Here's a thought: The new battery-box in the 787 is vented to atmosphere, so at altitude it's going to be at low pressure, and maybe quite cold (I don't know if the box is insulated, or how much of the aircraft's warmth will penetrate). On descent the box will "breathe in" the outside air, and if it's humid, will the cold battery have a tendency for condensation to form on it? I also wonder if they had to strengthen the battery's case to withstand the low pressure around it?