orel:
Thanks.Do you have any reference to suggest for a "non intelligent" power bank ?
Not really. Look for reviews, see what people write about them.
Also you have to be sure to get one that has proper battery protection, otherwise your Arduino will be able to drain the battery completely and destroy it.
Proper power banks have protection built in - when the battery inside goes too low, the boost converter will shut off and the cell will be disconnected. On the cheaper ones, the boost converter will stop working, but you will still have approx 3V at the USB-plug, enough for the Arduino to still keep operating and it will continue to do so while the cell gets dangerously low. Most "proper" USB-devices that people use with power banks (Cellphones etc.) will just stop charging when the voltage drops any lower than 4.5V
// Per.