Batteries for the sensors are flat lithium cells, CR1632, fairly widely available, and you can order them here. They should last at least a year in normal use.
Replacing them only takes a few moments, just unscrew the outer cover from the sensor and slip the new battery into place.
Are the sensors only ‘on’ while the receiver is on? in other words, will the battery last longer on a low-usage vehicle, and flatten quicker on a high-usage one? I assume the receiver will indicate a low-voltage, missing, or damaged sensor.
The sensors have a sophisticated power-saving design. They go into a ‘sleep’ mode when the vehicle is stationary and this sleep mode is independent of the status of the monitor.
The biggest power drain is in transmitting data, so although the pressures may be checked every few seconds, data is only transmitted if there has been any change.
The system includes a low battery warning system and an indication is given if transmission fails.