If your Hub temporarily loses its power source and has depleted its battery from 8.5V to 7.5V, a Trouble Condition will appear in the app and your Hub will enter Low Power Mode.
Within Low Power Mode, your Hub will decrease LED brightness, turn off the Z-Wave controller, and make other concessions to conserve power for life-saving operations until plugged in again.
If the power at the property had been lost but is now restored, plug the AC adapter into your Hub and allow up to 72 hours for the backup battery to recharge. Once the battery is fully charged again, the "Panel Low Battery" trouble condition will disappear.