There are a few write ups here for roadster battery replacement. But before going that route, make sure that your car is delivering 12v to the secondary battery. You didn't mention on 1.5,2.x version. I assume 2.x as the 1.5 did not have the additional battery but not sure if the software updates would assume any 12v code to differentiate between models.
Gruber does do some sweet replacement batteries but you may find something closer to home, shipping to Belgium may be cost prohibitive.
You are safe to drive it as the main pack provides 12v power in normal circumstances, a member here drives his without one and has had no issues. Personally I like the idea of having the emergency systems running all the time. Its worth getting acquainted with the search function as there is a lot of information already posted about the 12v system