Yes you dont turn the battery on the button is just to check charge status. Once connected when the bike is turned on it will use the extender first then seamlessly switch to the bike battery.
Would it be possible to use this to connect to i already have 1 on my road bike?
https://bike.shimano.com/en-EU/product/component/duraace-9070-di2/SM-EW90-A.html
There always seems to be problems after updating anything not just bikes. If its working well and there are no new features to be gained i wont update.