If you have solar panels connected to your BatteryPlus35, and the Trek monitor is displaying "--", it most likely means the solar panels have not been connected to the system properly.
Double-check the solar connection points on your BatteryPlus35 to confirm the solar is connected correctly.
If your solar is connected correctly, it could mean there is a fault with the panel or a fault with the wiring.
Contact your local dealer to have the panel and wiring checked.