Your battery twice has discharged to the point of no return. You can keep it alive for a while using the tender all the time but because of that it has built up sulfate on the plates.
Some people suggest shocking the battery to remove the sulfate but I will not do that. Your car is an electronic beast and as such locking the car shuts down the DME which will pulse the car every 4 hours for an entire status. Locking the car puts everything to sleep but the IBS.
This car has an overly complex charging system and heavy demand on the 180 watt alternator.
This is why BMW wanted to go with a 48 VDC charging system but abandoned it since it posed too high a safety risk according to BMW.
My 2007 is past 6 years battery life because of my battery tender and my M3 has now reached 9 years with the use of a battery tender.
I think you will get more use out of your battery which is pretty close to dead because the tender will keep it topped up as best it can. Since you had one for your motorcycle I am sure it must have helped that battery...