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Better regulated currvolties™ I guess. I've heard people charging Di2 with power packs have thought batteries are dead only to plug them in using a PC and it works again.
Di2 has some kind of PC interface software that does some performance stuff, doesn't it (not plugged mine into PC since 2 years ago)? Doubtful that the leccy from a PC usb port would be any better for a battery - but worth a try!
(Says he who forgot to charge Di2 and had to ride 80km singlespeed yesterday!)
Will give the PC method a go - any reason why’d it would be different to a USB plug though?