APC Symmetra LX model SYAF16KRMT switches to battery and back to building power

[GBarrett]

I have a Symmetra LX model SYAF16KRMT that is running on 200VAC. The input voltage is 201 volts and the output voltage is 207VAC with a load of 38%. Last March the UPS began to shift to battery and right back to regular AC power. The charge is always 99% and never reaches 100%. The UPS shows no faults and passes diagnostics and all self test. Would a phase imbalance cause this? Also I can't find and data on what voltage this model will shift to battery power.

Thank you,

Glenn