The batteries inside the hubs are rechargeable. You can expect around 40 hrs of run time or ride about 1 to 3 months with full batteries in normal use. To prevent issues, we recommend charging batteries before each use of your wheelset.
Inside the control unit, there is a replaceable coin cell battery (type CR1/3N), which lasts for about 6-12 months, depending on usage.
You can easily read the battery status of both wheels and the control unit from the Gravaa app.