Monitor backup sump pump

Putting this question here as assume I would want a Z-Wave technology as a solution. I have been struggling with monitoring the pit of my sump pump.

I tried putting a sensor into the pit but condensation or splashing would cause false warnings that the pit had gone more than an inch over where the primary sump pump usually would kick in.

I tried attaching a voltage monitor inline to the secondary sump pump to see if it triggers but now thinking that was dumb. My secondary is basically just a battery charger plugged into the wall. The charger connected to the batteries. The batteries are then connected to the sump pump. So if I am monitoring power usage I’m thinking what is being read from the charger is never going to change just because the secondary sump pump is going off since it’s drawing power from the batteries.

I’ve ordered a float but not sure it will work with my Fibaro flood sensor. Still want a secondary check. I’ve ready about some of these devices that clamp on the cables and detect power, but not sure if those would work either. Any thoughts on how to detect if the backup sump pump goes off?