Hi Luigi
I'm not sure what is causing your problem but it doesn't appear to be a problem with your controller settings or your batteries.
I suggest that you start by temporarily disconnecting both of the brake switch cables by unplugging the connectors and see if the throttle problem still exists.
Check out
this thread for a similar problem that was caused by a wire shorting out inside one of the brake switches.
If disconnecting the brake connectors cures your problem then plug them back in one at a time to determine which one is faulty.
If the problem still persists with the brake switches unplugged, it could be an internal short circuit within the main wiring harness, a faulty thumb throttle, or possibly the controller itself.
Try unplugging the brake cables first and let us know what happens.
Alan