Where we have batteries in use in our smart home we monitor their voltage and use this to assess their state of charge. The main example of this in our @smartest_home is the battery used for our Uninterruptible Power Supply (UPS). This is addition to the battery level monitoring built in to wireless sensors.
A related sensor is our smart home is our mains power sensor.
The main way we measure voltages in our smart home is by using an Arduino with its analogue input pins. These typically work with 5V dc (or less), so we have to use a voltage divider circuit to allow a higher range of voltages to be measured. This approach provides the ability to measure with a good level of accuracy and a decent resolution (typically we measure to the nearest 0.1V).
