How old is your battery backup? Is it possible to pass on the make and model of your battery backup/UPS? It's always advised to get a UPS that is rated to provide more power than your system will consume so to avoid any loss in terms of conversion. Rule of physics is that some energy will be lost and nothing ever runs at 100% efficiency unless manufacturers have found a way to eliminate restivity in the wires.
It'd help us further by passing on the make and version of your GTX980Ti cards and essentially your full system's specs inclusive of your OS and other components that may be connected to your battery backup. According to
this list your PSU shouldn't be overdrawing from the wall. I'd also suggest keeping Seasonic support in the loop. It may be a possibility that your unit may need to be RMA'd or the UPS alone or both.