I have a new Traccar install for a single vehicle.
The tracker is an FMC920, which also sends 2 voltages, called 'power' and 'battery', which I want visible in HA.

The standard 'battery' entity (of the 8 default entities added to HA) is 'Unknown'.

I tried adding power and battery to the 'custom attributes' but I still have only 8 entities per device in HA.

What step am I missing?
Go to the device_tracker.xxx entity, there u will see the attributes.
You can use a template helper to create a entity e.g.
{{ state_attr( 'device_tracker.xxxxx', 'power' )}}
I have a new Traccar install for a single vehicle.

The tracker is an FMC920, which also sends 2 voltages, called 'power' and 'battery', which I want visible in HA.
The standard 'battery' entity (of the 8 default entities added to HA) is 'Unknown'.

I tried adding power and battery to the 'custom attributes' but I still have only 8 entities per device in HA.

What step am I missing?