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I can change a SRAM battery in less than 5 s. The other can then be charged as you ride along from a battery pack.
@frank9755 SRAM batteries and chargers are surprisingly cheap compared to their D12 equivalents.
I have no skin in his long distance game but I have seen plenty of people stuck in one gear with flat Di2 systems. I have a £40 charged spare SRAM battery in my race bag all the time.
Keep a charger in your bag, and a spare battery. Not ideal I guess but won't take a lot of room