UPS-MIB upsSecondsOnBattery is zero whenever the UPS is on mains power, so
"On battery for 0s" read as a timer that had just started. The reading gets
its own unit, battery_seconds: zero says "No, on mains", anything else
"for <span>". The metric key stays on_battery_for, so stored history keeps.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>