Use a DC Multimeter and measure the potential between Pins 1 (V+) and 3 (GND) of the pack’s UW-6 plug connector.

UW-6 Plug pin connections

 

Healthy operational pack voltages range from 12V to approximately 15.1V, depending on the amount of direct sunlight and the age/health of the installed battery.

Significantly lower observed voltages may warrant a battery replacement (every 3-5 years standard) or solar regulator replacement (only in event of failure).