The B Max is clever (or stupid to some) in that when the battery charge level is low, the management system renders some heavy use features inoperative.
My B Max did need regular daytime use to keep the charge up.
Try an overnight trickle charge.
I just happened to have a battery I bought for another car and took it off before I sold that car.
This battery was the same size but was a bit taller, The original B-Max battery has this large plate over the top of it to hold it in and other metal straps, I Did Not fit back this large plate and it is still very secure with the other straps.
As you can not get at the Negative terminal I very carefully removed the positive terminal first (Not good practice as there is a risk of shorting it to earth (the cars body).
You have to remove the Engine air intake cover ( you can leave the filter in place but protect any dirt etc. dropping onto it.) then remove all the battery straps and the plastic front of the battery box.
You can only get the battery out by angling it at about 45 degree's.