A couple of thoughts....
Boeing's insistence on retaining the lithium batteries make me think that they weren't chosen just for their size and weight, but that some electrical characteristic (such a low internal resistance) make them central to the design of the electrical system. It's been said here that the diode essentially isolates the battery from the bus in normal operation, but this isn't strictly true: any undervoltage, even for a time measured in milliseconds, will cause the diode to conduct and the battery to attempt to prop up the bus. The 787 electrical system is huge by aircraft standards and may be subject to many transient effects as the various loads start and stop, within a time frame that the main generating system may not be able to compensate for. So the battery may have a more active function in the system than just a backup supply.
Secondly, I find the present focus on the batteries and chargers as isolated systems to be pretty naive. The engineers at the various plants involved can be assumed to be reasonably competent and that they would have picked up on any design fault that results in an MTBF of only a few months (or one that will be identified by outside, non-expert inspectors). Rather, the problem is likely to be a result of battery operation within the complete aircraft system, whereby electrical or environmental loads are being placed on the battery that were not envisaged in the specification or tested for during construction.
Edited: fixed typo
Last edited by fizz57; 31st Jan 2013 at 17:26.