Check the battery voltage. If the voltage is at a target charging voltage, then the module is adequately charging your batteries. For example, if you measure 13.7V on a 12V battery, then it has reached the Float charging voltage, and the battery is being adequately charged. Remember that the target voltages may be higher in cold weather and lower in hot weather due to temperature compensation.
Check the battery voltage over several minutes. If the voltage is increasing, the battery is being charged. If the voltage is increasing very slowly, then the module may not be receiving enough sunlight or another problem might be to blame. Check the terminal connections and any wiring connections of the PV module to the charge controller and the charge controller to the battery bank. Clean your panels and remove any obstacles causing shading, no matter how small.
Use a controller accessory with current measurement (if applicable) or a digital multimeter to measure the module current. Compare the measured value to the module datasheet. Check the battery manufacturer datasheet or user guides for recommended charge current or charge rates (C-rates).
