[Observium] how to set an alert based on a custom OID?

Hello all

For our UPS i have implemented some custom OIDs, here is an example:

Description: Battery Voltage OID: .1.3.6.1.4.1.818.1.1.10.2.5.0 Name: upsBatteryVoltage.0 Type: Gauge Unit: V

How to i now create an alert check that i.e. fires if the Value is below 300?

Best, Thomas