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Solar voltage lower when connect; theft or hidden conversion?

cordelclimbs

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Sep 7, 2021
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Hello Everyone,

Great Forum I have already learned a lot. Forgive me if this has already been answered but I couldn't see it

For my first setup, I have assembled a 40V 300W solar array attached to 200Ah sealed lead-acid batteries (2x 12v 100Ah in parallel) through an epsolarpv 30A MPPT.

Before hooking up the solar a quick test with a multimeter shows 39V good for a cloudy day. As soon as I connect it to the MPPT charger the display reading (and a test at the terminals) begins to drop to approximately 13V.
Is this the MPPT dropping the voltage to increase the AMPS to charge my battery or has the charger or I doing something we shouldn't?

Thank you for your help.

Cordel
 
Charging voltage needs to be independent of panel voltage. A battery must be charged with the correct voltage.

Whether you have 20 volts or 2000 volts from the panels, a 12 volt battery still needs to be charged at the appropriate ~13 to 14.4 volts.

That's the purpose of the charge controller.

Edit: if your voltage is dropping on the panel side that's a different thing going on and you'll need to check the output current and voltage, multiply then to get wattage, and see if that's in line with your panel rating.
 
Solar panel gives you maximum voltage at zero Amps and maximum current (Amps) at zero volts. In both cases you have a zero Watts. An MPPT charger is trying to squeeze maximum Watts by choosing the best Volts/Amps ratio.
 
Solar panel gives you maximum voltage at zero Amps and maximum current (Amps) at zero volts. In both cases you have a zero Watts. An MPPT charger is trying to squeeze maximum Watts by choosing the best Volts/Amps ratio.
What he(she?) said.

There will be voltage fluctuations on the panel input terminals.

The important factor once connected is the charging output to your battery.
 
Hello Everyone,

Great Forum I have already learned a lot. Forgive me if this has already been answered but I couldn't see it

For my first setup, I have assembled a 40V 300W solar array attached to 200Ah sealed lead-acid batteries (2x 12v 100Ah in parallel) through an epsolarpv 30A MPPT.

Before hooking up the solar a quick test with a multimeter shows 39V good for a cloudy day. As soon as I connect it to the MPPT charger the display reading (and a test at the terminals) begins to drop to approximately 13V.
Is this the MPPT dropping the voltage to increase the AMPS to charge my battery or has the charger or I doing something we shouldn't?

Thank you for your help.

Cordel
Imagine electricity in cables as water in pipes. Amps is flow (kg/h) and voltage pressure (bar). The solar panel is the water pump. What determines the voltage of the panels is the resistance to pressure which in turns is determined by the cells of the panel and the battery. Open circuit voltage is the voltage with only the resistance of the solar panel cells (imagine this is the resistance of the running pump). When the battery is added the voltage falls to a similar and superior battery voltage. Imagine now you make a hole on a pipe; water flow keeps getting out (amps) because there is always a little over pressure (volts) inside the pipe. If the whole is bigger (lower voltage battery) you get more flow but you still keep some overpressure.

If you use a 24V battery (the two in parallel that you have) with a 24V controller and you will see a voltage of 25-26V from the panel.
 
As soon as I connect it to the MPPT charger the display reading (and a test at the terminals) begins to drop to approximately 13V.
Its not clear which terminals were been measured and what the battery state of charge existed at the time.

If the battery is at a low state of charge or is loaded, the controller should be in MPPT mode. In MPPT mode the panels, assuming solar conditions are good, will deliver volts around 36 volt and up to 10 amps to the controller, thus 36 volts should be seen at the controller input terminals.
The conversion process will deliver to the battery the correct charging voltage 13 to 14.4 volts, ( depending on battery type and state of charge).
Once the battery is charged and the controller is float mode it will no longer search for the maximum power but adopt a loading on the panel to reduce the power needed to maintain the voltage at float level on the battery.

Mike
 
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