But that's an issue at very low voltages. The protection circuits should & did disconnect the batteries before they got that low.
If these batteries were being replaced then they weren't just going 'service low', they were going critically low ie the point at which they can no longer be recharged and basically become very expensive scrap!
The estimated number of replacement batteries is 100-150. Even taking the lower of those figures that is, as someone has already pointed out, more than two batteries per aircraft in just over a year in service. If these batteries were replaced by Boeing under warranty, then they have known that they have a problem with the battery/charging/monitoring system for some time.
I would be very interested to know the timescale of these replacements. If they started in October 2011 and the number replaced is nearer the 150 mark then that is potentially as many as six or seven batteries replaced per aircraft!
I would also be interested to know how the number of replacements break down between main and aft as they are identical units but are both connected to differing parts of the electrical system. This information would be a big aid to diagnosing the problem.
What can be said for certain is the that the two battery events probably came as no surprise to Boeing and that they must have been working on a resolution to this problem for some time while throwing $130,000 worth of battery at each aircraft per year.
