As a twonk who chooses to use his bow thruster for its designed purpose of assisting with manoeuvring, particularly in tight spaces between other craft in a crowded marina (after all why have on fitted if it is not to be used?) I often tie-up with a depleted B.T. battery.
The fact that on our craft, in common with many, the battery charger only serves the leisure batteries ensures a short life for the B.T. battery. Hence my comment that such a set-up is "just plain daft" and my advice to ensure that it gets probably charged and maintained.
Modifications to the B.T. charging system on my craft are in hand to allow charging via a land-line or solar panels.