I would have thought that there would be an exchange program for such an item, as they are too expensive just to scrap because of a eg: battery low diagnostic message.
I agree, although after a certain number of 'returns' I suspect Boeing/Thales/Yuasa would want to take a good look at these batteries rather than have them taken apart for reconditioning and put back into service.
I'd love to know the proportion of these replacements made to the 'APU' as opposed to the 'main'. If it is heavily weighted towards one or the other, that would be a big clue as to what was happening. Even if it was 50/50 it would help as that points towards a 'battery/charging' problem rather than a 'load/operation' problem.