connect the solar panel to the battery directly for a very short time just enought to check out the voltage ( 3-4 sec ) but the result was the same
                That is what is suppose to happen. You would be miles ahead just using a Diode to connect the panel directly to the battery and get rid of the regulator. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- What you do not seem to understand is how a battery operates, especially during a charge cycle. Example let's say you have a 12 volt 100 AH battery at 50% SOC open circuit voltage of 12.1 volts. Now apply a 5 amp charge current. What is the voltage of the battery and charger? ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- You do not know do you? Well if that battery is a Trojan 27TMX a 12 volt 100 AH battery has a Internal Resistance of roughly .005 Oms which means the charger and battery voltage has to be 12.1 + [5 amps x .005 Ohms] = 12.125 volts. Does not matter if the charge source voltage is 100 volts, if all it can supply is 5 amps it will drop from 100 volts to 12.125 volts period end of story. In about 10 hours the voltage will creep up to 13 volts. ------------------------------------------------------------------------------------------------------------------------------------------------------------------------- The mistake you have made is you assumed a Solar Panel is a Stiff Voltage Source, when in fact it is a soft Current Source, not voltage. ----------------------------------------------------------------------------------------------------------------------------------------------------------------- A PWM Controller is nothing more than a On/Off switch. When th ebattery demands a charge, the controller connects the panel directly to the battery. Your 100 watt battery panel is rated at 5 amps maximum. That means at most it can generate is 5 amps x 12 volts = 60 watts. That same panel with MPPT is a Power Converter. It converts the panels 20 volts @ 5 amps into 12 volts @ 8.3 amps or 100 watts from a 100 watt panel. --------------------------------------------------------------------------------------------------------------------------------------------------------------------------- So with a LM3XX regulator all you got to work with is 5 amps at very best. Voltage is irrelevant and is just a product of the current flowing into the battery resistance added to the battery open circuit voltage. Every battery works that way.
                MSEE, PE

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                • #9
                  Forgot to add. If you want to use the Regulator, set the output voltage to 15 volts. Unless you never use the battery, you will never see the voltage get to 15 volts. The Regulator will just turn Full-On passing maximum panel current to the battery. Exactly the same thing if you just tossed the regulator in the trash and replaced it with a Diode. The Diode is only needed to prevent the battery from back-feeding the panel at night, thus discharging the battery at night. The only thing the Regulator will do is Float the battery voltage and stop charging in the RARE event it ever became fully charged. Only a properly designed solar system with a generator is capable of ever fully charging a battery.
                  MSEE, PE

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