Solar Panel Voltage at Charge Controller

Re: Solar Panel Voltage at Charge Controller

If everything is connected correctly, and the batteries are pretty much fully charged... Then the controller is probably working fine.

The "MPPT" part of the controller (Maximum Power Point Tracking) occurs when the controller is charging a severely discharged battery (below ~85% or so state of charge)... The controller is trying to pump as much energy (current) into the battery as it can.

It does this by monitoring the Panel Voltage and Current... And maximizing the equation Power=Voltage*Current (adjust the current until Power is maximized). The controller then takes that Power and (at the panel's Vmp and Imp) and converts it to Pbatt=Vbat*Ibat (with a few percent loss of power in the controller). This is the "Bulk Charging" stage.

Once the battery is above ~85% State of Charge--the controller cuts the controller uses less than maxim current from the solar panels--which allows the solar panel voltage to creep up towards Voc (Voltage Open Circuit)--called "Absorb Stage". If you are reading 15 volts on the battery--it sounds like you are in this mode.

As the battery approaches full capacity, the charger will cut back on the battery voltage to around ~13.8 volts. "Float" stage. At this point, there is very little current flowing into the battery and the controller is just providing enough voltage and current to keep the battery fully charged. The PV panel voltage will approach Voc (open circuit) and the panel current will be very low (probably well under 1 amp).

-Bill