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Let's assume we have a 60 cell 260W PV panel with the following STC characteristics:

Voc= 38.1V Isc=8.98A Vmpp=31.1V Impp=8.37A

The panel is exposed to direct sun light and standard conditions are present (1000W/m2 and 25C). The panel is connected directly to an automotive type, 12V lead-acid battery without any dc-dc converter. The internal resistance of the battery is 100 milli-Ohms.

What would be the approximate operating voltage and current of the solar panel connected to the battery, assuming it is not fully charged.

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Given that 12V is largely lower than your Vmpp, your PV panel will operate in the constant current part of the characteristic. The current will be close to Isc. Is it then easy to calculate the voltage.

EDIT: I forgot I have a simulation of a PV panel here that could help explain what happens when you connect the panel to a rather low voltage battery. The characteristics are not the same as in your example, but you can simulate the same kind of operating points. Click on the switch to select the battery, and choose an emf (fém in the web page) below 12V. You will notice that the current stay close to Iscc (5.3A for 1000 W/m2 and 25°C).

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  • \$\begingroup\$ If the current will be close to Isc, the voltage generated should be very low too, much lower than the battery voltage. Would the battery be discharged by the solar panel then, since the current will flow from the battery to the panel? \$\endgroup\$
    – Simone
    Dec 13, 2018 at 17:35
  • \$\begingroup\$ The current is close to Isc, but that does not mean the voltage is close to zero. In this part of the characteristic, the PV panel behaves like a current source. \$\endgroup\$
    – Charles JOUBERT
    Dec 13, 2018 at 22:10

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