Power for the Smart Hub 2
If back-up power is required, and you do not want to incur the expense of a proprietary uninterruptible power supply (UPS), then one simple solution is a sealed lead-acid (SLA) battery on permanent charge. This is known as float charging. Charging voltage depends on ambient temperature and must be regulated in accordance with the battery manufacturer's instructions. A typical value is 13.8 Volts.
Can this slightly higher voltage be supplied to the Smart Hub without the risk of undue stress on any electronic components in the Hub? Since the design parameters of the Hub are unknown it is not possible to gauge the long-term effects. However, experience over a limited period of a few months has not revealed any problem.
Power supplied to the Smart Hub at 13.8 Volts causes a small reduction in the input current, presumably because there is an internal inverter generating the higher voltage supplies for the telephone port.
The UPS solution offered by BT is produced by CyberPower. BT's user guide states that the unit has a sealed lead-acid battery (12V; 7Ah) and has an "on battery" output of 12 volts (no mention is made of output voltage under other conditions). CyberPower's own website shows the same unit with technical specifications: SLA battery 12V 7.2Ah; output voltage in "line mode" 13.6V +/-5% (ie 12.92V to 14.28V); output voltage in "battery mode" 10.5V to 13.6V. It is assumed that "line mode" applies when the battery is charging, "battery mode" when mains input is off. Presumably BT are satisfied that the Smart Hub can be operated within the ranges specified by Cyberpower.
Power for an ONT
Customers with Fibre to the Premises (FTTP) have an Optical Network Terminal (ONT) provided by Openreach. The ONT also requires power. Various models of ONT by different manufacturers have been provided by BT. Earlier units did have a battery back-up which would provide continuity of service for a limited period. Power requirements of the different ONTs are not known. However, the website ISPReview has published details of one Nokia ONT apparently introduced in 2021. The article shows the power supply rated at 12V 1A DC. The actual current required will be lower.
Common Supply for Hub and ONT
If the ONT requires a 12-volt supply, it may be possible to power both Hub and ONT from the same UPS, provided that the supply can deliver the total current required. Resistance measurements on the Smart Hub 2 show that there is no path at DC between its power input socket and any pin of the LAN (Ethernet) ports. However, the negative line of the power supply input is connected directly to Pin 4 of the USB port.
Using a common supply may create, along with the data cable between the Hub and ONT, a loop capable of picking up radio frequency interference which could find its way into the data cable or ports. This might degrade the data signal and reduce transfer speeds.
