Primary battery
Over time the primary battery voltage level will drop where it only can supply the buoy while the solar panels are charging the battery. When the primary battery voltage level drops below approx. 10.5 volt the ToughBoy will switch to the secondary battery. In the "not in use" periods for the secondary battery, the voltage level rises a bit and the consumed current drops. Voltage levels are not really measured for the primary battery, when not in use, due to the switch circuitry.
It is advised to replace the primary battery when voltage drops under 11 with a freshly recharged battery to avoid too much drain on the secondary battery (non rechargeable).
Secondary battery
On default the warning sent to Onshore webUI is triggered at 9.5 volts. This warning is defined in the System.xml
The actual secondary battery drop off level should be around 6-7 volts after which the secondary battery is likely to be unable to boot/run the system.